Saturday, February 13, 2010

How to Broadcast Multimedia Contents? VI Open-Loop MIMO for Broadcast Multicast Services

What Is The Next for Mobile System Design? I A Single-Cell Model Perspective on Downlinks
[How to Broadcast Multimedia Contents? I Introduction]
[How to Broadcast Multimedia Contents? II Lessons from The Channel]
[How to Broadcast Multimedia Contents? IV Hierarchical Modulation]
[How to Broadcast Multimedia Contents? V Overloaded Transmission and IC]
[How to Broadcast Multimedia Contents? VII Network Layer or Steam Layer Design]

One most well-known space-time block coding (STBC) design is Alamouti code, which is the simplest open-loop orthogonal STBC. Alamouti code was designed for a two-transmit antenna system. It is a rate-1 code. It is the first open-loop encoding method with full diversity. Though orthogonal STBC has the advantages of relatively easy receiver design and full diversity, it is known that full-rate STBC don’t exist for more than 2 transmit antenna. From previous discussion, if two orthogobal STBCs are superimposed together and each of them experiences different channel fading, there would be multi-layer diversity in addition to potential superposition precoding gain. This means one STBC signal layer is in a bad channel condition, the other STBC signal layer may not. Therefore, the transmission of more than one layers may have higher achievable spectral efficiency.


Figure 1. A Quasi-Orthogonal Space Time Block Coding Example


One widely discussed example of quasi-orthogonal STBC is shown in Figure 1. From a receiver perspective, this quasi-orthogonal STBC obviously has a larger signal constellation size. In general, larger the signal constellation size is, high the spectral efficiency is achievable. Besides this, it may have so-called multilayer diversity when it experiences channel fading since each orthogonal STBC signal experiences different fading. However, there is no free lunch. The demodulation complexity may increase exponentially with the number of superimposed STBC layers.

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