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Saturday, July 20, 2013

PowerPoint Presentation On OFDM

OFDM Modulated Cooperative Multiple-Access Channel With Network-Channel Coding PPT

OFDM Modulated Cooperative Multiple-Access Channel With Network-Channel Coding Presentation Transcript: 
1.OFDM Modulated Cooperative Multiple-Access Channel With Network-Channel Coding

System model of  OFDM-CMAC
Diversity order of outage probability
Achievable Rates and Code Designs
Code Design methodology

consider the half-duplex cooperative multiple-access channel (CMAC) with frequency-selective block-fading.
Each link employs an orthogonal frequency division multiplexing (OFDM) system, where modulated symbols are drawn from a finite constellation set.
We first obtain the diversity order of the CMAC, as a function of the time sharing variables of the users and the rates of the code.

4.To achieve this rate diversity tradeoff, we use the principle of network coding where messages of the two sources are jointly encoded
Both separate and joint network-channel coding approaches are considered
Specifically, we design multiple turbo codes that minimize the outage probabilities of these approaches.
 We also give a code structure for the multiple turbo codes to achieve full diversity of the system.

Basic idea
Frequency selective fading -In any radio transmission, the channel spectral response is not flat. It has  fades in the response due to reflections causing cancellation of certain frequencies at the receiver. Reflections off near-by objects (e.g. ground, buildings, trees, etc) can lead to multipath signals of similar signal power as the direct signal. This can result in deep nulls in the received signal power due to destructive interference. For narrow bandwidth transmissions if the null in the frequency response occurs at the transmission frequency then the entire signal can be lost . This problem overcome by using OFDM trensmission.
» Using a large number of parallel narrow-band subcarriers instead of a single wide-band carrier to transport information.
 » Very easy and efficient in dealing with multi-path.
» Robust again narrow-band interference

6.System model of OFDM-CMAC
In this paper,  focus on the decode and-forward(DF) protocol under the framework of coded cooperation for cooperative multiple access channel (CMAC).
 We consider a time-division based half-duplex CMAC. With one antenna each, two sources S1 and S2 cooperate to deliver their messages to a common destination D.

7.We assume a block-fading channel where the channel coefficients remain constant over the duration of one block consisting of ?? channel uses, but are independently generated for each block.
For each channel use, one modulated symbol is transmitted. For each link, we assume a frequency selective fading channel with ?? multipaths, and an OFDM symbol that consists of ???? subcarriers, where ?? = ????.
 The normalized channel coefficients in the frequency domain for link ???? (channel from node ?? to node ??) are denoted by
g(????) = [??(????) 1 , · · · , ??(????) ????]??
 where the subscript denotes the subcarrier index

8.. Let ??|??(????)?? |2 = 1, where ?? ? {1, 2} and?? ? {1, 2,}.
 We assume a reciprocal channel between S1 and S2, so g(12) = g(21).
We denote the instantaneous SNR vector for link ???? as ??(????) = ¯??(????)g(????) ° (g(????))*, where ° is the element wise multiplication, ¯??(????) denotes the average SNR on link ????.
The ??th element of ??(????) is the SNR for the ??th

9.B. Achievable Rates and Code Designs
       Let ???? (??), or  ??(??)=  the mutual information as a function of the instantaneous SNR ??, based on modulated symbols from the set ?? of finite size 2??.

Assume that the corresponding ??(??) satisfies the following two properties.

P1: ??(0) = 0 and ??(??) is a monotonically increasing function that approaches?? <8 .="" br="">
P2: ??(??1) + ??(??2) > ??(??1 + ??2), for sufficiently high ??1 and ??2
10.consider the cooperation mode, where we assume  ??1 = ??1??12 and ??2 = ??2??21. Each source therefore can decode the other message successfully and cooperate to transmit to D in the third time slot using STBC. By using independent codewords and joint  decoding, an achievable rate region for (??1,??2) can be obtained as
??1 = ??1??1?? + ??3??STBC, (1)
??2 = ??2??2?? + ??3??STBC, (2)
??1 + ??2 = ??1??1?? + ??2??2?? + ??3??STBC.
Here, ??STBC =S???? ??=1 ??(??(1??) ?? + ??(2??)?? )/???? is the mutual  information achieved by the STBC coding

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