3.2 Closed Loop Transmit Diversity (CLTD)
In this section, we will discuss a CLTD technique. Here the Mobile Station (or User Equipment, UE)
calculates the weights to be applied to the Base station Transceiver Station (BTS) (or NodeB) in order
to maximize the instantaneous received power at the UE [79??“84]. The performance of this CLTD
technique is governed by three key issues.
1. BTS update rate. How often are the BTS weights updated? For fast time varying channels, we
would require a fast update rate in order to accurately track the fading phenomenon so it can be
effectively compensated.
2. Weight quantization. What information is required by the BTS? Once the UE calculates the
weights to be used by the BTS, it must transmit them to the BTS in an efficient manner. Hopefully
without loss of important information. This can take on the many forms of phase only, phase and
amplitude, progressive updating, and so on.
3. Weight confidence. How does the UE confidently know what weights were applied to the BTS?
Once the UE has quantized the weights it wants to feed back to the BTS, they are transmitted
through a multipath fading channel. So there is a chance that the BTS will apply the incorrect
weight, due to errors caused by the channel.
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