How fast can my LTE smartphone go?

Expert Opinion

Expert Opinion
Short LTE articles and opinions by our team of Subject Matter Experts (SMEs)

How fast can my LTE smartphone go?

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You just received one of those new fangled 4G LTE phones during the end of year sales!

You just picked up a brand new, unrefurbished “rPhone 999” from your local RayTel Wireless store….”Raytel Wireless, where rPhone is better than your phone!” .  It’s the latest, greatest ultra-fast high speed LTE wireless device in your hands.  It has the high def screen and ultra-super-color display for watching those action movies that you like. And the color of the phone is a hot red color!

Decisions, decisions. What movie to download first? That can wait. The first decision is which of your friends do you call first to brag about your new super high speed device.

Hmmmm?

But wait a minute…..Just how fast does your device really go? Is it really faster than your friend Herman’s phone. He always gets the latest technology.  And he can’t sleep unless he checks his smartphone speed on one of those speed testing web sites every night, so he knows what’s fast.

So just how fast can your device download videos? Let’s figure it out.

Your device is probably an LTE Category 3 device. LTE release 8 devices have a defined “UE category” that specifies uplink and downlink physical layer performance for each category value. These are defined in the 3GPP specification “3GPP TS 36.306 Evolved Universal Terrestrial Radio Access (E-UTRA);  User Equipment (UE) radio access capabilities (Release 8)”.  According to the table below, your new rPhone category 3 UE has a maximum downlink data throughput of 102 Mbps! That has to be better than anything Herman has.

Table 1. Maximum possible LTE airlink physical layer speeds for Release 8 devices (values based on information from 3GPP TS 36.306)

  

But wait, Herman keeps telling you about multiple antennas and MIMO and how it makes his device work so much faster. Let’s do some homework  before we make that call to Herman.

In Table 2 below, I have listed the maximum possible speeds based on different antenna techniques. You’re in luck, the rPhone category 3 device can still achieve up to 102 Mbps IF 2x2 MIMO is used. Also notice that 299 Mbps is only possible for a category 5 UE if 4x4 MIMO is used. But Raytel Wireless uses 2x2 MIMO like most operators today, so 102 Mbps is still possible!

Those maximum speeds have some built-in assumptions. Assumption #1: They assume the usage of multiple antennas, or more specifically multiple transmission layers reusing the same subcarriers on each antenna. Each transmission layer carries different data so you can increase your physical layer throughput by using more antennas with separate transmission layers. But you can only get MIMO if DL channel conditions are good (a high SINR value, probably something greater than 12 dB).  Notice that your maximum possible for a category 3 UE drops to 75 Mbps ( and probably something even slower) if you happen to have poor channel conditions or are a long distance from the transmitting antenna and there are probably obstructions between you and the cell tower antenna.

Assumption #2: must have great channel quality with zero redundancy bits and zero transmission errors. (Not likely for all of these 3 to happen in real life systems.)

Let’s call Herman.

Table 2. Maximum possible DL Physical layer speeds based on DL transmission techniques (maximum 20 MHz bandwidth)

  

But wait….Herman also told you about channel bandwidth. MIMO is great but your phone’s speed is also limited by your operator’s available channel bandwidth.  RayTel Wireless has both 5 MHz and 10 MHz channels depending on which city you are in. Looking at the tables below, your new rPhone category 3 device can achieve a maximum possible physical layer DL speed of 39.6 Mbps or 79.2 Mbps in 5 and 10 MHz channels, respectively. What happened to my 102 Mbps!?!?!?!  I’m using 2x2 MIMO!

Table 3. Maximum possible DL Physical layer speeds in 5 MHz channel

  

Table 4. Maximum possible DL Physical layer speeds in 10 MHz channel

  

Assumption #3: maximum possible speeds are also based on large channel bandwidths. Depending on the device, it can’t attain its maximum possible speed unless the channel bandwidth is large.

Based on the table below, you can see the category 3 UE won’t reach its maximum capability until it has at least a 15 MHz channel.

Table 5. Maximum possible DL Physical layer speeds in 15 MHz channel

  

The limiting factor here is if the channel bandwidth is not available, there just aren’t enough physical airlink resources (Physical Resource Blocks (PRBs)) available to get the highest possible speed.

Here’s the formula I used to make a quick calculation of maximum possible physical layer DL throughput:

Assumptions:

1. network is using normal Cyclic Prefix

2. network has 2 TX antennas

3. DL channel conditions are so good that no redundancy bits are needed (coding rate = 1) and no retransmissions since there are no errors

4.  due to the great channel conditions, the network uses 64-QAM modulation (6 information bits per data modulation symbol)

5. Only 2 OFDM symbols are used for the Physical Downlink Control Channel (PDCCH).

 

Formula: (# of PRBs)  x (# of available data modulation symbols per PRB pair)  x  (6 bits per mod symbol)  x (# of transmission layers)

Then multiply by 1000/1000000 to get a Mbps value.

Example for 10 MHz channel: 50 x 132 x 6 x 2 x 1000/1000000 = 79.2 Mbps

So even if the category 3 UE can attain DL data speeds of up to 102 Mbps, the 10 MHz network can only transmit at the maximum possible speed of 79.2 Mbps.

 

You can do a similar calculation for the UL speeds (see table below). Just use only one transmission layer since the UL only supports one TX antenna, use 16-QAM modulation for category 1-4 devices, and don’t allocate all of the UL PRBs because probably at least 4 of them will be allocated for Physical Uplink Control Channel (PUCCH). Number of available data modulation symbols per PRB pair will be 144 in the uplink.

Table 6. Maximum possible UL Physical layer speeds in 5 & 10 MHz channels

  

Obviously, you can see the maximum possible speeds are attainable if

1. you are the only user in the LTE cell

2. have great channel conditions

3. and the maximum bandwidth is available

 

So if you have to do a nightly “speed check” of your LTE 4G smartphone like my friend Herman does, don’t despair if the numbers don’t reach the maximum speeds we talked about. Those tools only count the application layer speed so they don’t take into account the overhead of LTE airlink protocol layers.

If you are looking for other high-tech toys that you desire , don’t forget the RayTel Wireless rPhone 999, now on sale for only $9.99 and hope that you always get high channel quality.


Ray

  • Wonderful post! It nicely breaks down many misconceptions.

    I previously thought that 73.6mbps was the peak throughput at the 10Mhz network. From my testing I used to peak at about 72mbps, but 79.2mbps is new to me.

    http://i.imgur.com/G6oKP.png

    www.youtube.com/watch

    Thanks for the article!