COMPARISON OF CODED AND UNCODED

VERTICAL AND HORIZONTAL LAYERED

STRUCTURE FOR MIMO OFDM SYSTEM

Khalida Noori,Electrical Engineering Department

Military College ofSignalsNational University ofSciences and Technology,

Rawalpindi, Pakistan.khliag cs edu.k

Abstract - This paper presents coded and uncoded vertical andhorizontal Layered structure for MIMO OFDM system. A turboencoder is used for encoding and MAP algorithm is used fordecoding. The results are simulated in both Raleigh and AWGNchannels. In this paper, coded and uncoded vertical andhorizontal Layered MIMO OFDM system is compared and theirperformance is analyzed in terms of bit error rate (BER). Weanalyze that BER of coded layered structure MIMO-OFDMsystem is lesser than that of uncoded layered structure MIMO-OFDM system. Simulation results are shown for two and threetransmit and receive antennas.

Keywords - Layered, MIMO-OFDM

I INTRODUCTIONOFDM technique greatly reduces receiver complexity by

transforming the frequency-selective fading channel into a setof parallel flat fading channels. One possible way to improvereliability of wireless communications is to employ diversity.Multiple-Input Multiple-Output (MIMO) antenna systems area form of spatial diversity. In a multi-path rich wirelesschannel, deploying multiple antennas, at both the transmitterand receiver, achieves capacity [1, 2].As a result, the combination of MIMO techniques with

OFDM (MIMO-OFDM) has a potential to enhance the datarate of future wireless communication systems [3, 4]. Space-time trellis codes (STTC) are proposed, together withconstruction criteria [5, 6]. Space- time block codes (STBC),are also attractive for their simplicity of detection [7]. A thirdpopular approach is layered structure [8].Layered structurehas shown its potential as a low-complexity low-cost solutionto future wireless communication systems with giganticincrease in spectral efficiency and throughput. In layeredsystems, the input data stream is de-multiplexed,independently coded, and sent via different transmitsantennas simultaneously. The low complexity of layeredstructure processing makes the layered structure a promisingcandidate for MIMO OFDM system with a large number oftransmit antennas and higher order modulation.

II LAYERED STRUCTURELayered structure is a technique that can attain a tight

lower bond on the MIMO channel capacity. The input datastream is divided into more independent streams, or layers of

Sami Ahmed HaiderElectrical Engineering Department

Military College ofSignalsNational University ofSciences and Technology,

Rawalpindi, Pakistan.samZigmllcs. edu.pgk

data, and is transmitted from all available transmit antennassimultaneously. This idea was originally suggested byFoschini. According to [7], spread data can exist in channelbetween transmit and receive antennas without conflictingeach other, and hence data can easily be exploited. Since eachpath is independent from point of view of the receiver, it canrecognize multipath as multiple individual paths. This isperformed by transmission of several copies that transmittedwith different path profile.

Concept of layering comes out as a way to achieve,possibly, full diversity and full rate gain. Main features oflayering schemes are that each layer encodes a certainnumber of symbols and different layers encode independentsets of symbols, which are successively decodedindependently of each other.

III VERTICAL LAYERED STRUCTUREIn vertical layered structure the input is first encoded via a

channel code.

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Figure 1: Block diagram of vertical layered MIMO OFDM system

The encoded stream is subsequently de-multiplexed intosub streams and each of them is independently modulated andassigned to a transmit antenna.

A- ImplementationThe Figure 2 shows the block diagram of vertical coded

layered structure MIMO OFDM system according to whichsimulation is done. First data is encoded with turbo encoderand then multiplexed. Each layer is individually modulated.After taking IFFT and inserting cyclic prefix, each layer istransmitted through transmit antenna After passing throughRayleigh channel and AWGN channel, data goes to receiver.

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Receiver works in same manner as transmitter but in reverseorder. Only change is that each layer is separately decoded.

RECEIE

Figure2: Block Diagram of Vertical Coded Layered MIMO OFDMSystem with two transmit and two receive antennas

B-Simulation resultsSimulation result in Figure 3 shows bit error rate (BER) of

the vertical uncoded LST architecture OFDM system having32 sub-carriers, with three transmit and three receiveantennas over a rayleigh fading with additive white Gaussiannoise (AWGN) channel. On x-axis is the signal to noise ratioand on y-axis is bit error rate. It is evident from Figure 5-13that bit error rate reduces as signal to noise ratio increases.BER at SNR value 15dB is greater than 10-°4 but at SNRvalue 20dB, BER is less than 10-0°6.

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Figure 3: BER plot of uncoded vertical layered MIMO OFDM system

Simulation result of Figure 4 shows bit error rate (BER) ofthe vertical coded LST architecture OFDM system having 32sub-carriers, with two transmit and two receive antennas over

a rayleigh fading with additive white Gaussian noise(AWGN) channel. On x-axis is the signal to noise ratio andon y-axis is bit error rate. It is noticeable that the bit error ratereduces as the signal to noise ratio increases

- ---

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5 IDa SNR i5Figure 4: BER plot of coded vertical layered MIMO OFDM system

C- Comparison of coded and uncoded vertical LayeredMIMO OFDM system

Figure 5 shows comparison of coded and uncoded verticallayered MIMO OFDM systems, having 32 sub-carriers, withthree transmit and three receive antennas over a Rayleighfading with additive white Gaussian noise (AWGN) channel.On x-axis is signal to noise ratio and on y-axis is bit errorrate. It can be seen from Figure 5, BER of coded system isless than that of uncoded system at a given SNR value. Thismeans performance of coded vertical layer MIMO OFDMsystem in terms of BER at given value of SNR is better thanthat of uncoded vertical layer MIMO OFDM system. Codedsystem achieves BER of 10-02 at SNR value between 20dBand 25dB, whereas uncoded vertical layered MIMO OFDMsystem achieves same BER at SNR value between 25dB and30dB.

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Figure 5: BER comparison of coded and uncoded vertical layeredstructure MIMO OFDM system.

IV HORIZONTAL LAYERED STRUCTUREIn HLST architecture shown in figure 6 the information

sequence is splitted into sub stream. Each sub stream isencoded independently by a channel encoder, interleaved,modulated and then transmitted via particular antenna. It isassumed that channel encoders for various layers are identical.However, different coding in each sub stream can be used.

ISBN 978-89-5519-131-8 93560

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Figure 6: Block diagram of horizontal layered MIMO OFDM system

A -ImplementationFigure 7 shows block diagram of horizontal coded layered

structure MIMO OFDM system according to whichsimulation is done. . Data is demultiplexed and then eachlayer is independently coded and modulated. After takingIFFT and inserting cyclic prefix, data in each layer isconverted from serial to parallel and transmitted throughantenna. After passing through Rayleigh channel and AWGNchannel, data goes to receiver. Same procedure is done atreceiver but in reverse order

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Figure 7: Block diagram of horizontal coded layered structure MIMOOFDM system

B-Simulation resultsSimulation result in Figure 8 shows bit error rate (BER) of

horizontal uncoded LST architecture OFDM system having32 sub-carriers, with two transmit and two receive antennasover a rayleigh fading with additive white Gaussian noise(AWGN) channel. On x-axis is signal to noise ratio and on y-axis is bit error rate. It is conspicuous that bit error ratereduces as the signal to noise ratio increases.

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Figure 8: BER of uncoded horizontal layered structure MIMO OFDMsystem

Simulation result of Figure 9 shows bit error rate (BER) ofhorizontal coded LST architecture OFDM system having 32sub-carriers, with three transmit and three receive antennasover a rayleigh fading with additive white Gaussian noise(AWGN) channel. On the x-axis is signal to noise ratio andon y-axis is bit error rate. It is noticeable that bit error ratereduces as the signal to noise ratio increases. It can analyzefrom the Figure 8 and 9 that coded system performs betterthan uncoded system. Uncoded horizontal layered structureMIMO OFDM achieves BER below 10-0°4 at SNR value 40dB.Coded horizontal layered structure MIMO OFDM systemachieves same BER at SNR value between 20dB to 25dB.

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Figure 9: BER of coded horizontal layered structure MIMO OFDMsystem

V CONCLUSIONIn this paper, a technique for layered MIMO OFDM is

proposed with low complexity that uses the RLS algorithmfor channel estimation. Turbo codes are used in coded layeredarchitecture which gives best complexity/performance tradeoff. The BER of different layered architecture is analyzedand the results of coded and uncoded layered space timearchitecture are compared. On basis of the simulated results,it was concluded that coded layered structure has low biterror rate as compared to uncoded architecture at a givenSNR value.

REFERENCES[I]Lim Wei Chee,B Kannan, Francois Chin, "MIMO Capacity Performancefor Both Narrowband and Wideband Systems", 8th international conferenceon communication system,vol. 1,pp. 426-430, Nov.2002.

[2] A.J. Paulraj, D.A. Gore, R.U. Nabar, H. Bolcskei, "An overview ofMIMO communications - a key to gigabit wireless" Proceedings of the IEEEinternational conference on communication ,vol. 92, pp. 198 - 218, Feb2004.

[3] Haideh M. Karkhanechi & Bemard C. Levy, "Spatial Multiplexing andDiversity Gain in OFDM-Based MIMO Systems", IEEE topical conferenceon wireless communication technology, pp. 299-30 1, Oct. 2003.

[4] Zhipeng Liu, Jeremy S. Parks, Scott A. Morrison and Karl S. Gugel,"Implementation and Evaluation of an OFDM-Based MIMO System", IEEEconference on signal, systems and computers, vol. 1, pp. 545-548, 2004.

[5] V. Tarokh, N. Seshadri, and A. R. Calderbank, "Space-time codes forhigh data rate wireless communication: Performance criterion and codeconstruction," IEEE transaction on information theory, vol. 44, pp. 774-765,Mar. 1998.

ISBN 978-89-5519-131-8 93560

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[6] Alamouti SM, Tarokh V, Poon P, "Trellis-coded modulation and transmitdiversity: design criteria and performance evaluation"., IEEE internationalconference on universal personal communication, vol. 1, pp. 703-707, Oct.1998

[8] G. J. Foschini, "Layered Space-Time Architecture for WirelessCommunication in a Fading Environment When Using Multiple Antennas",Bell Labs Technical Journal, Vol. 1, pp 41-59,1996.

[7] Tarokh V, Jafarkhani H, Calderbank AR, "Space-time block coding forwireless communications: performance results", IEEE journal on selectedareas in communications, vol. 17, pp.451-60, Mar. 1999.

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