reliability of higher mapping techniques over noisy channel with ofdm

Shivani Sharda et al. / International Journal of Engineering Science and Technology (IJEST)

RELIABILITY OF HIGHER MAPPING TECHNIQUES OVER NOISY CHANNEL WITH OFDM FOR WIRELESS SERVICES SHIVANI SHARDA M.Tech student Electronics & Communication Department Punjab Technical University Regional Campus DAVIET, Jalandhar Punjab, India

KIRAN AHUJA Electronics & Communication Department DAV Institute of Engineering and Technology, Jalandhar Punjab, India Abstract: - High error rate and amplitude/phase imbalances can degrade the performance of any OFDM based wireless networks. Also the proper recovery of the input at the receiver end becomes questionable. The effect of such impairments can be compensated by using high order mapping techniques. This paper analyzes the performance of M-QAM and M-PSK (where M=256) using OFDM over a noisy channel, which is AWGN particularly. The parameters are modelled and simulated using MATLAB. The mapping techniques are compared for the parameter Symbol Error Rate. The simulation is done for transreceiving the OFDM signal at lesser Symbol Error Rate. Keywords: - 256-QAM, 256-PSK, OFDM, SER, AWGN, SNR. 1.

Introduction

With the quick growth of many digital wireless communication services in recent years, a high speed mobile data rate transmission with appropriate QoS has become a necessity. Considering this large demand a low cost, a low power and fully integrated implementation of the wireless system becomes a need [1]. The wireless services such as third generation (High Speed Downlink Packet Access), WiMax (802.16), and FiOS (ATM passive optical network) has an ever growing demand for advance data rates and lesser error rates. These services often use digital baseband mapping techniques. Like a simple 64-PSK is being used in third generation mobile communication systems. Though the data rate it achieves is only acceptable. The high order mapping techniques (M-ary modulations, M=128 or 256, QAM, CPM) can be suggested for high speed communication systems like WiMax [2] and HSDPA to achieve the high transmission rate and less error rates. However the large M place stringent requirements like signal to noise ratio, and phase noise. It is thus desirable to design a model which gives optimum system performance considering phase and amplitude imbalances. In addition the decreased error rate is to be achieved. An M-ary quadrature amplitude modulation (QAM) has received interest for many communications such as IEEE 802.16 because with the use of a multi-carrier modulation i.e OFDM (orthogonal frequency division multiplexing) improve its high spectral efficiency which is recommended for immunity of noise [3]. So, the higher order (i.e. M=256) mappings can be used to get an improved symbol error rate, though with a little adjustment in the signal to noise ratio. So it is required to analyse this factor to improve spectral efficiency of wireless system for improving their quality of services. The paper is organized as follows. Next section describes some of the previous work done. The details of the proposed simulation of mapping techniques with orthogonal multiplexing using MATLab simulation tool are shown in the Section III. It gives details of the actual simulation. Also the methodology and the parameters are discussed in section III. In section IV are the scatter plots showing constellations of transmitted and received signals using PSK & QAM, observations of the comparison between symbol error rates and waveforms showing symbol error rate. Section V concludes the work done and the observations which followed.

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2. Related Work A simulation program [4] was developed for the Reed-Solomon codes with an error correction capability using simulation with quadrature amplitude modulation scheme in AWGN channel. The symbol error rate performance was evaluated and compared with its theoretical calculations. They gave [5-6] details of implementation and observations for 16-QAM and 4-QAM system. The model was implemented in simulink. Error bit rates as a function of SNR were studied using Monte Carlo estimation techniques. The implementation of orthogonal frequency division multiplexing was described in [7]. They studied and analysed the effect of inphase and quadrature-phase (IQ) imbalances on the achievable operating signal-to-noise ratio (SNR) at the receiver and consequently, the supported constellation sizes and data rates. The algorithms included fast Fourier transform. In [8] the expression for Closed- form outage probability and symbol/bit error rate with M-ray phaseshift keying (M-PSK) and M-ray quadrature amplitude modulation (M-QAM) signalling was derived. They analysed the effect on the performance of multiple input multiple output communication systems by driving approximate error rate expressions, however at high signal-to-noise ratios (SNRs) with M=64. So in this paper we propose a reliable and efficient design for OFDM communication using QAM and PSK with M=256 while considering SNR, SER, symbol period parameters as analysing factors with channel specifications (power) as well to see the impact of high order mapping schemes. 3.

The proposed Simulation Model The proposed model given here includes all the stages of transmitter and receiver. The block diagram shown in figure 1 models the simulation of mapping techniques like PSK, QAM and CPM [9] using orthogonal multiplexing. The random integer generator block generates uniformly distributed random integers in the Range [0, M-1], where M is the number of points in the constellation of mapping scheme. M-ary number can be either a scalar or vector. If it is scalar, the all the output variable are independent and identically distributed.

Random integer generator

Scatter Plot (constellation diagram) Complex to real

Work space

Integer to bit conv

PSD/Scope

Modulation (PSK,QAM)

Error Details

Fast Fourier Transform

Error rate calculation

AWGN channel

Scatter Plot (constellation diagram)

Phase Noise

Bit to integer conv

De-Mod (PSK, QAM)

Fig.1 Simulation model of M-PSK & M-QAM modulator/demodulator transceiver (M=256)

If the M-ary number is vector, then its length must equal the length of the initial seed. In this case the initial seed parameter is a constant and each output has its own range, so that the repeatable resultant noise can be introduced. The real or imaginary input serial data (real in this case) from the random generator is transmitted as a single symbol. Then it is converted into parallel form and is mapped to the phase angle or position based on mapping technique. After that each symbol is assigned to an orthogonal carrier while converting into respective time format using FFT [10] for transmission. A noisy channel is then introduced to transmitted signal which adds AWGN noise to the signal. The received signal is then demodulated to get back the original data. As the same frame generated integers that are simultaneously modulated using different modulations i.e 256PSK & 256-QAM exposed to the AWGN channel, a good approximation to the real performance can be observed over all the degradations applied to the input signal. The parameters like signal to noise ratio are changed but the power of the channel remains constant throughout the process to achieve consistent symbol error rate. The effects which a noisy channel induces are observed in the output constellations of mapping techniques. 4. Results and Discussion In this section the results for Symbol Error Rate SER and signal to noise ratio Eb/No are provided. To acquire the symbol error rate the parameter Eb/No is varied from 0.5 dB to 30 dB for both mapping techniques. The comparison between SER and Eb/No is shown on linear and semi-log scale respectively. For both the mapping

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Shivani Sharda et al. / International Journal of Engineering Science and Technology (IJEST)

techniques the symbols are transmitted repeatedly over a noisy channel i.e. AWGN. The error per symbol was calculated by comparing symbols at the receiver with the symbols generated at the transmitter. The figures 2 & 3 show the constellations of both mappings in the scatter plots at transmitter as well as receiver. The received scatter plots conceive the influence of AWGN channel.

(a)

(b)

Fig.2 (a) & (b) shows the transmitted and received constellations of 256-PSK respectively

(a)

(b)

Fig.3 (a) & (b) shows the transmitted and received constellations 256-QAM respectively PSK(+) & QAM(*) SER with AWGN Power I/P=1

0

10

-1

10

-2

SER

10

-3

10

-4

10

-5

10

-10

-5

0

5

10 15 Eb/No

20

25

30

35

Fig.4 Symbol error rate of PSK and QAM scheme with AWGN channel

Figure 4 represents the symbol error rate performance of mapping techniques to signal to noise ratio on a semi log scale. It shows a drop of 5 dB in the SER of 256-QAM which is more as compared to the previous constellations [11].

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Shivani Sharda et al. / International Journal of Engineering Science and Technology (IJEST)

Table 1: Observed Symbol Error Rate of M-ary PSK & QAM mappings for M=256

Eb/No (dB)

SER of 256QAM

SER of 256PSK

-8 0 6 12 18 24 30

0.95668 0.83154 0.38403 0.10438 0.00043999 0 0

0.95952 0.89962 0.7502 0.61769 0.32313 0.04938 0.00012

In table 1 result are given for the observation of both mappings schemes. As can be seen the symbol error rate is nearly equal at a minimal Eb/No for both the schemes. But the performance differs when high Eb/No values get involved. Table 2: Comparison of SER of high order (M=256) with low order (M=64) signaling.

Eb/No (dB)

64-QAM

256-QAM

64-PSK

-8

0.9260

0.9566

0.92142

0

0.777

0.899

0.79996

12

0.0727

0.1043

0.32235

24

0.0002

0.00043999

0.00014

256-PSK 0.95952 0.89962 0.61769 0.049379

There is performance comparison of high order mappings with previous work (low order i.e. M=64) in table 2. The selective values given in the table suggest improved symbol error rate while using high order signaling as compare to lower order. The results emphasize on the use of high order signaling.

5. Conclusion In this paper the symbol error rate of 256-PSK and 256-QAM is calculated at small interval variation of signal to noise ratio. The SER is compared on same logarithmic scale for both mappings. The main results show a good agreement between signal to noise ratio and an improved Symbol error rate. Because of distant massage points in case of 256-QAM, it performs better than 256-PSK. The simulation results clearly show that which mapping technique can be implemented for low SER at a reasonable signal to noise ratio.

6. References [1]

Chen, ZhenQi; and Dai, Fa Foster; (2010) “Effects of LO Phase and Amplitude Imbalances and Phase Transceiver Performance” IEEE Transactions on Industrial Electronics, vol. 57, NO. 5.

[2]

Prince, K.; Osadchiy, A.V.; Monroy, I.T.; (2009) “Full-duplex transmission of M-QAM WiMAX signals over an 80-km long-reach PON” Annual Meeting Conference Proceedings.

[3]

Al-Naffouri, T. Y.; and Mukaddam. A; (2007) “Frequency domain estimation of multiple access OFDM channels,” submitted to IEEE International Conference on Signal Processing and Communication. Ibrahim, Muhammad Bin; Ahmed, Norizan Binti; (2009) “SER Performance of Reed-Solomon Codes with QAM Modulation Scheme in AWGN Channel” 5th International Colloquium on Signal Processing & Its Applications (CSPA). Kisiel, K.; Sahota, D.; Swaminathan, G; (2005) “Quadrature Amplitude Modulation: A simulation study of a 16-QAM and 4-QAM system”Simon Fraser University, Canada. Tan, Shuang; Chen, Sheng; and Hanzo, Lajos; (2008) “Iterative multiuser minimum symbol error rate beamforming aided QAM receiver,” IEEE Signal Processing letters, vol. 15, pp 301-304. Tarighat, A; Bagheri, R; and Sayed, A.H.; (2005) “Compensation schemes and performance analysis of IQ imbalances in OFDM eceivers,” IEEE Transactions on Signal Processing, vol. 53, no. 8, pp. 3257-3268.

[4] [5] [6] [7]

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[8]

Zhao, Hongzhi; Gong, Yi; and Guan, Yong Liang; (2009) “Performance Analysis ofM-PSK/M-QAM Modulated Orthogonal Space– Time Block Codes in Keyhole Channels”, IEEE transactions on vehicular technology, vol. 58, NO. 2. [9] T. Rappaport, (1996) “Wireless communication, Principal & practic,” IEEE Press, New York, Prentice Hall, pp 169-17. [10] Mutyala Rao T., Govinda; Acharya, Bibhudendra; and Patra, Sarat Kumar; (2008) “Effects of fixed point FFT on the performance of OFDM in wireless LAN,” 1st International Conference on Advances in Computing, Chikhli, India. [11] Ansari, Ejaz A.; and Rajatheva, Nandana; (2009) “Exact SER Analysis of OSTBC MIMO-OFDM Systems over Uncorrelated Nakagami-m Fading Channels” IEEE AIT, Thailand

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