LTE University

LTE TDD (TD-LTE): How much different from LTE FDD?

Nishith Tripathi — Tue, 27 Jul 2010 21:08:00 GMT

By Dr. Nishith D. Tripathi

While initial commercial deployments have focused on FDD (Frequency Division Duplex) version of LTE (Long Term Evolution), interest in the TDD (Time Division Duplex) version of LTE has been rising. The TDD-based LTE is also known as TD-LTE (Time Division- LTE). We will discuss TD-LTE from the eyes of LTE FDD; the reader is assumed to be familiar with LTE FDD.

First and foremost, TD-LTE shares the same channel bandwidth between the uplink (UL) and the downlink (DL). As an example, the LTE FDD uses paired spectrum such as 10 MHz in the UL and separate 10 MHz in the DL. In contrast, the TD-LTE would use the same 10 MHz for both UL and DL. While FDD allows simultaneous transmission and reception at an entity such as the eNodeB or the UE, TDD involves either transmission or reception at a given time instant. The main implications of using TDD instead of FDD are that the service operator does not need large amount of spectrum to deploy TDD and that the average throughput is slightly lower in TDD due to relatively higher overhead. We summarize below key differences between TD-LTE and LTE FDD related to the frame structure, radio channels and signals, data transmission in UL and DL, and deployment aspects.

LTE FDD uses Type 1 Frame structure, whereas TD-LTE uses a Type 2 Frame Structure. While traditional FDD systems use symmetric spectrum in UL and DL (LTE FDD does allow asymmetric bandwidth), TDD has inherent support for an asymmetric use of UL and DL. The Type 2 frame structure defines various configurations that basically specify how much time is dedicated to the DL and to the UL. The UL:DL ratio varies from 3:2 ("uplink-heavy") to 1:9 ("downlink-heavy"). Within a 10 ms frame, subframe 0 and subframe 5 are always used for the DL in TD-LTE. TD-LTE defines one or two special subframes in a 10 ms frame. The special subframe has three parts- DwPTS (Downlink Pilot Time Slot), GP (Guard Period), and UpPTS (Downlink Pilot Time Slot). DwPTS and UpPTS are legacy terms from TDD version of UMTS (Universal Mobile Telecommunication System); formally, there is no "pilot" channel in LTE. The traditional "pilot" channel is called Reference Signal in LTE. DwPTS facilitates downlink synchronization, and UpPTS facilitates uplink synchronization. GP helps avoid interference between the uplink and the downlink and provides the transceiver adequate time to switch from transmit function to receive function and vice versa.

Roles of radio channels and signals remain the same in TD-LTE. However, structures of certain signals and channels are different in TD-LTE. The main reason for structure differences is to support different UL:DL ratios. The primary synchronization signal is sent in the third OFDM symbol in slot 2 (subframe 1) and slot 12 (subframe 6), and the secondary synchronization signal is sent in the last OFDM symbol of slot 1 (subframe 0) and slot 11 (subframe 5). Recall that the primary synchronization signal is sent in the last OFDM symbol of slot 0 (subframe 0) and slot 10 (subframe 5) and the secondary synchronization signal is sent in the second last OFDM symbol of these slots/subframes in LTE FDD. Multiple PRACHs (Physical Random Access Channels) (up to six) may be present in a given UL subframe in TD-LTE, whereas LTE FDD supports zero or one PRACH in a given subframe. While four random access preamble formats are available to TD-LTE and LTE FDD, an additional fifth format is also available for use in TD-LTE for small cells. The PDCCHs (Physical Downlink Control Channels) can use up to 2 OFDM symbols in subframes 1 and 6. In other downlink subframes, up to 3 or 4 OFDM symbols can be occupied by the PDCCHs in LTE TDD just like LTE FDD. Number of PHICH (Physical HARQ Indicator Channel) groups differs as a function of UL:DL ratio. HARQ is Hybrid Automatic Repeat reQuest. Sounding reference signal in the UL has different configurations in TD-LTE.

The overall DL/UL data transmission approach remains the same for TD-LTE and LTE FDD. There are additional parameters in the DCI (Downlink Control Information) messages carried over the PDCCHs to support resource allocation in TD-LTE. The UL resource allocation (and UL power control command) specified by the PDCCH in subframe "n" is valid for the UL subframe "(n+4)" in LTE FDD and subframe "(n+k)" in TD-LTE, where k ranges from 4 to 7. HARQ and semi-persistent scheduling are also affected due to different UL:DL ratios. While DL HARQ supports up to 8 HARQ processes in LTE FDD, TD-LTE supports up to 15 HARQ processes in the DL. To support DL transmission, the TD-LTE UE could use ACK/NACK bundling to send a single response to multiple processes or use ACK/NACK multiplexing to provide process-specific HARQ responses. While UL synchronous HARQ has eight HARQ processes in LTE FDD, the number of TD-LTE HARQ processes ranges from 1 to 7. Semi-persistent scheduling has additional constraints in LTE TDD. Due to the channel reciprocity in TD-LTE, the channel conditions in the UL and DL are likely to be similar. Such knowledge can be exploited by the eNodeB scheduler to make decisions about the DL packet scheduling by observing the UL.

From the perspective of deployment, the availability of unpaired spectrum is needed for TD-LTE. Due to the tight timing synchronization requirements for TDD, eNodeBs would need a network synchronization mechanism such as GPS (Global Positioning System). LTE FDD may or may not use GPS. In addition to the inter-cell interference "management" schemes of LTE FDD such as adaptive modulation and coding, HARQ, UL power control, and ICIC (Inter Cell Interference Coordination), LTE-TDD can exploit GP to reduce inter-cell interference.

In summary, TD-LTE utilizes unpaired spectrum and reuses many of the LTE FDD features and mechanisms. Differences between LTE FDD and LTE TDD are primarily due to the fundamental TDD/FDD difference and different UL:DL ratios supported by LTE TDD. Many of the LTE FDD and LTE TDD differences exist at the physical layer, allowing LTE FDD and LTE TDD to benefit from the same LTE ecosystem. Countries such as India and China may see early widespread deployments of TD-LTE. TDD-based legacy networks, such as TD-SCDMA (Time Division- Synchronous Code Division Multiple Access) in China, can evolve to TD-LTE to achieve higher spectral efficiency.

Counting Down to LTE

Award News — Fri, 23 Jul 2010 19:08:00 GMT

Long Term Evolution (LTE) networks are now just months away from going live in the USA, so let's run down what you can expect in the way of this proto-4G technology in the coming year.

MetroPCS
MetroPCS Inc. (NYSE: PCS) has previously claimed that it will have "the first LTE wireless network featuring smartphones" in the US. The operator plans to launch in the "second half of this year." It hopes to have a smartphone that supports both CDMA and LTE ready for the network. So far, the operator has only said for sure that it will be launching in Las Vegas, although other unspecified markets are on the roadmap.

Verizon Wireless
Verizon Wireless reiterated on its second-quarter earnings call Friday that it is on track to launch what will be the largest initial deployment of LTE in the US sometime in the fourth quarter. The deployment is expected to cover some 25 to 30 cities with average data download speeds of between 5 and 12 Mbit/s by the end of 2010. In fact, Engadget is suggesting that November 15 might now actually be the launch date. (See CTIA 2010: Verizon LTE Gets Friendly.)

The operator is planning to launch with data cards and LTE modems. Smartphones, however, should be available by the first half of 2011. (See CTIA 2010: The LTE Smartphone Scramble.)

LTE might also move away from unlimited data plans for Verizon: "We have indicated in the past, as we move to an LTE world and LTE pricing, we will probably look very hard at tiered pricing, and that continues to be our thinking right now," CFO Joe Killian said on Friday's call. (See Verizon Likes Its Droids.)

AT&T
AT&T Inc. (NYSE: T) CFO Rick Lindner said this week that the operator is readying LTE test markets now. "We plan to begin our trials of LTE in the next few months and will start commercial service in 2011," he said on the operator's second-quarter call this week. Very little is yet known about what markets AT&T will initially launch in or what kind of devices it plans to offer.

LightSquared

The dark horse of early LTE providers, it emerged this week that LightSquared , which is a Harbinger Capital Partners LP venture, wants to launch LTE markets in the second half of next year. The operation is little different from other carriers, as it plans to use a combination of satellite and terrestrial broadband spectrum to launch wholesale services to sell to other operators. LightSquared will launch trial LTE networks in the first half of next year in Denver, Phoenix, Las Vegas, and Baltimore. (See Harbinger Hatches LTE Challenger in US.)

Original Source: Dan Jones, Site Editor, Light Reading Mobile at http://www.lightreading.com/document.asp?doc_id=194847&f_src=lightreading_gnews

Nokia Siemens Networks to supply LTE for LightSquared

Award News — Wed, 21 Jul 2010 21:57:00 GMT

Nokia Siemens Networks (NSN), a 50-50 joint venture between Nokia Corp. (NOK - Analyst Report) and Siemens AG (SI - Analyst Report), won an eight year contract from LightSquared. The deal is worth more than $7 billion. This is closely followed by NSN's announcement just a day before to buy Motorola Inc.'s (MOT - Analyst Report) network-equipment business unit for $1.2 billion in cash.

As per the terms of the contact, Nokia Siemens will install and maintain a new U.S. 4G LTE mobile broadband network with around 40,000 base stations that will cover 92% of the U.S. population by 2015.

LightSquared is a new telecom network venture launched by New York-based hedge fund Harbinger Capital Partners. Nokia Siemens plans to use its remote network operations centre in India extensively to manage the new network.

In the past, NSN has already received some contracts for supplying LTE equipments to AT&T (T - Analyst Report) and T-mobile U.S. With the acquisition of the Motorola's unit, NSN will become the third largest telecom network solutions provider in the U.S., after L.M. Ericsson AB (ERIC - Analyst Report) and Alcatel-LucentALU - Analyst Report).

Industry sources estimate that by far this over $7 billion contract is the single largest contract in the global wireless gear manufacturing industry. From early 2010, NSN has taken several steps to improve its financials. This helped the company to win 15 LTE contacts throughout the world. Notable among them are TeliaSonera AB, (NTT DoCoMo (DCM - Analyst Report) and Shaw Communications Inc. (SJR - Analyst Report).

This contract win together with NSN's proposed acquisition of Motorola's wireless infrastructure assets will significantly boost its presence in the lucrative U.S. market, which accounted for more than 40% of the $82 billion global wireless network infrastructure business.

Source: Zacks Equity Research (http://www.zacks.com/stock/news/37260/NSN+Wins+$7+Billion+Contract)

1xEV-DO vs. LTE: Connection Setup

Don Hanley — Wed, 21 Jul 2010 18:29:00 GMT

So far in this ongoing series comparing 1xEV-DO with LTE, I've covered initial system acquisition and the random access process. Although the specific details obviously differ, 1xEV-DO and LTE have some remarkable similarities in these functions.

The next step in setting up a call is the actual message exchange between the mobile device and the network. This is where resources are allocated and the radio link is configured for the particular service being requested; this is also where 1xEV-DO and LTE start to diverge, due to some of the fundamental differences between the technologies.

The basic connection setup sequences for 1xEV-DO and LTE are illustrated here:

Although both 1xEV-DO and LTE are solely packet-oriented solutions, 1xEV-DO has its roots in its circuit-oriented predecessor, cdma2000 1x. In particular, this means that the AT will be in soft handoff with up to 6 sectors, based on the measurements reported in Route Update messages. An LTE UE only supports hard handovers, and communicates with only 1 eNB at a time.

Since (1) connections in 1xEV-DO are pre-defined during the DO session configuration exchange between the AT and the RNC, (2) there's really only one type of service supported in 1xEV-DO (high speed packet data), and (3) all ATs are functionally the same, there's no need to include any of that information in the signaling messages. This keeps the setup exchange short and simple; in fact, the first part of the setup messaging occurs during the random access process itself. Other than the Route Update and Traffic Channel Assignment, the messages used in a 1xEV-DO connection setup contain little or no extra information. In LTE, however, the requested service, radio link configuration, and UE characteristics must all be explicitly defined, which can make for some fairly lengthy messages.

1xEV-DO forward link traffic channels are identified by an assigned MAC index, one for each member of the active set; this pre-allocation reduces the signaling overhead needed for transmitting data to the AT, but may lead to MAC index blocking under high traffic conditions. LTE resources are basically assigned on the fly.

The operation of the 1xEV-DO forward link relies on the successful receipt of Data Rate Control (DRC) covers and values sent from the AT over the DRC channel, so confirmation that the DRC channel is working properly is built into the setup process before traffic begins to flow. LTE does not perform a similar check for its Channel Quality Indicator (CQI) values. (I'll cover the details of traffic operations in a later blog.)

What's the bottom line? 1xEV-DO is optimized for best effort, delay tolerant, packet data applications, and is designed to keep the signaling necessary to set up and maintain a call short and to the point. LTE, in contrast, is intended to be flexible, supporting a wide variety of packet services and Quality of Service requirements, which requires long and complex messages to properly assign the appropriate radio resources to the call.

Does LTE Support Voice Calls?

Chris Reece — Wed, 07 Jul 2010 22:05:00 GMT

Yes. There are a number of proposals for LTE voice. They include Circuit Switched (CS) Fallback, Single Radio Voice Call Continuity (SRVCC), and the IP Multimedia Subsystem (IMS). The key stage 2 specs that discuss these topics are 23.272 for CS Fallback, 23.207 for SRVCC, and 23.228 for IMS. A video discussing these options can be found at http://www.lteuniversity.com/media/p/7736.aspx.

Multiple Antenna Techniques in LTE - Part II

ndalal — Tue, 29 Jun 2010 15:14:00 GMT

LTE provides the following methods to achieve above goals:

1. Transmit Diversity: This method is utilized in the downlink of LTE using 2 or 4 transmit antenna at the eNB. The receiver (UE) may have 1 or more receive antenna. Here, similar modulation symbols are transmitted to improve the signal quality (SINR).

Figure 2. SFBC Transmit Diversity in LTE

This method does not require any feedback information from the receiver and is effective when the receiver is in a low SINR radio environment. Transmit Diversity improves the SINR and thus reduces required retransmissions attempts and/or allows the transmitter to utilize aggressive coding & modulation scheme. The specific method LTE uses for transmit diversity is SFBC (Space Frequency Block Coding), providing both spatial and frequency diversity. SFBC improves cell coverage and/or improves cell-edge throughput.

In this figure, one transmit antenna is transmitting a reference signal (RS) from subcarrier frequency (f0) and the second transmit antenna is transmitting another reference signal (RS) from another subcarrier frequency (f3). Since, the first transmit antenna is not transmitting any signal on frequency f3 and the second transmit antenna is not transmitting any signal on frequency f0, the receiver receives the reference signals without much interfering signals on both f0 and f3. This facilitates the demodulation of the unknown signals on the rest of the subcarriers such as f1, f2, ...

Here, one transmit antenna transmits modulation symbols S1 and S2 and other transmit antenna transmits phase shifted versions of these modulation symbols (S2* and -S1*). Thus, utilizing two subcarriers to transmit two modulation symbols doesn't double the data rate but it certainly improves the signal quality (SINR) of the transmitted signal and thus increasing the achievable data rates.

Since modulation symbol S1 is transmitted from one antenna on frequency f1, and its phase shifted version (-S1*) is transmitted from another antenna (space diversity) on another frequency f2 (frequency diversity), this method is known as Space Frequency Block Coding (SFBC) in LTE.

1xEV-DO vs. LTE: Random Access

Don Hanley — Wed, 23 Jun 2010 20:51:00 GMT

In my previous blog, I compared how 1xEV-DO and LTE devices perform an initial system acquisition. Now I'll continue with the next step in setting up a data call, random access.

In virtually every mobile wireless system, mobiles attempting to get on the system have to overcome a Catch-22: they need a radio channel in order to request a radio channel. The usual solution to this is to define a common, or shared, random access channel, which allows devices to transmit their initial messages without permission. Since these random accesses are not coordinated with the network, there is a risk of colliding with other users; in addition, there is only limited information available from the network to help the mobile determine how much power it should use for its transmission.

3G and 4G systems have taken a remarkably similar approach to managing collisions and initial power. Based on the system parameters sent over the broadcast channels, each mobile selects a random access time, and estimates the amount of power its transmission should require. If it receives a positive acknowledgement from the network (indicating that its transmission was successfully received and accepted), then the mobile proceeds with the rest of the signaling needed to set up the call; otherwise, it waits a small amount of time (in order to avoid colliding with another user) and tries again with a little more power (to improve its chances to be heard by the network). How many times it tries and how loud it gets are determined by the system configuration parameters.

The random access sequences for 1xEV-DO and LTE are illustrated here:

These sequences are very similar, but there are a few important differences:

1) Random access content. A transmission over the 1xEV-DO Access Channel (ACH), called a probe, consists of an initial preamble (containing only the pilot) followed by the actual messages themselves (typically a Route Update and a Connection Request). In LTE, the UE transmits a simple preamble signal over the Physical Random Access Channel (PRACH); the actual RRC Connection Request message is sent only after a positive acknowledgement is received from the network.

2) Initial power estimate. 1xEV-DO determines its initial power level by subtracting the measured forward link signal strength (Mean RX Power) from its nominal power level (Open Loop Adjust) and then adding the downlink path loss (Pilot Strength Nominal minus the measured pilot strength) and a "fudge factor", Probe Initial Adjust; note that the downlink path loss estimate is constrained within configured minimum and maximum values. LTE simply adds the configured Preamble Received Target Power to an uplink path loss value determined by the UE, and then ensures that the initial power does not exceed the allowed maximum power P_CMAX.

3) Access retries. In both systems, if a positive acknowledgement is not received in response to the access attempt, the device will try again, increasing its power by a small amount (Power Step in 1xEv-DO, Power Ramping Step in LTE). A 1xEV-DO AT will try a total of Probe Num Step probes, wait a randomized amount of time, and then repeat the entire sequence Probe Sequence Max times, recalculating the initial power at the start of each sequence. In LTE, the UE will send up to Preamble Trans Max preambles, but will not repeat the sequence if it does not receive an acknowledgement.

1xEV-DO and LTE use similar processes to make their first contact with the network, but 1xEV-DO works a little harder at it, giving the AT more opportunities to be heard. Of course, the more accurate the initial power estimate is, the more likely it is that the transmission will be heard and acknowledged.

SIP vs. GTP/MIP

Chris Reece — Fri, 18 Jun 2010 21:34:00 GMT

Hello All,

I had a question recently about different types of mobility that I thought might be of interest. The question was about using SIP mobility instead of GTP or MIP in LTE. This is a good question. On the surface both are solving mobility issues so why have different solutions. The problem is that SIP mobility and MIP/GTP are really trying to solve different problems. MIP (or Mobile IP) and GTP (or GPRS Tunneling Protocol) are trying to solve geographical mobility. As a mobile moves through the network, we need to direct the packets to its current S-GW and eNodeB. MIP is an option for sending packets from the P-GW to the current S-GW of the mobile. I don't see many people using MIP. Most are looking at using the GTP. SIP Mobility is used to map the current IP address of the UE to a logical name. Since we can use dynamic IP addresses in LTE, we need a way to map the current IP address assigned by the P-GW to the UE to a logical name that is static (i.e. Chris@LTEISCOOL.com). So they are just trying to solve different problems.

I found this an interesting question and I hope you find this helpful.

Take care,

Chris

1xEV-DO vs. LTE: System Acquisition

Don Hanley — Thu, 10 Jun 2010 16:18:00 GMT

LTE (Long Term Evolution) has been designed from the very beginning to provide a natural evolution path for GSM/GPRS and UMTS networks. As a consequence, someone knowledgeable about UMTS will find many aspects of LTE to be familiar. But how does someone used to the unique design of a 1xEV-DO network learn about how LTE works? Earlier blogs and white papers have discussed the different terms used in 1xEV-DO and LTE networks; let me put those into context now, comparing key LTE network operations with their 1xEV-DO equivalents.

I'll begin at the beginning. When a mobile device (an AT in 1xEV-DO, or a UE in LTE) first powers on, it must first go through a process called "system acquisition", where it locks on to a suitable radio channel and learns some basic information about the network. The illustration below lays out the key system acquisition steps for 1xEV-DO and LTE, side by side.

At first glance, they don't look very much alike; however, both sequences perform the same basic functions:

1) Locate a channel. 1xEV-DO devices search for the distinctive DO pilot signal in each slot, while LTE UEs look for the Direct Current (DC) subcarrier at the center of the LTE channel.

2) Synchronize with the radio link. 1xEV-DO uses the timing of the pilot signal and the Control Channel preamble to determine slot and frame timing, while LTE uses explicit Primary and Secondary Synchronization signals.

3) Learn the system configuration. A 1xEV-DO AT will read the Synch, Quick Config, Sector Parameters and Access Parameters messages in the Control Channel, while an LTE UE reads the Master Information Block (MIB) and System Information Blocks (SIBs) 1 and 2 in the Physical Broadcast Channel (PBCH). Note that an AT learns the cell's identity (the PN offset) from the Synch message, while a UE derives the physical cell ID from the synchronization signals.

At this point, both devices (1xEV-DO and LTE) are ready to send their first message to the network. I'll cover that process in the next entry in this series.

Award Solutions is a 4G World Training Sponsor

Award News — Tue, 01 Jun 2010 19:12:00 GMT

4G World is the first and only conference and expo covering the entire ecosystem of next-generation technologies that enable the mobile Internet revolution including mobile network infrastructure, advanced devices, applications and content. 4G World delivers a world-class comprehensive four-day conference program that highlights the rapidly evolving interdependencies between operator business models, mobile network technology and architecture, and mobile Internet applications and services.

Award Solutions, Inc. provides exceptional training in advanced wireless and Internet technologies and is excited to announce that we will be running the following courses in partnership with Yankee Group, the organizers of 4G World:

- LTE Technology Overview: October 18-19, 2010
- LTE-Advanced: October 20, 2010
- Ethernet Backhaul Essentials: October 21, 2010

Detailed course outlines along with prices and registration for these courses are available at the 4G World website: 4gworld.com. Register before June 25th to qualify for the Early Bird discount.

Award Solutions, Inc. provides training and consulting in advanced wireless and Internet technologies such as UMTS LTE, GSM/GPRS/EDGE, 1xEV-DO, WiMAX, and IP. We provide clients with innovative, cost-effective solutions that rapidly boost workforce productivity and competence, to more quickly meet their market demands.

The training and consulting solutions delivered by Award have proven successful for telecommunications and Internet equipment manufacturers, service providers, and management consulting firms since 1997.

UE Identifiers in LTE - The Big Five

Vishal Dhar — Mon, 31 May 2010 15:01:00 GMT

In my previous blog I had motivated the idea behind the Registered State/Deregistered State and the Connected Mode/Idle Mode. I would like to continue to build on those ideas and briefly talk about five key Identifiers assigned to the UE by different entities during the UEs lifetime. This is not a comprehensive list of all the identifiers but their discussion broadly covers the end to end data connectivity concepts in LTE.

The IMSI (International Mobile Subscriber Identity) and IMEI (International Mobile Equipment Identity) are permanent identifiers assigned to the USIM card and the Mobile Equipment, respectively. They are permanently associated with the subscriber and stored in a permanent provider database like the HSS (Home Subscriber Server) and will be used by other nodes in the network to identify the user. Similar to 2G and 3G technologies, for reasons of security, efficiency and practicality - the LTE network minimizes the exchange of these two identifiers with the UE.

During the Initial Attach procedure between the UE and the LTE Network the UE is assigned three additional dynamic identifiers by different LTE Network nodes that have varying scopes of use.

The eNodeB (Evolved Node B) assigns the UE a C-RNTI (Cell Radio Network Temporary Identifier) to identify the UE during exchange of all information over the air. The C-RNTI is assigned during the setup of the RRC Connection (Idle Mode à Connected Mode transition) between a UE and an eNodeB and is valid only for that RRC Connection. Once the UE leaves the coverage area of an eNodeB the RRC Connection must be moved (Inter-eNodeB Handover) and the "new" eNodeB will assign a "new" C-RNTI to the UE. The C-RNTI is an E-UTRAN (Evolved Universal Terrestrial Radio Access Network) specific identifier and the EPC (Evolved Packet Core) Network has no visibility to it.

The MME (Mobility Management Entity) assigns the UE a GUTI (Globally Unique Temporary Identifier) to identify the UE during all message exchanges and procedures with the EPC. The GUTI is assigned during the Attach procedure (Deregistered State à Registered State transition) between the UE and the MME and is valid only as long as the UE is attached to the MME that assigned the GUTI. Once the UE leaves the Tracking Area(s) of an MME the "Attachment" has to be moved (Inter-MME handover) and the "new" MME will assign a "new" GUTI to the UE. Embedded within the GUTI are the PLMN ID of the service provider and the MME Identity. Thus, the GUTI uniquely and globally identifies a UE attached to a specific MME in a specific Service Providers LTE Network in a specific Country. The MME may choose to periodically re-assign a "fresh" GUTI to a UE that is attached to it.

The PGW (Packet Data Network Gateway) assigns the UE an IP address to facilitate data connectivity between the UE and any internal or external PDN (Packet Data Network). This could be an IPv4, IPv6 or Dual Stack IP address and the PGW could use a variety of IP address allocation schemes associated with the type of IP Address. The UE may set up PDN Connections with more than one PGW and may be assigned more than one IP address. The first IP address is assigned to the UE during the Initial Attach procedure and it stays with the UE as long as the UE is attached to the LTE Network. Unlike the other temporary identifiers the IP address is more "persistent" or "sticky" and does not change as long as the UE is attached - thus uninterrupted IP connectivity is provided to the UE. For all practical purposes, the UE is assigned an IP address when it powers on and loses its IP connectivity when it powers off. It is important to recognize that the eNodeB, MME and the SGW do not have any use for this UE IP address for connectivity purposes. It is used for IP forwarding decisions by the PGW and all nodes "north" (between the PGW and the PDN) of the PGW.

These ideas have been captured in the picture below.

To summarize the above:

1. IMSI - International Mobile Subscriber Identity:

The IMSI is a permanent identity assigned by the Service Provider
It is valid as long as the Service is Active with the Service Provider
It is stored on the USIM card and on the HSS (Home Subscriber Server)
It globally and uniquely identifies a user on any 3GPP PLMN (Public Land Mobile Network)

2. IMEI - International Mobile Equipment Identity

The IMEI is a permanent identity assigned by the Device Manufacturer
Valid as long as the Device is in Use
Stored on the Device hardware and on the HSS (Home Subscriber Server)

3. C-RNTI - Cell Radio Network Temporary Identity

Dynamic Identity assigned by the eNodeB
Valid as long as the UE is Connected to the eNodeB that assigned the C-RNTI
Stored in the UE and the eNodeB

4. GUTI - Globally Unique Temporary Identity

Dynamic Identity assigned by the MME (Mobility Management Entity)
Valid as long as the UE is Registered with the EPC (Evolved Packet Core) and Attached to the MME that assigned the GUTI
Stored on the UE and the MME

5. IP Address

Dynamic Identity assigned by the PGW
Valid as long as the UE is Registered with the EPC (Evolved Packet Core)
Stored in the UE and the PGW and any other node "north" of the PGW

The HetNet - Pico-cells make a comeback

Hooman Razani — Sat, 22 May 2010 01:05:00 GMT

How many RATs (Radio Access Technologies) can you name? Lets see, there is GSM with 4 billion subscribers and then there is W-CDMA with its released flavors and evolutionary cousin LTE of course, a few still clinch on to their efficient 1xEVDO and its peculiar flavors, then we have the IEEE family from 802.11s and the newly launched mobile versions of those, i.e. WiM... (forbidden word in my blogs) , there are Satellite based RATs, and a few RATs facing the skies mainly aimed at commercial airliners, .... to name a few!

First, we can wonder what is going to play the bagpiper tune to which all these heterogeneous RATs will dance to in unison, (assuming there is a need for such a dance!)

One important unifying scheme that has been discussed extensively here and elsewhere is the IP Multimedia Subsystem (IMS), playing the role of a (super) glue. No matter what type of RAT is available or offered by the service provider(s), the mobile station can get to the service plane by choosing the right RAT for making the service plane connections. In the most generic scenarios, little or no Radio Resource Management is involved across these RATs and the choice of the "best" available network is a mobile device responsibility. Nevertheless, Inter-RAT handover decisions are usually made across RATs belonging to the same domain of standards or after extensive harmonization between different standardization bodies.

Most recently the term HetNet (Heterogeneous networks) is now being applied in a different sense to one RAT as a novel way to increase (data) capacity and coverage within that RAT. But in what sense can a single RAT network be heterogeneous?

A uni-RAT hetnet is a mobile wireless network where cooperating "smart" pico-cells work together within the larger Macro-cell coverage, to dramatically improve performance. The step can be seen as one that falls between the femto-cell evolution and the existing ubiquitous Macro-cell structure. To me, the core of this idea is close to the hierarchical cell-structures that have been used in the GSM networks in the past, with a new twist of increased coordination and intelligence among the pico-cells.

Many vendors such as Samsung, Ericsson, NSN, Huawei and others are very busy filing patents in this arena and see great opportunities in the US market. It is a well known fact that the North American landscape has a void in pico-cell deployments compared to European/Japanese networks. The HetNet concept is seen by these and others as a keystone in attacking the conspicuous congestion issues that are widely experienced in big cities.

An interesting and open question is about the future role of the femto-cells in the shadow of the coming HetNet pico-cells. It may turn out that the femto-cell will be pushed to a secondary niche application such as the indoor coverage problem.

The HetNet initiative is at an embryonic stage as far as the standards are concerned. Most likely the 3GPP R10 (LTE) will be the place where we will see the first standardization attempts being made. Although most of the HetNet functions are network-centric, many actors believe that the success of the HetNet will depend on standardization for allowing multi-vendor solutions. Otherwise the HetNet might face the same fate as many other good ideas that were never deployed!

/Hooman Razani

LTE and “The Holy Grail” of “Always On”

Vishal Dhar — Wed, 19 May 2010 13:48:00 GMT

“Always On” is a term that has caught on in the industry in the last few years. From the perspective of wireless data access it simply refers to the user device “always being on an IP network”. Why do I call it “The Holy Grail”?

Previous generations (2G and 3G) of cellular data technologies have provided this experience of “Always On” to the mobile device but have left it largely to the hosts involved in the end to end connectivity. The hosts periodically push information to prevent the Wireless Network from de-allocating the users “IP access connection” – whether it is a 3GPP based PDP Context or a 3GPP2 based PPP session. I think of the Wireless Core Network and Wireless Access Network as having different motivations for determining the length of time a user session should be kept up. The Wireless Core Network wants to keep the user session on “forever”, if possible (obviously with minimal resource reservation), since longer lived sessions are positively correlated with more revenue or opportunity for revenue. The Wireless Access Network wants to keep the user session only as long as the user is actively exchanging information with the network. The balance between these two divergent desires is the optimal solution or the sweet spot for the service provider. Let’s not forget, that while the Core Network and the Access Network are balancing this challenge of radio resource management, capacity and revenue, the users of the service should have a “feeling” that an uninterrupted data connection exists between the mobile device and the far-end host.

LTE calls the users “IP access connection” an EPS Bearer, which is a connection between the UE (User Equipment or mobile device) and the P-GW (PDN Gateway). The P-GW is the default gateway for the UEs IP access. LTE has defined a Default EPS Bearer to provide “Always On” connectivity. The design of the EPS Bearers and the supporting procedures relieves the hosts involved in the end-to-end connectivity from having to periodically push information for the reasons already mentioned.

At least ONE Default EPS Bearer to ONE P-GW is setup for a UE by default, when the UE powers on. As part of the Default EPS Bearer being setup the UE is assigned an IP address. The Default EPS Bearer does NOT guarantee any specific throughput so the Core Network could support a large number of users each with a Default EPS Bearer. The Core network part of the Default EPS Bearer and the IP address assigned to the UE, for all practical purposes, stays with the UE as long as the UE stays powered on. The Default EPS Bearer is the first bearer that gets established and the last bearer that gets released. This allows the UE (specifically the applications using the IP connection) to transmit information to the server and the server to transmit information to the UE at any time. This state of the UE is called the REGISTERED state.

While the Core Network keeps the Default EPS Bearer resources and the IP address “persistently” allocated to a specific UE as long as the UE is in the REGISTERED state, the Radio Network adopts a different approach. It dynamically allocates and releases radio network resources to support the Default EPS Bearer with transitions between the two modes of the REGISTERED state, CONNECTED and IDLE modes, respectively. These transitions are made in about 100 milliseconds.

The two diagrams provide a pictorial view of the traffic/bearer/IP connections representing resources for a UE while in these two modes. As a result, this still allows the UE (specifically the applications using the IP connection) to transmit information to the server and the server to transmit information to the UE at any time.

In summary, the Default EPS Bearer structure and procedures provide the LTE network to provide “Always On” capability to the UE while meeting the “needs” of the LTE Access Network and the LTE Core Network.

Compare and contrast of UMTS and LTE Mobility

Chris Reece — Mon, 17 May 2010 14:15:00 GMT

As I think about mobility in LTE, I always go back to making comparisons with UMTS. I thought t might be good to do a comparison chart of the two. Below is a list of the key mobility terms/concepts and their equivalent terms in UMTS and LTE. Let me know what you think of this and if you have any questions/comments.

Thanks,

Chris

Mobility Concept	UMTS Concept	LTE Concept
Paging Zone	Location Area, Routing Area, UTRAN Registration Area	Tracking Area
Types of Handover	Soft, Softer, and Hard	Hard Only
Basic dBm measurement	Received Signal Code Power (RSCP)	Reference Signal Received Power (RSRP)
Basic dB measurement	Energy per Chip over Noise (Ec/No)	Reference Signal Received Quality (RSRQ)
Measurement Configuration sent to UE	In multiple Measurement Control Messages. One message is needed per event configuration.	Included in RRC Connection Reconfiguration. One message can include multiple measurement configurations.
Intra-Frequency Measurement events	e1 event types	A event types
Inter-Frequency Measurement events	e2 event types	A event types
Inter-RAT Measurement events	e3 event types	B event types

Measurement Event Comparison

Chris Reece — Wed, 12 May 2010 22:43:00 GMT

While sitting in a colleague’s class this week we were discussing the UMTS measurement event types. I thought it might be good to do a comparison of UMTS’s measurement events with the measurement events in LTE. There are a couple of key differences between the two. The first is that UMTS supports a soft handover and LTE does not. That is important in looking at this comparison as there is no active set in LTE. The second difference is that LTE does not highlight the differences between intra-frequency measurements and inter-frequency measurements as strongly as UMTS does. The following table gives a comparison and comments on how the LTE measurements compare with UMTS.

I hope it is helpful.

Thanks,

Chris

LTE Measurement Events and Description	UMTS Measurement Events and Description	Comments
Event A1 - Serving becomes better than threshold	e2f - The estimated quality of the currently used frequency is above a certain threshold	In LTE this may be used to stop looking for a cell on a different frequency or technology, as e2f is used in UMTS.
Event A2 - Serving becomes worse than threshold	e2d - The estimated quality of the currently used frequency is below a certain threshold	In LTE this may be used to start looking for a cell on a different frequency or technology, as e2d is used in UMTS.
Event A3 - Neighbor becomes offset better than serving	e1a - A Primary CPICH enters the reporting range	This comparison is a bit of a stretch. The reason I have equated them is because in both cases a neighbor should be considered for a handover, either soft or hard depending on the technology.
Event A4 - Neighbor becomes better than threshold	e1e - A Primary CPICH becomes better than an absolute threshold
Event A5 - Serving becomes worse than threshold1 and neighbor becomes better than threshold2	e2b - The estimated quality of the currently used frequency is below a certain threshold and the estimated quality of a non-used frequency is above a certain threshold	This comparison is good for saying that the current cell is below an absolute threshold and the new cell is above a threshold. This would mean that the current cell is not good enough and the new one is good, which is a good reason to do a hard handover (in either technology).
Event B1 - Inter RAT neighbor becomes better than threshold	e3c - The estimated quality of other system is above a certain threshold
Event B2 - Serving becomes worse than threshold1 and inter RAT neighbor becomes better than threshold2	e3a - The estimated quality of the currently used UTRAN frequency is below a certain threshold and the estimated quality of the other system is above a certain threshold	This comparison is good for saying that the current cell is below an absolute threshold and the new cell on a different technology is above a threshold. This would mean that the current cell is not good enough and the new one is good, which is a good reason to do a hard handover to the new technology.

LTE-Advanced in 5 Minutes!

Nishith Tripathi — Tue, 04 May 2010 20:51:00 GMT

For those who are too picky on the definition of "true 4G," hold your breath...4G version of LTE is taking shape as LTE-Advanced! LTE-Advanced is a Release 10 feature of 3GPP. Recall that Release 8 defined LTE. Release 9 makes some enhancements to LTE such as support for emergency calls using IMS. We'll summarize below motivating factors behind LTE-Advanced and briefly introduce main features of LTE-Advanced.

ITU (International Telecommunication Union) has defined requirements for IMT-Advanced (just a prettier name for 4G?). LTE-Advanced aims to meet and often exceed IMT-Advanced requirements. Remember that LTE is a Lon...g Tem Employment (LTE!). Hence, LTE-Advanced needs to aim for an even longer term employment! IMT-Advanced requires the support for 100 Mbps for high mobility and 1 Gbps for low mobility. While Release 8 can already meet 100 Mbps peak rate requirement, enhancements are needed in Release 10 to support 1 Gbps. Peak spectral efficiency targets in IMT-Advanced are 15 bps/Hz and 6.75 bps/Hz for DL (downlink) and UL (uplink), respectively. LTE-Advanced aims to achieve the peak spectral efficiency of 30 bps/Hz and 15 bps/Hz in DL and UL, respectively. IMT-Advanced needs to support wider bandwidths such as 40 or 100 MHz. LTE-Advanced allows 100 MHz bandwidth. While IMT-Advanced specifies the delay of 100 ms for an idle mode to connected mode transition, LTE-Advanced intends to achieve a 50 ms delay. Recall that even Release 8 LTE can meet the 100 ms transition delay requirement. In addition to IMT-Advanced requirements, LTE-Advanced attempts to achieve higher performance with reduced cost. Furthermore, LTE-Advanced facilitates meeting future operator and user needs. Note that wireless data traffic has been experiencing explosive growth. Of course, LTE-Advanced cannot forget the competition...LTE-Advanced needs to be prepared for its arch rival- WiMAX! In summary, IMT-Advanced requirements, lower cost per bit, and competition are major motivating factors driving the design of LTE-Advanced.

Let's turn our attention to the main features of LTE-Advanced. LTE-Advanced is primarily an air interface enhancement, and, hence the features that we would briefly highlight below are related to the radio link between the UE and the eNodeB. Main features of LTE-Advanced are Carrier Aggregation, DL MIMO enhancements, UL SU-MIMO, CoMP (Coordinated Multipoint), and Relay. Main benefits of these features are higher peak and average cell and user throughput and lower cost per bit.

Carrier Aggregation means that multiple carrier frequencies are aggregated to increase the overall bandwidth and hence data rates. These carrier frequencies are called component carriers (CCs). The component carriers may or may not be contiguous in frequency domain. A CC could have any of the channel bandwidths defined for Release 8, which ranges from 1.4 MHz to 20 MHz. The eNodeB and the UEs may be capable of transmitting/receiving one or more CCs. The standard aims to support the total channel bandwidth of up to 100 MHz. Depending upon the UE capabilities, the eNodeB may allocate multiple CCs to the UE for the DL and the UL.

While Release 8 already supports (4x4) DL SU-MIMO, LTE-Advanced further increases it to (8x8) SU-MIMO in the downlink. Additionally, beamforming is enhanced in the downlink using enhanced reference signals to improve MU-MIMO performance. Release 8 does not support SU-MIMO in the uplink. However, Release 10 extends SU-MIMO to the UL with the support for up to four layers of spatial multiplexing.

If you were missing soft handoff/handover and fast cell (or sector) switching in LTE, we have good news for you! CoMP introduced by LTE-Advanced works similar to soft handover/handoff. CoMP feature is available for the DL and the UL. In the DL, it is CoMP Transmission, while it is CoMP reception in the UL. Multiple cells could now be involved in communication with the UE. Two main CoMP transmission approaches are Joint Processing (JP) and coordinated scheduling and beamforming (CS/BF). The JP transmission approach offers two methods: (i) The eNodeBs transmit the same information from two cells in the "joint transmission" method. The UE then combines these two signals similar to a UMTS UE combining signals in soft handover. (ii) In the "dynamic cell selection" method, one cell among a set of cells is dynamically chosen for the DL transmission to the UE. In the CS/BF approach of CoMP transmission, beams are formed in individual cells while reusing the subcarriers near cell edge. Scheduling in different cells would need coordination to realize such beamforming. The CoMP reception in the UL involves reception of the UE signal at more than one cells. One of the cells would be a "central" cell responsible to combine signals received at multiple cells.

A relay can be thought of as an enhanced repeater, where the cell coverage (and hence cell-edge throughput) can be extended/improved. A new interface, Un interface, exists between the traditional eNodeB and a relay node. An example of relaying in LTE-Advanced is Layer 3 relaying with self backhauling. The eNodeB uses the help of a relay node that takes care of some users. Such users would be outside the coverage area of the eNodeB but inside the coverage area of the relay node. LTE-based air interface can indeed be used as the wireless backhaul between the eNodeB and the relay node. Such backhaul could share the same bandwidth with users or could use different spectrum bandwidth.

LTE-Advanced is fully backward-compatible with Release 8 LTE. A Release 8 UE can work with a Release 10 E-UTRAN, and a Release 10 UE can work with a Release 8 E-UTRAN. LTE-Advanced would support interworking with legacy technologies such as UMTS and 1xEV-DO.

We hope that you have digested LTE-Advanced in 5 minutes. If you finished up digesting LTE-Advanced in less than 5 minutes, congratulations! If you took longer than 5 minutes, well...what can we say...you are a slow reader and you need to work on your reading skills! "See" you next time!

Sounding Reference Signal

Hongyan Lei — Tue, 20 Apr 2010 17:53:00 GMT

LTE defines an optional sounding reference signal (SRS) in the UL. What is it for? UL channel quality, timing advance, and more. SRS is transmitted by the UE using a known sequence, similar to UL demodulation reference signal (DM RS), so the eNB can use it to estimate the UL channel quality. You may have a question: UL DM RS is already there and the eNB can decode the UL information with its assistance, what's special about SRS? Well, UL DM RS is transmitted together with the UL data and both locate exactly in the same RBs, so the channel quality information the eNB extracts from the UL DM RS is for that transmission. In terms of SRS, it may be transmitted periodically in a wider bandwidth (beyond PUSCH RBs allocated for UL data transmission) and when there is no UL data transmission, so the channel information obtained from SRS is a good input to UL scheduler. It's like CQI report from UE for DL scheduler. Also, since SRS can be transmitted periodically, the eNB can use it to check the UE timing alignment status and send time alignment command to the UE accordingly.

Where is SRS located in a UL PHY frame? It is transmitted in the last symbol of a subframe if scheduled.

Figure 1: Illustration of SRS location in UL PHY frame

In the time domain, SRS can transmit once or periodically based on eNB scheduling. If periodically, the UE-specific periodicity can be 2 / 5 /10 / 20 / 40 / 80 / 160 / 320 ms as defined in TS36.213 section 8.2. The eNB defines the SRS transmit time instance (specific subframes) in TS 36.211 section 5.5.3.3, which is shared by all UEs.

In the frequency domain, the SRS from a UE is transmitted in the unit of 4RBs, defined in TS36.211 section 5.5.3.2-1/2/3/4. For example, with system bandwidth of 50 RBs, we should check Table 5.5.3.2-2. There are 8 possible configurations. Here we show the configuration 1 on the left and the configuration 2 on the right. With C_SRS = 1, the SRS for a UE can occupy 4 RBs, 8 RBs, 16 RBs, or up to 48 RBs. It's a tree-based structure and the allocation scheme would be similar to OVSF allocation in WCDMA. So, multiple UEs share the RBs in frequency domain.

Figure 2: Example of SRS bandwidth configuration

You may notice that the same RBs are shared by two UEs. Why? It's because SRS uses Interleaved SC-FDMA (IFDMA) where consecutive subcarriers can be allocated to several UEs alternatively. LTE decides to multiplex two UEs in one RB as shown in Figure 3, i.e., subcarriers in one RB are allocated to two UEs alternatively. This structure provides comb-like effect and expands the SRS channel bandwidth of a UE. Frequency hopping can also be used to help cell edge UE (with power limitation) spread sounding channel bandwidth by using different frequency locations in different subframes/transmit opportunities.

Figure 3: Multiplexing of two UEs in one RB (12 subcarriers over one slot)

Next, what is content of the SRS? SRS uses the same Zadoff-Chu sequence as UL demodulation reference signal (DM RS). Since the cyclic shift versions of the Zadoff-Chu sequence are orthogonal, several UEs (up to 8) can transmit using different cyclic shifts on the same physical radio resource.

In summary, SRS can be used to estimate UL channel quality and provide input to UL scheduler, as well as help timing advance. SRS is transmitted in the last symbol of a subframe periodically and occupies one comb leg of several RBs (multiple of 4). The content of SRS is Zadoff-Chu sequence. Multiple levels of UE multiplexing are supported: subframe (time), RBs (frequency/ RB level), comb leg (frequency/ subcarrier level), cyclic shift (code).

Award Solutions Speaker at LTE Forum 2010

Award News — Tue, 06 Apr 2010 15:45:00 GMT

Award Solutions' Senior Consultant John McKeague will be speaking at the pre-conference workshop LTE Essentials at the LTE Forum on Tuesday, April 27, 2010 in Stockholm, Sweden.

Join HanseCom Media and Communications and LTE Forum 2010 at this one-day workshop, which provides a comprehensive high-level view of LTE and is intended for those in sales, marketing, product line management, and executives who need to understand LTE and its place in the 4G wireless landscape.

The LTE Essentials workshop provides an overview of LTE from both application and technical aspects, including the network architecture, the underlying technologies of OFDM and multiple antenna techniques, and the call setup procedure. In addition, deployment and interworking issues are explored along with the competitive landscape by comparing features and services with other 4G systems such as WiMAX.

As an Award Solutions representative, John McKeague will also be leading as a panel moderator on LTE Performance Expectations, with discussions in performance assessments in LTE and an outlook into LTE-Advanced, voice over LTE, and SMS and voice services on LTE on Wednesday April 28, 2010 in the late afternoon session.

LTE Forum 2010 will help operators identify the evolutionary steps to 4G LTE, understand the drivers for migrating, and determine what they need to do to ensure their network remains competitive. The conference will bring together operators, vendors, end-users, regulators, experts, analysts and the media to discuss, debate, and discover trends and strategies of the Next Generation Mobile technology, the most crucial industry challenges facing LTE.

For more information about LTE Forum 2010, visit www.lteforum2010.com.

About Award

Award Solutions, Inc. provides training and consulting in advanced wireless and Internet technologies such as LTE, UMTS/HSPA/HSPA+, GSM/GPRS/EDGE, 1xEV-DO, WiMAX, and IP. We provide clients with innovative, cost-effective solutions that rapidly boost workforce productivity and competence to more quickly meet market demands.

For additional information, please visit our website at www.awardsolutions.com or call us toll-free at +1.972.664.0727.

About HanseCom

HanseCom Media & Communication is a leading IT and Telecom conference organizer that has earned a reputation of excellence, specializing in specific themes pertaining to the Information Technology community. Led by an experienced management team headquartered in Porto, the company was established in Portugal in 2004.

Our conferences provide not only valuable updated information, but also insight on how a given technology is likely to pan out in the future. In this way, a HanseCom event is often an interface between research and industry. It is an excellent occasion for bringing together these two worlds and ensuring an extremely strong networking opportunity for delegates and speakers.

A Mobility Question

Chris Reece — Mon, 22 Mar 2010 16:39:00 GMT

Hello All,

Sorry for my lack of blogging lately. Projects can be tough. With the apologies out of the way, I had a colleague ask a couple of questions recently and I thought it would be good to share the answers with LTE-U.

His main question was about mobility. Mobility in LTE can be quite tricky. The main point of his question was about the frequency band size. In LTE there is a scalable OFDM channel. The bandwidth can vary from 1.4 MHz to 20 MHz. There are also a number of frequency bands that are going to be used for LTE (the new 700 MHz as well as the old PCS and Cellular bands).

The question basically was can a mobile move between frequency bands and the answer is absolutely yes. The second part of the question regarded movement between different sizes of bands. The best way to approach this question is to define two terms. The two terms are intra-frequency and inter-frequency mobility. We are use to these terms in other technologies (i.e. GSM and UMTS), but LTE puts a new spin on things. Intra-frequency mobility is when a mobile moves between two cells and both cells have the same frequencies (I know, not the most profound statement). If there is a change in the center frequency this is now called inter-frequency mobility. In LTE both intra-frequency and inter-frequency mobility are supported. (5/4/10 Note - I has come to my attention that if the bandwidth changes, but the center frequency does not change it is still a intra-frequency handover.)

For example, let's say that an operator has in one city a 10 MHz block in the 850 MHz band that they are using for LTE. In a city that is only a few miles away this same operator has only 5 MHz available in the 850 MHz band. When a subscriber drives from city A to city B the size of the frequency band is changing (as will probably the DC carrier) and the mobile will have to do inter-frequency mobility measurements to provide information on the new cells signal strength.

The reason this is an important discussion is that when the mobile does intra-frequency measurements the network does not need to allow any time to do the measurements. The mobile can just do them whenever they want to. For inter-frequency measurements the mobile must be given time to stop listening to its current cell and listen to a new cell to take the measurements. That increases the level of complexity and the amount of work the mobile will need to do.

I hope this helps give some insight into mobility. Please feel free to send questions if you have any.

Take care,

Chris

LTE Forum 2010 to Take Place in Stockholm Home of the First Commercial 4G/LTE Network Worldwide

Award News — Wed, 17 Mar 2010 21:05:00 GMT

	NewswireToday - /newswire/ - Porto, Portugal, 03/16/2010 - Speakers include representatives of 3GPP, GSMA, Telenor and TeliaSonera and leading analysts of idate and Ventura team.

	The wireless business is undergoing a major shift from voice-driven to data-driven services. Studies indicate that data revenue has grown by more than 30% per year, whereas voice revenue grew by just 4%. When mobile phones became popular, the 2G cellular networks they utilized did not provide enough bandwidth to support data-heavy applications. The wireless industry responded with 3G technologies to give greater bandwidth to enable more services, applications and wireless devices. 3G helped drive the adoption of data applications. Today, to meet the requirements for more bandwidth and richer applications, the mobile industry is responding with an exponentially faster 4G technology, so called Long Term Evolution (LTE) – an output of the 3rd Generation Partnership Project (3GPP). The LTE Forum 2010 will help operators to identify the evolutionary steps to 4G LTE, to understand the drivers for migrating, and to determine what they need to be doing now to ensure their network remains competitive. Presentations from mobile operators, analysts, regulators and other industry experts will help delegates to understand why, when and how to migrate to 4G LTE, and what they should be doing now to ensure their networks remain competitive into the future. The LTE Forum (lteforum2010.com) is scheduled to take place on 27th – 28th April 2010 in Stockholm, for more information and to register please visit the event website. The special focus of this conference covers critical areas for LTE migration: - Session 1 “The LTE Business Environment” - Chairman: Frédéric Pujol, Mobile Broadband Practice Manager, Idate. - Session 2 “LTE Deployment, the infrastructure supplier’s perspective” - Chairman: Hans Kuropatwa, Partner, Ventura Team. - Session 3 “LTE Business Models” - Chairman: Alan Hadden, President, GSA. - Session 4 “LTE Performance expectations” - Chairman: John McKeague, Sr Technical Consultant, Award Solutions. The LTE Forum 2010 brings operators, vendors, end-users, regulators, experts, analysts and the media together to discuss, debate, and discover trends and strategies of the Next Generation Mobile technology, the most crucial industry challenges facing LTE. The Speakers list of the LTE Forum 2010 in Stockholm/Sweden include: - Randall Schwartz, Wireless 20/20, Founder; - Alan Hadden, GSA, President; - Hans Kuropatwa, Ventura Team, Partner; - Hassan Claussen, Hansecom, Founder & CEO; - Xiaodong Zhu, ZTE, CTO of ZTE Western European Marketing Platform; - Richard Savage, Qualcomm, Director Business Development; - Sami Jokinen, Nokia, Senior Manager in Nokia Devices R&D; - Martin Ljungberg, Ericsson, Product Manager, Mobile Broadband, Business Unit Networks; - Hanna Maurer Sibley, LTE/SAE Trial Initiative (LSTI), Member of Steering Group; - Luc Beylkens, Alcatel-Lucent; - Kevin Holley, 3GPP, 3GPP SA Vice-Chairman; - Mats Lundbaeck, TeliaSonera, Director of Mobile Network Architecture and Strategies; - Meik Kottkamp, Rohde & Schwarz, Technology Manager, Test & Measurement Division; - Rune Harald Rakken, Telenor, Telenor Group Business Development and Research; - Dan Warren, GSM Association, Director of Technology; - John McKeague, Award Solutions EMEA, Sr Technical Consultant; - Frederic Pujol, Idate, Head of the radio technology and spectrum practice. About HanseCom HanseCom Media & Communication is a business media company. The company, led by an experienced management team headquartered in Porto, was established in Portugal in 2004. HanseCom Media & Communication, a leading IT & Telecom conference organizer, has earned itself a reputation of excellence, specializing in specific themes, pertaining to the Information Technology community. Our conferences provide not only valuable updated information, but also insight on how a given technology is likely to pan out in the future. In this way, a HanseCom event is often an interface between research and industry. It is an excellent occasion for bringing together these two worlds and ensures an extremely strong networking opportunity for delegates and speakers alike. For more information about The LTE Forum 2010 Contact: Guy Redmill HanseCom Media and Communications T: +44 774 088 1279 Submitted by NeonDrum on behalf of HanseCom Media and Communications Nicky Denovan / T: +44 7747 017654.

Multiple Antenna Techniques in LTE – Part I

ndalal — Wed, 24 Feb 2010 16:30:00 GMT

One of the key goals of LTE systems is to improve the end-user experience while keeping the overall cost low for the service provider. One way to achieve this goal is to improve the spectral efficiency of the system.

The use of multiple antenna techniques allows us to improve the efficiency and performance of the wireless system and thus lowers the cost in terms of bits/sec/Hz delivered. Here, the goal is to increase the throughput while keeping the use of the air interface resources to a minimum. We will discuss various multiple antenna techniques in this discussion series.

Multiple antenna techniques address the following scenarios:

1. Cell edge data rate of a user: Utilizing multiple transmit antennas from the cell-site, LTE can improve the transmitted signal strength and thus the received signal quality quantified by SINR (Signal to Interference and Noise Ratio) for the mobile user. This improved received SINR results into fewer retransmissions and/or higher Modulation and Coding Scheme (MCS) offered to the cell-edge users. These factors result into higher achievable data rates to cell edge users with fewer air interface resources. One can justify that the improved cell edge throughput is similar to increased cell throughput as well as improved cell coverage since the users at the cell edge locations or indoor locations are now getting better SINR and have ability to receive higher data rates and thus improved coverage in these locations. Space Frequency Block Coding (SFBC) is an example of a transmit diversity (multiple transmit antenna) mechanism utilized in LTE to improve the cell edge data rates.

2. User's Peak data rate: The use of multiple transmit antennas (e.g. 2 or 4) allows LTE to linearly increase the peak data rate by transmitting different modulation symbols from each transmit antenna using the same air interface resource (i.e. same sub-carrier at the same time). Such antenna technique is known as Single User Multiple Input Multiple Output (SU-MIMO) or Spatial Multiplexing (SM). The higher peak data rate increases the average throughput for the user and thus improves the end-user's Quality of Experience (QoE). In LTE, for a category 5 device, we can achieve 300 Mbps in the DL, if we utilize 4 transmit antennas at the eNB and allocate all resources of the 20MHz channel to a single user.

3. Cell Capacity: LTE provides means to increase the cell capacity (or cell throughput) or the number of users supported simultaneously in a given cell. Even though LTE is not a circuit-centric system where we measure the Erlang capacity, we still want to improve the cell throughput or support increased number of simultaneous users, which in turn, results into increased number of VoIP user capacity. LTE provides support for Multi-user MIMO (MU-MIMO) or Space Division Multiple Access (SDMA) in the downlink where same air interface resources are assigned to two different users simultaneously and these users' traffic is transmitted from two different transmit antennas at the eNB. Similarly, the eNB can also assign same resources in the uplink to two different users in a same cell and thus double the cell throughput and UL capacity.

Let's summarize various multiple antenna techniques supported by LTE. We will discuss them individually in subsequent segments.

Figure 1: Landscape of Multiple Antenna Techniques in LTE

How to calculate peak data rate in LTE?

Hongyan Lei — Thu, 18 Feb 2010 16:35:00 GMT

You may hear it many times that the peak data rate of LTE is about 300Mbps? How is the number calculated? What are the assumptions behind? Let's estimate it in a simple way. Assume 20 MHz channel bandwidth, normal CP, 4x4 MIMO.

First, calculate the number of resource elements (RE) in a subframe with 20 MHz channel bandwidth: 12 subcarriers x 7 OFDMA symbols x 100 resource blocks x 2 slots= 16800 REs per subframe. Each RE can carry a modulation symbol.
Second, assume 64 QAM modulation and no coding, one modulation symbol will carry 6 bits. The total bits in a subframe (1ms) over 20 MHz channel is 16800 modulation symbols x 6 bits / modulation symbol = 100800 bits. So the data rate is 100800 bits / 1 ms = 100.8 Mbps.
Third, with 4x4 MIMO, the peak data rate goes up to 100.8 Mbps x 4 = 403 Mbps.
Fourth, estimate about 25% overhead such as PDCCH, reference signal, sync signals, PBCH, and some coding. We get 403 Mbps x 0.75 = 302 Mbps.

Ok, it is done through estimation. Is there a way to calculate it more accurately? If this is what you look for, you need to check the 3GPP specs 36.213, table 7.1.7.1-1 and table 7.1.7.2.1-1. Table 7.1.7.1-1 shows the mapping between MCS (Modulation and Coding Scheme) index and TBS (Transport Block Size) index. Let's pick the highest MCS index 28 (64 QAM with the least coding), which is mapping to TBS index of 26. Table 7.1.7.2.1-1 shows the transport block size. It indicates the number of bits that can be transmitted in a subframe/TTI (Transmit Time Interval). For example, with 100 RBs and TBS index of 26, the TBS is 75376. Assume 4x4 MIMO, the peak data rate will be 75376 x 4 = 301.5 Mbps.

Table 7.1.7.1-1: Modulation and TBS index table for PDSCH (3GPP TX 36.213)

Table 7.1.7.2.1-1: Transport block size table (3GPP TS 36.213)

[Q: Want to try a small exercise? Here you are: what is the peak data rate if MCS 20 is used? Assume the channel bandwidth is 10 MHz and 2x2 MIMO is configured.]

We also know that there are different device capabilities, which is defined in 3GPP TS 36.306, Table 4.1-1 and table 4.1-2. For example, with a cat 2 device, the supported peak data rate is about 50 Mbps in the DL and about 25 Mbps in the UL. All UE categories should support all channel bandwidths (1.4/3/5/10/15/20 MHz) and all duplex modes (FDD/TDD/H-FDD) in LTE. Cat 1~4 devices can support up to 2x2 MIMO in the DL. Only cat 5 device can support 4x4 MIMO in the DL and 64QAM in the UL.

Now, if a cat 3 device is used in a 10 MHz channel with 2x2 MIMO configuration, can we get the peak data rate of 100 Mbps in the DL? Let's calculate. We know that, in the network side, the peak data rate is 300 Mbps for 20MHz channel with 4x4 MIMO, so the peak data rate is 75 Mbps for 10 MHz with 2x2 MIMO. Therefore, in case of cat 3 device in 10 MHz channel with 2x2 MIMO, the expected peak data rate over the air interface is: min (device capability, network capability) = min (cat 3, 10MHz with 2x2 MIMO) = (100, 75) = 75 Mbps.

One more thing before you claim you've mastered the peak data rate calculation, QoS profile can also impose constraint on the actual peak data rate a user expects.

Ans: 39.7 Mbps.

LTE Myth Busters

Hooman Razani — Mon, 23 Nov 2009 22:23:00 GMT

Since the website face-lift a year ago, 3GPP is actively involved in more than just producing Technical Specs and Reports. I regularly read the 3GPP News column and the latest one from Nov. 4^th has the conspicuous title “Dispelling LTE Myths” . I wonder if these so called Myths are the product of competing technology campaigns or the result of self-inflicted “accidents” such as CS Fallback (CSFB) and Single Radio Voice Call Continuity (SRVCC). Well, with UMB in ruins and mobile WiMAX shrinking in size to the footprint of a loaded UMTS cell (ouch!), there is little to fear from the competition nowadays…

The article dispels four “common myths” in LTE. (1) No Voice, (2) No SMS, (3) IMS isn’t ready and (4) No E911 calls. My aim in here is to enhance the descriptions with references to the 3GPP documents and add a few neutral and hopefully insightful comments.

Myth 1: No Voice in LTE. It is true that the carriers have been reluctant in throwing out their CS core and jumping on IMS as the universal all-IP service platform. For this and other reasons, LTE has full IMS solution arrives. CS-Fallback is one such solution. In CSFB the call is handed over to 2G/3G at the beginning of the call. This involves signaling between an upgraded 2G/3G MSC and the MME. IMS standards have become more VoIP friendly without compromising the emergency call requirements. Finally, one-way handover from IMS to legacy 2G/3G is supported via SRVCC. This can be done during an ongoing call. The following is a quick reference to 3GPP specs for further reading.

(For a guide about how to find your way around the 3GPP site, take a look at my earlier post)

a. CS-Fallback TS 23.272 Rel8

b. Enhanced VoIP in IMS R7 TS 23.228 and TS 24.229 Rel7

c. IMS to 2G/3G handover for voice TS 23.216 (SRVCC) Rel8

Myth 2: No SMS in LTE. SMS is supported in a couple of ways in LTE. In one approach, the SMS is carried on the user plane over IP (unlike 2G/3G for example). This is SMS over IMS and was standardized in 3GPP in Release 7. The other approach keeps SMS on the control plane and uses the new SGs interface between the MMEs and at least one MSC that is upgraded with the required protocol. Even supplementary services (SS) can be supported in either way.

d. SMS over IP TS 23.204, TS 24.341 and TR 23.804 Rel7

e. SMS over SGs TS 23.272, TS 29.118 and TS 24.301 Rel8

Myth 3: IMS is not here! IMHO this is not much of a myth, but a sad state of affairs. IMS specs have been around for a looong time and certain operators on both sides of the pond have very ambitious deployment programs. However IMS has still some convincing to do as a reliable voice anchor (and other CS based services) in mobile environments. The Rich Communication Suite initiative (RCS) may be the final push needed to get over the hurdle.

f. IMS: IMS is covered over a large number of specifications. A good reference is TS 21.202 from which you can jump to all other IMS related specs

g. RCS: Rich Communication Suite is not a 3GPP feature. RCS is more like a service package that will use the ‘always-on’ paradigm of IMS based services and provide feature rich services to end-users and will make mobile applications easier to develop and deliver for the providers – How about getting your SMS on the TV screen in the hotel room….The RCS initiative is now under GSMA (www.gsmworld.com).

Myth 4: No Emergency calls in LTE. This is a genuine problem in Rel8 LTE. In fact emergency call handling has been pushed to Release 9 in the specifications. UE positioning techniques in LTE are not fully defined yet and until Rel9 is in place, carriers will have to redirect E911 calls to 2G/3G. It is interesting to speculate on how this deficiency will impact the provision of VoIP in LTE. Today, many carriers redirect emergency calls in 3G to legacy 2G where they do support (sometimes proprietary) positioning techniques which satisfy the stringent regulatory requirements. Early LTE deployments will have to deal with the same problem. Meanwhile carriers with UMTS who are on the LTE path are facing a difficult choice (if any) about support of full emergency-class positioning infrastructure in their 3G networks.

h. Location Services in LTE TS 23.271, TS 44.031, TS36.455

i. UE Positioning in E-UTRAN TS 36.305

/Hooman

Cellular Ethernet Backhaul is Easy! – Its’ getting there that’s hard

bbest — Thu, 08 Oct 2009 22:22:00 GMT

After all, we are just trying to transport a simple Ethernet frame from the base station (BTS, NodeB, eNB) to the base station controller (BTS, RNC, S-GW, ASN-GW). As the cellular industry moves toward "all IP networks", they are laying the groundwork for an effective unified backhaul solution. The solution that seems to have universal acceptance is the use of "Metro Ethernet". That is the deployment of a carrier grade Ethernet network solution capable of supporting both the backhaul needs of today (e.g. T1/E1 TDMs, ATM, HDLC, IP) as well as backhaul needs to be presented by 4G (and 5G) networks of the future.

To appreciate how complex this "simple" task of backhauling Ethernet is, you need to be a fan of the Rube Goldberg Machine. A Rube Goldberg machine is a deliberately over engineered apparatus that performs a very simple task in a very complex fashion, usually including a chain reaction. The picture below is an example of such a machine which has the simple objective of "de-mothing" the clothes in the closet.

To see the parallel with the Rube Goldberg Machine, consider the chart below depicting the various solutions to address Ethernet backhaul. This is a characterization of what Metro Ethernet Forum calls the Ethernet Services Model. This is a three layer model with the applications supported (e.g. TDM, ATM, etc.) at the top. The middle layer represents the different standardized services supported by Metro Ethernet. To date that have three services types; 1) a standardized point to point services type called E-LINE, 2) a standardized multipoint to multipoint services called E-LAN, and 3) a rooted point to multipoint services type called E-TREE.

The third layer is the killer. It is called Carrier Ethernet Transport (CET) and describes various transport solutions that couple with Metro Ethernet Applications and Metro Ethernet Services, to provide a Metro Ethernet backhaul solution. By the way, the terms "Metro Ethernet" and "Carrier Ethernet" are sometimes used interchangeably. In fact the "Metro Ethernet" Forum in the organization that defines the "Carrier Ethernet" standards.

The CET solutions can be viewed is three sets. The solutions on the left are the "Multi-technology" solutions. These are the Rube Goldberg Solutions. The solutions on the right include a Pure Ethernet Solution (Provider Backbone Bridging - with Traffic Engineering - PDD-TE) and an MPLS solution (Multi Protocol Label Switching - Transport Profile - MPLS-TP). These are the mode straight forward solutions.

To appreciate the Multi-technology alternatives, consider the possible use of Resilient Packet Rings - RPR (thought by some to become the SONET/SDH of the future). There are numerous ways RPR can be deployed but one realistic contender is this:

Embed the Ethernet frame being transported inside an MPLS header to get an MPLS frame
Embed the MPLS frame inside an RPR header to get an RPR frame
Embed the RPR frame inside a Generic Framing Protocol (GFP) header to get a GFP frame. Note: GFP used to map packetized data into synchronous frames
Map the GFP frame into a SONET Payload Envelope (SPE) which is then encapsulated with a SONET header to get the SONET frame. Now every 125 milliseconds, SONET frames will be carrying your Ethernet data

A natural question is, "If the PBB-TE solution or the MPLS-TE solution is a better solution, why not just go there initially?". There are actually several good answers to that. First, neither PBB-TE nor MPLS-TE are yet approved standards. The PBB-TE proposed standard is the closest is finalized. A search of the web suggests that MPLS-TE may be the preferred between the two alternatives.

A second reason that carrier do not instantly jump to the "better solution" is that new solutions require new commitments to network equipment and software. Migration to new networking solutions in large networks takes years of planning and preparation. As cumbersome as some of the Multi-technology solutions may seem, they provide the carrier an opportunity to begin migrating to Ethernet backhaul today using today's equipment.

A third reason not to jump to a new backhaul solution involves the natural evolution expected toward an all-IP network. This will happen over time. The 4G solutions like Mobile WiMax and Long Term Evolution (LTE) claim to be capable to supporting up to 300 Mbps of traffic from the cell site. Few doubt that these claims will be eventually realized, but it will take some time.

Finally a primary tool to be used by the carriers to extend the life of their current network infrastructure, and grow the backhaul in proportion with increase in demand is "Bundling". Bundling is techniques where multiple "links" are "bundled" together os appear as a single link having larger capacity. Consider the fact that E1 has the capacity to transport 2 Mbps. If this is insufficient, the standards define the next higher transport container to be an E3 with a capacity of 16 E1s (i.e. 32 Mbps). Bundling with a technique called MultiLink Point to Point Protocol (MLPPP), would enable the carrier to bundle 5 E1s to create a "personalized" container of 10 Mbps. On other words, the carrier could grow with the traffic rather than needing to make a quantum jump in the deployed capacity.

As one person once said, there is a light and the end of the tunnel for Ethernet Backhaul (e.g. PBB-TE and MPLS-TP) and it is not a train!!

CQI Reporting in LTE

Hongyan Lei — Thu, 06 Aug 2009 22:44:00 GMT

CQI (Channel Quality Indication) report is an important element of LTE and has significant impact on the system performance. There are two types of CQI report in LTE: periodic and aperiodic. The periodic CQI report is carried by PUCCH. But if the UE needs to send UL data in the same subframe as the scheduled periodic CQI report, the periodic CQI report will use the PUSCH, together with UL data transmission. This is because a UE can’t transmit on both PUCCH and PUSCH simultaneously. In this case, the periodic PUCCH resource will be idle. Since periodic CQI report brings in the “always on” signaling overhead, the report granularity is relatively rough. In order to get more detail CQI report, the eNB can trigger aperiodic CQI report when needed. The aperiodic CQI report is transmitted on PUSCH, together with UL data or alone.

The granularity of CQI report can be divided into three levels: wideband, UE selected subband, and higher layer configured subband. The wideband report provides one CQI value for the entire downlink system bandwidth. The UE selected subband CQI report divides the system bandwidth into multiple subbands, selects a set of preferred subbands (the best M subbands), then reports one CQI value for the wideband and one differential CQI value for the set (assume transmission only over the selected M subbands). The higher layer configured subband report provides the highest granularity. It divides the entire system bandwidth into multiple subbands, then reports one wideband CQI value and multiple differential CQI values, one for each subband.

If closed loop MIMO is used, PMI (Precoding Matrix Indicator) and RI (Rank Indication) are also reported. PMI indicates the codebook (pre-agreed parameters) the eNB should use for data transmission over multiple antennas based on the evaluation of received reference signal. RI indicates the number of transmission layers that the UE can distinguish. Spatial multiplexing can be supported only when RI>1. For spatial multiplexing, CQI is reported based on per codeword. The maximum number of codeword in LTE is two.

There are seven transmission modes in LTE, each one is corresponding to certain multiple antenna techniques. For each transmission mode, certain combination of CQI report is defined in the specs, based on periodic/aperiodic, wideband/UE selected subband/higher layer configured subband, No PMI/single PMI/multiple PMI. Since RI changes slower than CQI/PMI, it is reported with a longer interval on periodic report. It is reported together with CQI/PMI on aperiodic report since the resource on PUSCH is less limited.