從發射端傳輸的信號除了會受到雜訊干擾之外，也會受到多重路徑衰退影響，
使得原始訊號經過通道後變得品質不良。合作式通訊系統主要是在發射端與接收端
之間建立數個中繼站，而透過這些中繼站轉傳發射端訊號給接收端以彌補點對點的
傳輸可能受到通道衰減的不足，而每個中繼端只會有一根天線，因此建立成本也較
低，此方法透過這些散佈的天線之間的合作，可以達成等同於MIMO 的效果，提供
了小型用戶裝置受限體積無法容納多天線的解決之道。
在中繼端轉傳的協定大致可分為放大後轉傳(Amplify and Forward) 和解碼後轉傳
(Decode and Forward)，而在接收端的訊號合併則多採用最大比率合併(Maximun
Ratio Combining MRC)、選擇合併(Selection Combining SC)，在本論文也採用混合
SC/MRC 架構(Hybrid-SC/MRC)。
本文利用在中繼端解碼後轉傳(Decode and Forward) 的中繼協定並且利用發射端到接
收端通道響應的諧和平均(Harmonic Mean)，有別於單純考慮中繼端到接收端的通道
響應，能有效改善系統錯誤率。

In the wireless communication, the signal transmitted from the source to the destination
may suffer the multipath fading besides the noise interference. This reduces the
quality of the received signal at the destination. The cooperative communication system
exploits several inactive relay mobiles between the source and destination, such that
these relay nodes can help re-transmit their received signals to the destination. We assume
that each relay has only one antenna, so the construction cost is low. This method
achieve an equivalent effect of the MIMO system through the cooperation between these
scattered antennas, since the limited volume of small user devices cannot accommodate
multiple antennas. The forwarding protocol at the relay station can be roughly divided
into ”Amplify and Forward” and ”Decoding and Forwarding”. For the signal combination
at the destination, one can apply the Maximum Ratio Combing (MRC), or Selection
Combining (SC), or the Hybrid SC/MRC. In this thesis, we apply the relay protocol of
Decode and Forward at the relay stations. We propose the harmonic mean of channel responses
between source-relay and relay-destination. These harmonic means are applied
in the destination to replace conventional the channel responses between relay and destination.
The harmonic mean of channel responses can reflect the overall channel effects
from the source to a relay and the relay to destination. We apply this harmonic-mean
approach in several schemes, and we observe that better performance can be achieved.
We also discuss how to efficiently transmit the estimated channel responses at a relay
node to the destination.

[1] Lingzhi Cao and Norman C Beaulieu. Bit error rate analysis of hybrid selection/maximal-ratio diversity combining with channel estimation error. In IEEE
Global Telecommunications Conference, 2004. GLOBECOM’04., volume 1, pages
446–451, 2004.

[2] Ming-Xian Chang and Yu T Su. Model-based channel estimation for ofdm signals in rayleigh fading. IEEE Trans. Commun., 50(4):540–544, 2002.

[3] Sheng-Lun Chiou, Kuo-Wang Hsieh, and Ming-Xian Chang. Csi reduction of mimoofdm systems by parameterization. In 2008 IEEE 19th International Symposium on Personal, Indoor and Mobile Radio Communications, pages 1–5, 2008.

[4] A Dinamani, Swagata Das, L Bijendra, R Shruti, S Babina, and B Kiran. Performance of a hybrid mrc/sc diversity receiver over rayleigh fading channel. In 2013 International conference on Circuits, Controls and Communications (CCUBE), pages 1–4, 2013.

[5] J Nicholas Laneman, David NC Tse, and Gregory W Wornell. Cooperative diversity in wireless networks: Efficient protocols and outage behavior. IEEE Trans. Inf. Theory, 50(12):3062–3080, 2004.

[6] Hoyong Lee, Sang Kyu Park, and Yujae Song. Performance analysis of a hybrid sec/mrc diversity scheme over rayleigh fading. In 13th International Conference on Advanced Communication Technology (ICACT2011), pages 1115–1118, 2011.
[7] Qian Li, Rose Qingyang Hu, Yi Qian, and Geng Wu. Cooperative communications for wireless networks: techniques and applications in lte-advanced systems. IEEE Wirel Commun, 19(2):22–29, 2012.

[8] Ahmed K Sadek, Weifeng Su, and KJ Ray Liu. Multinode cooperative communications in wireless networks. IEEE Trans. Signal Process., 55(1):341–355, 2006.

[9] Tairan Wang, Alfonso Cano, Georgios B Giannakis, and J Nicholas Laneman. Highperformance cooperative demodulation with decode-and-forward relays. IEEE Trans. Commun., 55(7):1427–1438, 2007.

[10] Moe Z Win and Jack H Winters. Analysis of hybrid selection/maximal-ratio combining in rayleigh fading. In 1999 IEEE International Conference on Communications (Cat. No. 99CH36311), volume 1, pages 6–10, 1999.

[11] Moe Z Win and Jack H Winters. Virtual branch analysis of symbol error probability for hybrid selection/maximal-ratio combining in rayleigh fading. IEEE Trans. Commun., 49(11):1926–1934, 2001.