LTE Network Tech Explained

If you look at the evolution of wireless networks from generation to generation, one of the clear generational delimiters that sticks out is how multiplexing schemes have changed. Multiplexing of course defines how multiple users share a slice of spectrum, probably the most important core function of a cellular network. Early 2G networks divided access into time slots (TDMA—Time Division Multiple Access); GSM is the most notable for being entirely TDMA.

3G saw the march onwards to CDMA (Code Division Multiple Access) where each user transmits on the entire 5 or 1.25MHz, but encodes data atop the spectrum with a unique psueodorandom code. The receiver end also has this pseudorandom code, decodes the signal with it, and all other signals look like noise. Decode the signal with each user’s pseudorandom code, and you can share the slice of spectum with many users. As an aside, Qualcomm initially faced strong criticism and disagreement from GSM proponents when CDMA was first proposed because of how it seems to violate physics. Well, here we are with both 3GPP and 3GPP2 using CDMA in 3G tech.

Regardless, virtually all the real 4G options move to yet another multiplexing scheme called OFDMA (Orthogonal Frequency Division Multiple Access). LTE, WiMAX, and now-defunct UMB (the 3GPP2 4G offering) all use OFDMA on the downlink (or forward link). That’s not to say it’s something super new; 802.11a/g/n use OFDM right now. What OFDMA offers over the other multiplexing schemes is slightly higher spectral efficiency, but more importantly a much easier way to use larger slices of spectrum and different size slices of spectrum—from the 5MHz in WCDMA or 1.5MHz in CDMA2000, to 10, 15, and 20MHz channels.

We could spend a lot of time talking about OFDMA alone, but essentially what you need to know is that OFDMA makes using larger channels much easier from an RF perspective. Engineering similarly large channel size CDMA hardware is much more difficult.

In traditional FDMA, carriers are spaced apart with large enough guard intervals to guarantee no inter-carrier interference occurs, and then band-pass filtered. In OFDM, the subcarriers are generated so that inter-carrier interference doesn’t happen—that’s done by picking a symbol duration and dividing it an integer number of times to create the subcarrier frequencies, and spacing adjacent subcarriers so the number of cycles differ by just one. This relationship guarantees that the overlapping sidebands from other sub-carriers are nulls at every other subcarrier. This results in the interference-free OFDM symbol we’re after, and efficient packing of subcarriers. What makes OFDMA awesome is that at the end of the day, all of this can be generated using an IFFT.

If that’s a confusing mess, just take away that OFDMA enables very dense packing of subcarriers that data can then be modulated on top of. Each client in the network talks on a specific set of OFDM subcarriers, which are shared among all users on the channel through some pre-arranged hopping pattern. This is opposed to the CDMA schema where users encode data across the entire slice of spectrum.

The advantages that OFDMA brings are numerous. If part of the channel suddenly fades or is subject to interference, subcarriers on either side are unaffected and can carry on. User equipment can opportunistically move between subcarriers depending on which have better local propagation characteristics. Even better, each subcarrier can be modulated appropriately for faster performance close to the cell center, and greater link quality at cell edge. That said, there are disadvantages as well—subcarriers need to remain orthogonal at all times or the link will fail due to inter-carrier-interference. If frequency offsets aren’t carefully preserved, subcarriers will no longer be orthogonal and cause interference.

Again, the real differentiator between evolutionary 3G and true 4G can be boiled down to whether the air interface uses OFDMA as its multiplexing scheme, and thus support beefy 10 or 20MHz channels—LTE, WiMAX, and UMB all use it. Upstream on LTE uses SC-FDMA which can be thought of as a sort of precoded OFDMA. One area where WiMAX is technically superior to LTE is OFDMA on the uplink, where it in theory offers faster throughput.

There are other important differentiators like MIMO and 64QAM support. HSPA+ also adds optional MIMO (spatial multiplexing) and 64QAM modulation support, but even the fastest HSPA+ incantation should be differentiated somehow.

Again, OFDMA doesn’t implicitly equal better spectral efficiency. In fact, with the same higher order modulations, channel size, and MIMO support, they’re relatively similar. The difference is that OFDMA in LTE enables variable channel sizes and much larger ones. This table from Qualcomm says as much:


Keep in mind, the LTE device category here is category 4.
Launch LTE devices with MDM9600 are category 3. 

LTE heavily leverages MIMO for spatial multiplexing on the downlink, and three different modulation schemes—QPSK, 16QAM, and 64QAM. There are a number of different device categories supported with different maximum bitrates. The differences are outlined in the table on the following page, but essentially all the Verizon launch LTE devices are category 2 or 3 per Verizon specifications. Differences between device categories boil down to the multi-antenna scheme supported and internal buffer size. Again, the table shown corresponds to 20MHz channels—Verizon uses 10MHz channels right now.

One of the limitations of WCDMA UMTS was its requirement of 5MHz channels for operation. LTE mitigates this by allowing a number of different channel sizes—1.4, 3, 5, 10, 15, and 20MHz channel sizes are all supported. In addition, all equipment supports both time division duplexing (TDD) and frequency division duplexing (FDD) for uplink and downlink. Verizon right now has licenses to the 700MHz Upper C-Band (13) in the US, which is 22MHz of FDD paired spectrum. That works out to 10MHz slices for upstream and downstream with an odd 1MHz on each side whose purppse I’m not entirely certain of.

All of what I’ve described so far is part of LTE’s new air interface—EUTRAN (evolved UMTS Terrestrial Radio Access Network). The other half of the picture is the evolved packet core (ePC). The combination of these two form LTE’s evolved packet system. There’s a lot of e-for-evolved prefixes floating around inside LTE, and a host of changes.

Introduction to Cellular Network Evolution More about LTE and Implementation Details
POST A COMMENT

31 Comments

View All Comments

  • OctavioShaffer - Tuesday, November 13, 2018 - link

    this very informative, i can now open my vpn server, through my Ip address 192.168.1.1. because recently i spend days just to search a to fix my problem. Thanks alot! Reply
  • strikeback03 - Wednesday, April 27, 2011 - link

    I live in an LTE market and would be happy to accept some LTE devices if you don't want to be driving to Phoenix ;)

    For that matter I also have a Droid X with the stock 2.2 build.

    Playing with a Thunderbolt at a Verizon store the data speeds are really quick. Will be interesting to see how much they drop off though with more users.
    Reply
  • Penti - Wednesday, April 27, 2011 - link

    They use 20MHz in 2.6GHz here in Sweden (few cities so far) so you can actually see speeds up to about 80 Mbits here, LTE-Adv on 800MHz is in the works of being deployed here now, but they will be using 10MHz spectrum. I'm guessing people in major cities will see 20-80Mbits and people in areas only covered by 800MHz will see 10-40Mbits. Latency is where it clearly matters though. Though 50 Mbits on 2x10MHz 700MHz is clearly at the top. It's not often you will see much of high speeds any way.

    To bad they pretty much price themselves out of the market though. A 16Mbit Turbo3G connection is less then half of what 4G costs here. For limitless traffic at least.
    Reply
  • xp3nd4bl3 - Wednesday, April 27, 2011 - link

    Love the graphing. Reply
  • mars2k - Wednesday, April 27, 2011 - link

    Not good, I need USB tethering. This is a deal breaker for me. I need outside access in several places where wi fi is not allowed. Reply
  • Lord 666 - Wednesday, April 27, 2011 - link

    Brian,

    Currently have an open ticket with VZW about the lack of public addresses. Have several LTE cards used with cradlepoints that are used for DMVPN backup connections and need public addresses. In testing, would randomly get nat'd address bring up a complete tunnel, but it was very rare. All of the IPs issued were 10.xxx in NYC.

    Was told static public IPs will be available around May.
    Reply
  • nerdydesi - Wednesday, April 27, 2011 - link

    I'm curious on what you meant by this.

    "Note that the Thunderbolt is a 2x1 device while the others are 2x2, which explains some of the upstream throughput distribution difference"

    Do you mean that the other devices have more antennas than the Thunderbolt and thus why their speeds seemed to be faster than the Thunderbolt? Regarding the phone and its "unlimited data", I used 30gb in my last billing cycle and so far 70gb now with no peep from Verizon. It could also be that I'm currently a VZW employee. I hope that because I bought the phone, I can be grandfathered into the plan.

    Also as a note, if you take the sim card from the Thunderbolt with its full voice and data plan and put that into a mifi or USB modem, you get the unlimited data as well. Just keep in mind you pay more per month due to having the voice along with it (which is useless on the modem devices), but still better than the current 5gb and 10gb caps. If you do vice versa, take the card from a modem to an LTE phone, you are charged for each minute of voice and each text unless you change your plan.
    Reply
  • DanNeely - Wednesday, April 27, 2011 - link

    " AT&T on the other hand has a sprinkling of lower block B and C licenses that are both 12MHz. AT&T also purchased Qualcomm's licenses to blocks D and E, which are both 6MHz unpaired, though it's not entirely clear how AT&T will integrate both blocks of unpaired spectrum. All total that gives AT&T between 24 and 36MHz of 700MHz spectrum, again depending on market."

    Since the only blocks that they own nationwide are the 6mhz D and E blocks shouldn't it be 12 to 36mhz of spectrum. Looking at auction maps it appears there're fairly large areas where ATT didn't win the A or the B blocks.

    http://www.cellularmaps.com/700_auction.shtml
    Reply
  • Brian Klug - Thursday, April 28, 2011 - link

    That's a good point. If they can manage to either TDD or FDD both of those it should be 12 to 36. Just don't forget about the lower C block which was involved prior to this latest auction, that's the 24 that I'm thinking of.

    With 12 MHz of spectrum they can run 5 MHz FDD channels which really won't be much faster than current WCDMA systems. I guess that's why I mentally discounted it.

    -Brian
    Reply
  • Lothsahn - Wednesday, April 27, 2011 - link

    I notice that with my Sprint aircard, I maintain the 3G connection even during extended trips across the nation. However, the 4G connection on the same aircard appears to be unable to handoff and loses its connection while traveling constantly. I find that my connection disconnects every 2-3 minutes when actually moving. However, if I'm stationary in a building, it'll maintain the connection for hours.

    What results did you have with LTE for these sorts of usage scenarios?
    Reply

Log in

Don't have an account? Sign up now