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Wimax network architecture

The diagram below shows the Wimax network architecture. This is a clickable map; click on each entity to go to the corresponding article.

Introduction to wimax

What is wimax ?

Wimax (and its predecessor) Wifi are two new radio-technologies developed for personal communication systems. Based upon OFDM, these technologies allow for broadband wireless communications between individuals. They admit a multiplicity of network hierarchies, potentially allowing multi-tiered networks, mesh networks and other variants.

So what is the big deal?

If we compare wimax with the previous generation of wireless PCS technologies, the big deal is the broadband. GSM allowed individual users a maximum of 200khz per user, and DSSS CDMA improved this to 1.25Mhz - 5Mhz. wimax may increase this to 20Mhz per user.

So what happened to make this possible?

There are many ways to increase receiver bandwidth.

One is to simply try and scale the receiver; this has several problems. As the bandwidth increases, the symbol duration decreases inversely, leading to the requirement for extremely high-speed linear circuitry. Second, the receiver faces frequency selective fading, whereby the symbol experiences different levels of attenuation for different parts of the spectrum. The third is that discrete components which operate over a large frequency band have varying responses across the spectrum - a ripple. This adds to the received noise. UMTS goes for this approach - all data was spread by a 2 Mcps chip rate, before transmission.
Instead of simply scaling a single receiver, it is possible to operate multiple receivers in tandem. While this avoids many issues above, it is expensive and also difficult to synchronize symbols between receiver chains - this causes inter-symbol interference. Also, if we are using standard FDM, there is a large guard band between channels, which makes for poor spectral efficiency
The wimax technique is closer to the previous method than the first one - it uses a long duration symbol which is transmitted over multiple carriers. The key trick is to do an inverse FFT with the symbol over the entire transmission band and use the iFFT output to modulate each carrier. OFDM allows us to pack channels closely together - since a symbol has a natural roll-off with a zero at a multiple of the carrier, you can put the next band with its peak at this zero, as long as you can make sure there is no ISI

How do we compare radio-technologies?

There are several ways to compare two radio-communication technologies. From the user side, the comparision is obviously in terms of peak throughput, average and peak service levels and throughput as a function on cost (including both terminal cost and airtime cost). From the network side, we compare in terms of spectral efficiency and capacity (active users per sq km ).

So, how does wimax stack up against a previous generation of radio-technology, say UMTS?

Comparisions of wimax against UMTS can have good learning potential, as long as we keep an eye on the details and don't look merely at the top picture. First, let us note that these are not exactly similar. For example, UMTS was targetted for handheld user terminals; recently they have been available for PDAs and PCMCIA cards. In all these environments, there are severe restrictions on transmission power. On the other hand, Wimax is meant to be used in portable CPEs, of the size of a laptop or similar - transmission power restrictions are far lower in these cases. Also, many technologies expected to be used in wimax were never targetted for use in UMTS; MIMO comes to mind as an example, simply because putting multiple antennae on a hand-held phone is not much of an option.
A good method of comparision is spectral efficiency. Spectral efficiency measures the aggregate transmission capacity per unit of allocated spectrum, both for an individual user as well as an average for the network. Several factors come into play here:
- The channelization, including guard bands, dedicated pilot bands, etc.
- Modulation and coding available
- The target SIR.
- Whether we are considering the peak spectral efficiency or the average.
- What is the environment under consideration? Are we only considering internal interference i.e. ACI/CCI from other terminals or external sources. This is important because unlike UMTS, wimax works on the unlicensed band, with significant potential for external interferors.

UMTS spectral efficiency has been estimated by many researchers using different techniques. We take our numbers from [gilhousen]. This article does a fairly detailed estimation, dealing not only with allocation of spectrum, but also allocation of power and the effect of pilot signal transmission. The result is that, given full cell loading, the UMTS can handle approximately 36-45 users per sector in a 3-sector cell; each user transmitting an 8kb/s coded voice signal with an activity factor of about 40%. In terms of raw spectral efficiency, this translates to a value of 0.3 bps/Hz.

[Kaiser] provides a comparision of OFDM vs DS-CDMA using simulation techniques. While Kaiser's model does not exactly match that of [Gidovani], it is reasonable and the comparision seems valid. This shows that OFDM outperforms DS-CDMA by a factor of nearly 3 in terms of spectral efficiency, achieving about 0.8 bps/Hz against 0.21 by DS-CDMA, with a 1% blocking probablity.

But still, both the efficiency numbers are rather low. Compare for example, DVB-S, which achieves spectral efficiencies of 2-3 or higher.

Will it be technically viable?

Yes, says any number of marketing experts. For once, they seem to be supported by this article by [Smura]. Smura has broken up the potential Wimax market into three categories, urban, suburban and rural and analyzed the returns potential for overall network deployment. Using different concentration models, Smura forecasts IRR of upto 30% for urban markets and 24% for the other markets. This is based on a 10000 euro base-station product and a CPE cost varying between 300 and 500 euros. For some reason, this study focusses on DSL as the only competing technology for Wimax and ignores 3G, as well as newer evolution of the 3G technologies, such as TD-CDMA.

Related articles: wimax base station, ASN G/W design, Wimax air interface, Wimax core network specifications, ims wimax interworking issues

References

[Gilhousen] Gilhousen, Padovani, Jacobs, Viterbi, Weaver, Wheatley, "On the capacity of a cellular CDMA system", IEEE Transactions on Vehicular Technology, 1991

[Kaiser] Kaiser, "OFDM-CDMA vs DS-CDMA: performance evaluation for fading channels" IEEE Internaltion Conference on Communications, 1995. ICC 95 Seattle.

[Smura] Smura, "Competitive Potential of Wimax in the broadband access market - A techno-economic analysis", Helsinki University of Technology

Maintainer: abheek.saha@hsc.com
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