Saturday, May 12, 2007

[Wireless] Why OFDM for WiMAX and B3G?

In the beginning of 3G (Third Generation) era, CDMA technique was widely employed for most systems. For instance, 3GPP selected WCDMA (Wideband CDMA) for FDD mode, TD-SCDMA for TDD mode; 3GPP2 extended cdmaone to CDMA2000. The main reason is to support higher data rate wireless service in 3G systems than that in 2G systems.

Wideband CDMA system has a large bandwidth, which can be used to achieve frequency diversity. However, there exists a contradiction between the large bandwidth and high data rate which can be shown in the following simple diagram:

Wide bandwidth -> frequency selective channel -> causes large delay spread in time domain
High data rate -> small symbol interval -> experiences serious Inter Symbol Interference

For CDMA system, in order to combat this multipath delay spread, usually it has to resort to the sophisticated RAKE receivers, which resolves multipath using muliple fingers.

However, when wireless system goes to the broadband era, wideband CDMA may not be adequate. Even 3GPP proposed HSPDA/HSUPA to support data rate up to 14.4M/5.8Mbps in 5MHz channel , 3GPP2 proposed 1xEV-DO for data rate 3.1M/1.8Mbps (revision A), 4.9M/1.8Mbps (revision B) in 1.25MHz channel, their capability of supporting much higher data rate is limited. One reason is that they have to use multiple codes simultaneously, which may result in self-interference.

That's why in B3G (Beyond 3G), LTE (Long-term evolution) of 3GPP and AIE (Air Interface Evolution) of 3GPP2 both pick OFDM combining MIMO instead of CDMA. Similarly, in IEEE802.11 series, the low date rate is working on CDMA (802.11b) but high date rate uses OFDM (802.11g and 802.11a). WiMAX is also using OFDM.


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