Don Hanley

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.

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.

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.

LTE and 1x/1xEV-DO Terminology and Concepts

Don Hanley — Tue, 10 Feb 2009 18:25:00 GMT

By Don Hanley, Senior Consultant - 2/2009

1xEV-DO and LTE networks are surprisingly similar in many respects, but the terms, labels and acronyms they use are very different. How can a 1xEV-DO operator make sense of this new jargon?

Introduction

As 4G technologies like Mobile WiMAX and Long Term Evolution (LTE) move closer to commercial reality, operators are beginning to understand the differences and the similarities between what they have currently deployed and what is coming down the road. Service providers who are contemplating the transition from 1xEV-DO to LTE will have to contend not only with new radio technologies and new network architectures, but with a whole new set of terms and concepts as well.

Both 1xEV-DO and LTE are designed to offer high-speed packet data services to mobile subscribers, so it should not be surprising that they have taken similar approaches to solving some of the challenges they both face. An engineer familiar with 1xEV-DO can get a head start with understanding LTE simply by learning the meaning of key LTE terms and associating them with their 1xEV-DO counterparts.

The following sections take various LTE concepts, grouped into related categories, and provide a brief explanation of each, along with the corresponding 1xEV-DO equivalent. In some cases, there is a one-to-one match between LTE and 1xEV-DO; in others, there simply is no equivalent concept. In most cases, however, there is generally something within 1xEV-DO that does the same thing as its LTE counterpart, under a different name or in a different location. We will identify the similarities and differences of LTE-EPC and 1x/1xEV-DO networks in various categories, including Air Interface, Access and Core Networks, Identities and Operations.

General

LTE is an evolution of the UMTS system defined by the 3G Partnership Project (3GPP), which is an offshoot of the European Telecommunications Standards Institute (ETSI). 1xEV-DO, on the other hand, is designed by the 3G Partnership Project 2 (3GPP2), which is associated with the North American Telecommunications Industry Association (TIA). Both 3GPP and 3GPP2 have mandates to develop specifications for wireless networks, but they have adopted rather different design philosophies, which are reflected in the resulting standards:

a) Flexibility versus optimization: In general, 3GPP prefers to create standards which are very open and flexible, allowing them to incorporate a variety of options, and to easily extend the interfaces to accommodate new features and capabilities. In contrast, 3GPP2 tends to define very optimized interfaces, which perform specific tasks as efficiently as possible. 1xEV-DO, for example, takes far fewer (and much shorter) messages to set up a data session than UMTS requires, but new features tend to require new sets of messages.
b) Authentication and security: 3GPP takes privacy very seriously, and very little information is sent over the air in its original form; encryption, temporary identifiers, message integrity checking, and user verification are basic elements of LTE signaling. 3GPP2 also includes security functions in the definition of 1xEV-DO, but they are optional extensions to the basic operation of the system.
c) User information: 3GPP makes extensive use of the Subscriber Identity Module (SIM), which stores user subscription data and related information separately from the phone itself. This allows a user to make use of a different device without losing their features and contacts. In 3GPP2 systems, the subscriber's identity and the phone's identity are usually tightly linked.

Despite the different mindsets behind the specifications, however, both 1xEV-DO and LTE do what they were designed to do quite well: deliver high-speed packet data to mobile users.

Air Interface

Not surprisingly, the greatest differences between LTE and 1xEV-DO lie in the air interface. 1xEV-DO is a CDMA-based system, using fixed 1.25 MHz channels, while LTE is a scalable OFDMA system, capable of using anywhere between 1.4 MHz and 20 MHz, divided into 15 kHz subcarriers. 1xEV-DO devices are assigned timeslots for downlink traffic, but can transmit at any time on the uplink (the hallmark of a CDMA system); LTE terminals must be explicitly allocated uplink and downlink non-overlapping resources to send and receive traffic. The Physical Layer descriptions of these two technologies are as different as night and day.

Nonetheless, they must both be capable of supporting multiple users simultaneously, of allowing new users to access the network, of tracking the terminal's location and redirecting traffic as the user moves. Key LTE terms relating to the air interface, and their 1xEV-DO equivalents, are listed here.

LTE Term	Meaning and Usage	1xEV-DO Equivalent
OFDMA	Orthogonal Frequency Division Multiple Access, physical layer of LTE Downlink	CDMA
SC-FDMA	Single Carrier Frequency Division Multiple Access, physical layer of LTE Uplink	CDMA
Subcarrier	A single 15 kHz radio channel	Radio channel
Symbol	A single 66.67 µs time period	Chip (0.81 µs)
Resource Element	The smallest unit of radio resources, one subcarrier for one symbol	n/a
Resource Block	The smallest block of resources that can be allocated, 12 subcarriers for 7 symbols (84 resource elements)[1]	n/a
Timeslot	7 consecutive symbols¹	Slot
Subframe	2 consecutive timeslots	n/a
Frame	10 consecutive subframes, the basic transmission interval	Frame
Synchronization Signal	Periodic signal for synchronizing with and identifying cells	Sync message
Reference Signal	Periodic signal for transmission quality measurements	Pilot Channel
PBCH	Physical Broadcast Channel	Control Channel
PDSCH	Physical Downlink Shared Channel	Forward Traffic Channel
PDCCH	Physical Downlink Control Channel	Preambles + MAC channels
PCFICH	Physical Control Format Indicator Channel	DO Session
PHICH	Physical Hybrid ARQ Indication Channel	ARQ Channel
PRACH	Physical Random Access Channel	Access Channel
PUSCH	Physical Uplink Shared Channel	Reverse Traffic Channel
PUCCH	Physical Uplink Control Channel	MAC Channels

Access Network

Figure 1 illustrates an LTE eUTRAN, the radio access network. The eUTRAN has a flat architecture, with no centralized controller; instead each eNode B manages its own radio resources, and collaborates with other eNode B's over the X2 interface. The eNode B's connect to the core network over the S1 interface, to allow users to register with the network and send and receive traffic.

Key LTE terms relating to the access network, and their 1xEV-DO equivalents, are listed here:

LTE Term	Meaning and Usage	1xEV-DO Equivalent
eUTRAN	Evolved Universal Terrestrial Radio Access Network	AN
eNode B	Evolved Node B	Base station + RNC
Physical Layer Cell ID	Unique cell identifier	Pilot PN offset
UE	User Equipment	AT
X2	eNode B <-> eNode B interface	A13/A16/A17/A18
S1	eNode B <-> core network interface	A10/A11/A12
Uu	LTE air interface	Specified per 3GPP2 C.S0024 (IS-856)
Attach	A configured signaling path between the UE and the eNode B	DO Session
Radio Bearer	A configured and assigned radio resource	DO Connection

Core Network

The LTE and 1xEV-DO core networks are more similar than they are different; Figure 2 shows a view of the LTE Evolved Packet Core (EPC). Both are based on IP protocols, and support seamless access to packet-based services; both make use of Mobile IP to redirect traffic as the user moves through the network.

Key LTE terms associated with the core network, and their 1xEV-DO equivalents, are listed here:

LTE Term	Meaning and Usage	1xEV-DO Equivalent
EPC	Evolved Packet Core	Packet Data Network
MME	Mobility Management Entity	RNC + PDSN + AN-AAA
S-GW	Serving Gateway	PDSN + PCF
PDN-GW	Packet Data Network Gateway	HA
HSS	Home Subscriber System	AAA
PCRF	Policy Charging Rule Function	PCRF
MIP	Mobile IP	MIP
S1 Bearer	A configured traffic path between the eNode B and the S-GW	A10 + R-P Session
S5/S8 Bearer	A configured traffic path between the S-GW and the PDN-GW	MIP
EPS Bearer Service	A configured end-to-end traffic path between the UE and the PDN-GW (Radio Bearer + S1 Bearer + S5/S8 Bearer)	PPP + MIP

Operational Terms and Identifiers

When a mobile device arrives in the network, it must be recognized, configured and assigned resources, and its services must be maintained as it moves from cell to cell. Various terms associated with LTE operational functions, and their 1xEV-DO equivalents, are listed here:

LTE Term	Meaning and Usage	1xEV-DO Equivalent
UE	User Equipment (the mobile device)	Access Terminal (AT)
IMSI	International Mobile Subscriber Identity	IMSI [Mobile Country Code (MCC), Mobile Network Code (MNC) and Mobile Identification Number (MIN) or Mobile Directory Number (MDN)]
IMEI	International Mobile Equipment Identity	Mobile Serial Number (MSN) or Mobile Equipment Identity (MEID)
Downlink (DL)	Transmissions from the network to the mobile	Forward Link (FL)
Uplink (UL)	Transmissions from the mobile to the network	Reverse Link (RL)
Ciphering	Over-the-air privacy	Encryption
Attach	Initial registration process	UATI Assignment + DO Session Establishment + MIP Registration
MIB, SIB	Master Information Block and System Information Block	Quick Config + Sector Parameters + Access Parameters + DO Session
DCI, UCI	Downlink Control Information and Uplink Control Information	Traffic Channel Assignment
C-RNTI	Cell Radio Network Temporary Identifier	MAC Index
CQI	Channel Quality Indicator	DRC value
HARQ	Hybrid ARQ	HARQ
Handover	Redirection of traffic from one base station to another	Handoff
Measurement Control events A1, A2, A3, A4, A5, B1, B2	Thresholds for cell selection and handover	Pilot Add, Pilot Drop, Dynamic (Soft Slope) Thresholds

Conclusion

A simple description in a table does not convey the full complexity of a concept; a detailed understanding of LTE's technologies, architectures and interfaces is needed to fully appreciate both the similarities and the differences it has with 1xEV-DO. Nevertheless, the fact that LTE and 1xEV-DO concepts can be laid out side-by-side in this way should help to reassure 1xEV-DO operators that the step from 3G to 4G is not as big a leap as they may have thought.

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The 3GPP and LTE logos are the property of Third Generation Partnership Project (3GPP). The 3GPP2 logo is property of Third Generation Partnership Project (3GPP2) and its organization partners. The TIA logo is property of Telecommunications Industry Association (TIA). The content of this document is based on 3GPP/LTE, 3GPP2 and TIA specifications which are available at www.3gpp.org, www.3gpp2.org, and www.tiaonline.org.