WiMAX II – 802.16m – Chasing the Ghost

Looking at presentations from a recent LTE meeting I found it quite interesting at how many of them mention WiMAX 802.16m. I haven’t heard much about 802.16m yet but since they all refer to it I thought it might be time to find out a bit more about it.

It seems to be a bit early for that search however. First announced in early 2007 the only facts so far known about 802.16m is that the IEEE would like to create a standard as much backwards compatible as possible to the current version of the WiMAX (802.16e or 820.16-2005) but with peak data rates of up to 1 GBit/s (that’s around 1.000 MBit/s).

Compared to current systems deployed in live networks today such as HSDPA with a theoretical top speed of 14 MBit/s and about 2 MBit/s with a Cat-6 HSDPA mobile today in live networks, these numbers are staggeringly impressive. So how can such data rates be achieved? As not much is known so far, let’s speculate a bit.

Between today and WiMAX II, there’s systems such as WiMAX and LTE which promise faster data rates than those available today by mainly doing the following:

  • Increase the channel bandwidth: HSDPA uses a 5 MHz channel today. WiMAX and LTE have flexible channel bandwidths from 1.25 to 20 MHz (Note: The fastest WiMAX profile currently only uses a 10 MHz channel today for the simple reason that 20 MHz of spectrum is hard to come by). So by using a channel that is four times as broad as today, data rates can be increased four times.
  • Multiple Input, Multiple Output (MIMO): Here, multiple antennas at both the transmitting and receiving end are used to send independent data streams over each antenna. This is possible as signals bounce of buildings, trees and other obstacles and thus form independent data paths. Both LTE and WiMAX currently foresee 2 transmitting and 2 receiving antennas (2×2 Mimo). In the best case this doubles data rates.
  • Higher Order Modulation: While HSDPA uses 16QAM modulation that packs 4 bits into a single transmission step, WiMAX and LTE will use 64QAM modulation under ideal transmission conditions which packs 6 bits into a single transmission step.

By using the techniques above, LTE and WIMAX will be able to increase today’s 2 MBit/s to about 20-25 MBit/s. That’s still far away from the envisaged 1.000 GBit/s. To see how to get there let’s take a look at what NTT DoCoMo is doing in their research labs, as they have already achieved 5 GBit/s on the air interface and have been a bit more open at what they are doing (see here and especially here):

  • Again increase of the channel bandwidth: They use a 100 MHz channel for their system. That’s 4 times wider than the biggest channel bandwidth foreseen for LTE and 20 times wider than used for today’s HSDPA. Note that in practice it might be quite difficult to find such large channels in the already congested radio bands.
  • 12×12 MIMO: Instead of 2 transmit and receive antennas, DoCoMo uses 12 for their experiments. Current designers of mobile devices already have a lot of trouble finding space for 2 antennas so a 12×12 system should be a bit tricky to put into small devices.
  • A new modulation scheme: VSF spread OFDM. This one’s a bit mind bogelling using CDMA and OFDM in combination. Wikipedia contains a description of something called VSF-OFCDM which might be a close brother.

A four times wider bandwidth with six times the number of antennas results in a speed increase factor of 24. So multiplying 25 MBit/s * 24 results in 600 MBit/s or 0.6 GBit/s. That’s still a factor of 8 away from what DoCoMo has said they have achieved, so I wonder where that discrepancy comes from!? I guess only time will tell.


For the moment, the wireless world’s pretty much occupied with making LTE and WiMAX a reality. Pushing beyond that is not going to be an easy thing to do in the real world as bands that allow a single carrier of  100 MHz will be even harder to find than for the 20 MHz envisaged for LTE. Also, cramming more than 2 antennas into a small device will also be a formidable challenge.

More about 4G, LTE and WiMAX can be found here.

5 thoughts on “WiMAX II – 802.16m – Chasing the Ghost”

  1. Hi Martin,

    Interesting entry. It seems though that all these higher speeds are being pushed by manufacturers (because they want to sell new equipment). The biggest cost for an operator is the transmission. Operators still count their transmission in terms of E1 links which they usually lease and cost a LOT of money. How you are going to get 1000Mbps back to the core network? And who wants 1000Mbps worth of bandwidth anyway? Although the technology is quite impressive to read about I don’t think it has much value in a real world environment.. Just my opinion..

  2. Hello Mark,

    you mention an interesting point. Even with HSDPA today, using 2 MBit/s E-1 copper links to the base station is quite a stretch as they are very expensive.

    I recently talked to some guys who deal with backhaul transport. For WiMAX, LTE and also for HSDPA (in the forseeable future) there’s a radical change in store. Instead of E-1 over copper, IP links (e.g. Metro Ethernet) will be used with much faster link speeds than current E-1’s. This won’t work over copper cables however, so the base station needs to either get a fiber link (expensive again if not already there…) or needs to be connected via mircowave (already done today but usually only to bridge E-1’s).

    Concerning the use of the bandwidth. Well, maybe a single person won’t need 1 GB/s. However, a base station severs potentially hundreds of simultaneous users which brings you back to reality very quickly. Or, let me put it in the words of an optical network architect whom I met recently: “5 MBit/s is not broadband” 🙂


  3. It is only half way true that LTE is mainly pushed by operators. In fact operators have realized that end users prefer flat rates over complex tariff models and that the introduction of flat rates causes a tremendous increase in traffic (duh!). With current networks, however, they have to pay more for each additional bit they transfer. Not only in backhauling but also in capacity licenses at the RNC or several other network components.
    For them every increase in traffic means additional costs. Not a very comfortable situation if you want or need to introduce flat rates to stay competitive. Thus, operators are in urgent need for a network technology that can bring more bits to more end users at less cost.
    To find those networks they have even jointly founded a company – the NGMN Ltd. (http://www.ngmn.org) which searches for the best and most cost efficient next generation network. Manufacturers indeed propose LTE as the possible answer to what the NGMN Ltd is searching for as LTE significantly reduces the cost-per-bit.
    It is doing so not only by an all-IP backhaul concept but also by a simplified flattened architecture, an increased spectrum efficiency and the fact that existing spectrum can flexibly be re-used causing only few changes in the existing antenna sites which are another cost factor next to backhaul.

  4. The calculation isn’t exactly right, with S-OFDMA, and probably increased subcarriers number, the spectral efficiency can be increased. so the base is no longer 2xnn, and 1Gbps is achievable, which is also demoed by Samsung and some other companies.

  5. Hello Bruce, I don’t quite understand yet how spectral efficiency is increased by using more bandwidth!? This just increases the number of subcarriers that can be used. This however does not increase spectral efficiency. So I am still missing something.


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