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In this article, we shall focus on the wimax base-station. The base-station acts as the network controller and terminates the access radio network. It interconnects wimax terminals to the terrestrial network using the ASN GW node. In the mesh configuration, there may be multiple base stations in a single network and terminals can combine some base station functionalities.

Functional decomposition of the Wimax base station

The diagram below shows the key blocks of the Wimax base station.

Introduction to Wimax BSS

The Wimax base station is a very challenging component to describe and design. It simultaneously meets needs of high capacity, low cost (both capex and opex) and manageability. Many new technologies have to be successfully identified and used in order to make a successful component design. In this article, we have tried to identify the key technologies that we believe are going to be involved. A short description is given below; for more details click on the appropriate block.

Direct Conversion Radio replaces the typical superhet design of standard wireless recievers. Using high performance wideband DAC/ADC technologies, the DCR directly samples and digitizes the incoming RF signal for further base band processing.
Adaptive Antenna Systems are used to increase the capacity of the BTS by dynamically steering the coverage to the most optimal configuration. MIMO capabilities also help in increase receive performance using space time diversity.
RF to baseband interface design in the Wimax Base station undergoes significant changes due to the use of DCR at the front-end.
Time and frequency synchronization remains a significant challenge in the Wimax/Wifi domain. This module executes the ranging function
Channel Estimation is a key challenge for broadband wireless systems. It remains to be seen whether standard linear techniques suffice for multi-carrier environments or newer techniques have to be developed
The Wimax baseband codec is fairly standard, except that it has to support new features like ACM, Hybrid ARQ and deal with throughputs of 150Mb/s or above. Use of SDR in the codec may mean that standard FEC algorithms have to be ported to a GPU environment.
The Wimax baseband to CPU data interface will both depend on and drive the platforms on which Wimax basestation is built. Use of high performance data processing platforms and the functional decomposition of the layer 2 will influence this choice.
The L2 implements the Wimax link layer and media access layers. This is an extremely heavyweight component of the base station. Other than being able to handle throughputs of 150Mb/s in uplink and downlink, the L2 also has to support numerous related functions for

Authentication, Authorization and Accounting
QoS management,
media access for contention channels, PMP mode and mesh modes
network acquisition and ranging services.

Capacity and dimensioning of a Wimax bss

The Wimax BSS is the key component if fixed wireless access networks are to provide viable competition to the DSL, cable modem and other wired broadband access systems. That there is a vast market available at the correct capacity/cost point is evident - OECD data for January 2005 shows that penetration of broadband in Western Europe varies from 2% to 27% with an average of 13.6%. The situation is somewhat better if we consider the Asia Pacific: Japan and Korea combined have an average of 19%.

However, for the Wimax market to succeed, the Wimax network must offer a correct combination of system capacity, coverage and throughput. The following table shows the typical configuration for three classes of base-stations. The first class is for Urban networks, the second for suburban/semi-urband networks and the third is for rural networks. A mixture of retail and SME consumers are assumed.

Feature	Network Type: Urban	Network Type: Suburban	Network Type:Rural
Sectors	6	3	3
Range	2.0 km	2.8 km	10.0 km
Coverage area	15 km²	25 km²	300 km²
Modulations	BPSK-64QAM	BPSK-16QAM	BPSK-QPSK
Adaptive Antennae	Required	Required	Not Required
Average tput per user	2Mb/s	1-2Mb/s	256kb/s-512 kb/s
Peak throughput (best case)	20Mb/s	10 Mb/s	5 Mb/s
Subscriber density	5000-1000 per km²	500-100 per km²	20-5 per km²

The above figures are from [Smura05]. They assume a Wimax Base station operating in the 3.5GHz band, with a channelization of 7Mhz.

Engineering challenges

References

[OECD broadband] [http://www.oecd.org | OECD broadband statistics, Dec.2004]
[Smura05] Timo Smura "COMPETITIVE POTENTIAL OF WIMAX IN THE BROADBAND ACCESS
MARKET: A TECHNO-ECONOMIC ANALYSIS"

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