Diversity Receiver

by allenlu2007

What is Diversity Receiver

Diversity receiver (或翻譯為分集接收器) 在現代通訊中常使用。

一些是 (intentional) 在 transmitter 就把 information 分散在不同的載體上 (frequency, time, space, phase, polarization). 一些則是 (unintentional) 因為 channel 的特性自然產生 (e.g. multi-path 產生 time and space diversity; dispersion 產生 time diversity or ISI; frequency fading or response 產生 ISI or frequency diversity).  不論是 intentional or unintentional 產生的 information 分散作用。都需要 diversity receiver 來接收以及重組回原來的 information.

Some Diversity (or like diversity) receiver or transceiver

* Frequency diversity (OFDM)

* Time diversity (DSSS, Rake receiver)

* Space diversity (beam-forming or MIMO)

* Phase diversity (similar to time diversity, but relative to frequency; well defined time and make orthogonal receiver)

* Polarization diversity (similar to phase diversity; orthogonal receiver for planar wave?)


add figures!!


Diversity receiver structure:  

f1, f2, .. fn (orthogonal)

t1, t2, …, tn (t1, t2, .., tn are random variables for Rake receiver)

s1, s2, .., sn?

p1, p2 (sin and cos, orthogonal)

pol1, pol2 (any more branch?)


Why Diversity Receiver or Diversity Transceiver

1. Diversity provides redundancy (i.e. robustness).  If one branch has trouble, we can use other branches to recover information.

2. Diversity provides gain after aligning and combining multiple branches together, either using soft decision and decode, or hard decision and decode.

3. Facilitate implementation without reference or reference may be uncertain (due to noise or imperfection).   E.g.   phase diversity receiver can reconstruct signal without reference phase; polarization diversity receiver can reconstruct signal without reference polarization.




How to Implement Diversity Receiver (unintentional diversity) or Diversity Transceiver (intentional diversity)

理論上 diversity transmitter 的每一 branch 收到一樣的 signal (好像送 repetition code).  例如 Rake receiver 

in multi-path environment 自然收到 replicas of time delayed signals (echos).  可以 soft decision and decode, 或是 hard decision and decode.  

但 repletion code 的 efficiency 太低,另一方式是做更有效率的 coding (RS, Viterbi, turbo, or LDPC), 就可以在不同的 branch 傳不同的 signals, 如 COFDM transmitter/receiver, i.e. coding + OFDM.  

如果是純 orthogonal bases (frequency, phase, or polarization) 沒有 redundancy 也沒有加 coding, 則沒有任何 diversity gain or redundancy 的好處。只是 information 的 encoding 方式。舉例而言,如果 phase diversity 是 sin(x), cos(x),  則沒有 redundancy.  如果是 sin(x), sin(x+120), sin(x+240) 則有 redundancy 效果嗎 ? (in power system or motor PWM control).

另一個例子:  s1, s2, .., sn 都是相同 signal, 則是 beam forming.  如果不同,則是 MIMO.

當然 MIMO 多半有 coding (in case some branch has problem), STBC (space time block code).  MIMO+STBC is very common.  就像 OFDM –> COFDM