Tuesday, June 19, 2007

[WiMAX] Fundamentals of WiMAX - (4)

Today we are talking about another important technique used in WiMAX - OFDM.

From the characteristic of the channel propagation in wireless environment, we know that delay spread caused by multipath fading will generate severe Inter Symbol Interference (ISI) for high data rate transmission, because the symbol time is inversely proportional to the data rate. As we mentioned before (Why OFDM for WiMAX and B3G?), you can use CDMA technology with Rake receiver to solve this problem, but it elicits two new problems: one is that the complexity is high, another is that you may need a huge bandwidth to get the essential spreading gain. For WiMAX system, the second problem is more difficult to handle with. But for UWB (Ultra Wide Band) system, it is achievable. So that's why one of the UWB's candidate PHY proposal is using spreading spectrum technique (DS-UWB).

Fortunately, a smart multi carrier modulation scheme called OFDM can support high data rate effectively. The basic idea of OFDM is simple: if you can not change the channel, you can change the signal to adapt to it.

We know in order to have a channel that does not have ISI, the symbol duration should be larger than the delay spread (or equivalently, the signal bandwidth should not exceed the coherence bandwidth). If we can divide the high rate date stream into M lower-rate substreams, then each substream will have larger subsymbol duration. If the subsymbol duration exceeds the delay spread, it hence is ISI free. Euqivalently, in this scenario, each substream's bandwidth is lower than the coherence bandwidth of the channel. However, simply dividing the high rate stream to lower-rate streams is just like the conventional mulit carrier technique, you have to insert guard band between subcarriers, which leads to very inefficient use of the available spectrum. OFDM solves this problem by overlapping subcarriers. Of course, you can not overlap subcarriers arbitrarily, because overlapping introduces crosstalk between subcarriers. But, if those subcarriers can overlap in orthogonality, great, the crosstalk vanishes!

A block diagram of OFDM system is shown in the following figure:

The modulator part is illustrated as follows:

And a simplified baseband model of passing an OFDM signal through the channel is demonstrated in the next figure:

Finally, the time-frequency block structure of the OFDM signal is shown in the figure below (where CP means cyclic prefix, whose length should be larger than the maximum delay spread of the channel):

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