Novel Approach: Codec Design for WiMax System Kishor Kulat
Rajeshree Raut Assistant Professor, Electronics & Communication Department, Ramdeobaba Kamla Nehru Engg. College, Nagpur . e-mail:
[email protected] Abstract - Wireless communication is the fastest growing segment of the communication industry. New services are being added and data is provided at higher bit rates to the end users. With these advancements any communication system has to critically consider data integrity. This requires, maintaining a lower bit error rate. Present work focuses on the Broadcast Wireless Access standard named WiMax (Worldwide Interoperability for Microwave Access). Possible options for maintaining a lower bit error rate in WiMax System are worked out. In particular a Novel Approach which uses a concatenation of RS (ReedSolomon) and Turbo Codes for the Codec design in The WiMax Communication System is presented. The paper also discusses use of OQPSK Modulation Technique in place of the conventional QPSK system, for performance improvement. The comparative simulation results of existing WiMax System and the system using the novel approach are also provided. These results are used to draw useful conclusions for reducing the bit error rate in the WiMax System. Key Words- Bit error rate, WiMax System, RS codes, Turbo codes,
OQPSK modulation technique. 1.
INTRODUCTION
WiFi (Wireless Fidelity) is one of the most popular wireless communication standards in the market. WiFi technology was almost solely used to wirelessly connect laptop computers to the internet via local area networks. However this technology has certain limitations. Security and interference are the main issues with current WiFi standards, as well as its inability to reliably stream high definition audio and video. The problem with WiFi access is that hot spots are very small, so coverage is sparse. To overcome these problems, we need a new technology that would provide: 1. The high speed of broadband service. 2. Wireless rather than wired access, so it would be a lot less expensive than cable or DSL and much easier to extend to suburban and rural areas. 3. Broad coverage like the cell phone network instead of small WiFi hotspots. This system is actually coming into being right now, and it is called as WiMax. WiMax is short form for Worldwide Interoperability for Microwave Access, and it also goes by the name IEEE 802.16. WiMax has the potential to provide broadband Internet access what cell phones have added to phone access. The result is that many people have given up their "land lines". Similarly WiMax is expected to replace
Professor, Electronics & Computer Science Department, VNIT, Nagpur. e-mail:
[email protected]
cable and DSL services, providing universal Internet access just about anywhere you go. WiMax will also be as painless as WiFi -- turning your computer on will automatically connect you to the closest available WiMax antenna [1], [2]. The IEEE 802.16 i.e. the WiMax System has the followin2 Specifications Range - 30-mile (50-km) radius from base station Speed - 70 megabits per second Line-of-sight- not needed between user and base station Frequency bands - 2 to 11 GHz and 10 to 66 GHz (licensed unlicensed bands. The following table [1] gives the comparison of WiMax and WiFi. TABLE 1
COMPARISION OF WiMax WITH WiFi WiMax
WiFi
Standard
802.16
802.11
RanJ!e Speed
30 mile radius 70Mbps 1.75 -20 MHz 2- 11 & 10- 66GHz
4-6 mile radius 2-54 Mbps At least 20 MHz 2.4-5 GHz
MAN
LAN
TDMlFDM Vehicular
TDM Pedestrian
Channel Size Freg Bands Networking emoloved MultiDlexin~
Mobilitv
This work presents optimization approach for the CODEC design in a WiMax Transceiver system. Next to follow is: • The WiMax transceiver system • Novel approach of using Turbo in concatenation with RS in CODEC design of WiMax system. • OQPSK Modulation technique proposed to be used in WiMax system. • Implementation of the four models viz.1. RS in concatenation with convolution codes (present system) 2. RS in concatenation with Turbo codes 3. CODEC employing only Turbo Codes 4. The OQPSK Model • The comparative simulation results of the bit error rate for each of the above system and finally the conclusion.
Proceedings ofthe 2008 International Conference on Computing, Communication and Networking (ICCCN 2008) 978-1-4244-3595-1/08/$25.00 co 2008 IEEE
II. TRANSCEIVER SYSTEM IN WIMAX The receiver and antenna could be a small box or they could be built into a laptop the way WiFi access is today. A single WiMax tower can provide coverage to a very large area - as big as 3,000 square miles (--8,000 square km).
Data Source
Convolution Encoder
Data Randomization
Interleaver
dimensional array buffer, such that the data enters the buffer in rows, which specify the number of interleaving levels, and then, it is read out in columns [5]. The result is that a burst of errors in the channel after interleaving becomes in few scarcely spaced single symbol errors, which are more easily correctable. This data is superimposed on high frequency carrier in the modulator.
QPSK Modulator
QPSK
Phase shift keying is a modulation process whereby the input signal, a binary PCM waveform, shifts the output waveform to one of a fixed number of states. The general analytic expression for PSK is
si(t)=(2E/T)/\l/2*cos[OJot+qJi(t)}
o< t < T , i=I, ...,M ,Where the phase term ~":L D :11IIII ~
..
Fig4.l Process of shortening and puncturing ofthe RS code.
~ Random Integer
Fig 4.3 MATLAB Simulation of Convolution Encoder
The generator polynomials for this encoder are gO = 1710ct and gl =133oct. The encoder can easily be implemented in hardware shift registers. The first step is to represent the input bit string as a polynomial. Any sequence ofO's and 1's can be represented as a binary number or a polynomial. The convolutional encoder for WiMax (gO = 1710ct and gl = 133oct) can be represented as follows:
• gO = J + n + n + n + n 2 3 6 • gJ = J + n + n + II + n 2
3
6
The convolutional encoder basically multiplies the generator polynomials by the input bit string, as follows: • A(x) = gO(x) * I(x) = abc g
• B(x) = gJ (x)
* I(x) = P Q R
V
Interleaving the two outputs from the convolutional encoder yields E(x) = aPbQcR ... gV, which can also be written as:
E(x) = (aO bO cO ... gO) + (OPOQOR ... OV) = A (x2) + x*B(x2) Therefore, E(x) = A(x2) + x * B(x2) and A(x2) = gO(x2) * I(x2) and B(x2) = gJ (x2) * I(x2), with the following: E(x) = gO(x2) * [(x2) + x * gJ (x2) * [(x2) = I(x2) * (gO(x2) + x * gJ (x2)) = [(x2) * G(x) where G(x) = gO(x2) + x * ¥J (x2) 7 lJ l2 l3 4 5 6 i.e. G(x) = J + x + x + x + x + x + x + x + X + X .
1
I xlI
Logical Operator1
'---_...J
Random·lntege Generator
Tine Veclor
Scope
Fig 4.2 MATLAB Simulation ofRS Encoder-decoder for WiMax
Turbo Encoder It consists of two convolutional encoders. The outputs of the turbo encoder are the information sequence, together with the corresponding parity sequence produced by first encoder and the parity sequence produced by the second encoder block, the input to second encoder is through interleaver, which scrambles the data bit sequence. Simulation model of Turbo encoder -decoder is shown in fig 4.4
v.
1 - -. . . . .---.:
1
Out1
Convolutionoll Encoder1
RESULTS AND CONCLUSION
The Simulation models were prepared for following four systems: 1. WiMax using RS+ Convolutional Codec and OQPSK 2. Existing WiMax system(RS + Convolutional) and QPSK 3. WiMax using RS+ Turbo Codec and QPSK 4. WiMax using Turbo Codec and QPSK Each of them was observed for bit error rate at different values of SIN (signal to noise ratio). Graphs ofS/N versus FER (Frame error rate) were plotted and are as shown in fig 5.1. WMAA l6i~ (RS+CarGutiormO am OQPSK
Existi~ Wi~system
(RS+ CoooUknII) &QPSK
1.2..-----~-___r_-__r---,
0.8 0.8 a:
w
u.
0.6
0.6 a:
0.4
~0.4
0.2 0.2 -0.210
15
20
25
30
0 10
15
20
SNR(lB)
25
~
St\R(lB)->
WMAXUsiYll RS+Turtx> COI:EC
0.58
o.oo~
WMAXusiRJ TurtxlCodngTlK:/ri1tIl
0.56 0.54 ~0.52
~0.65
0.5 0.64
0.48
Fig. 4.4 SIMULINK Model of Turbo Encoder-Decoder
Turbo Decoder Turbo decoder shown above in Fig 4.4 uses iterative decoding. The turbo code decoder is based on a modified Viterbi algorithm that incorporates reliability values to improve decoding performance. The turbo decoder consists of M elementary decoders - one for each encoder in turbo encoding part. Each elementary decoder uses the Soft Decision Viterbi Decoding to produce a soft decision for each received bit. After an iteration of the decoding process, every elementary decoder shares its soft decision output with the other M - 1elementary decoders. Channel The AWGN Channel block adds white Gaussian noise to a real or complex input signal. Each of the major blocks mentioned above have individual sub blocks which are configured to meet the WiMax Specifications (After scaling, keeping in mind the mathematical constraints of modeling a real time system). Fig 4.5 shows the entire WiMax Transceiver System using RS in Concatenation with Turbo codes.
0.63 0
10
15 SNR(lB)
20
25
30
0.460
10
15
~
25
~
SNR (dB)
Fig 5.1 Plots of SIN vs. Frame Error Rate (FER) for different model of WiMax System
Conclusion 1. As seen from fig 5.1 the turbo concatenation with RS gives desired performance of decreasing the bit error rate with increasing SIN. 2. Using only Turbo codes for the Codec design, works perfect as far as simulation is concerned But real time systems will introduce Burst errors. To reduce burst errors RS coding is a must. 3. More over, it was observed that every system requires different signal power for the receiver to interpret the data. The Turbo concatenated system works fine with low power signals as compared to only RS codec system. 4. Also the OQPSK system can respond to much low level signals in comparison with the existing QPSK systems and produce a much lower BER
..... _".":~', fcIIDj,rii .... ,
3i II.I~I., ••,--'---,.----...._,-..._.. -,
'0 r"'l.l~ ~81~-~ l'....-.:::.~-~::-l• • ~i~~
Convoldou' Intlrle. .,
COlWOlulion.' Encode,
Tx
Ax
Conwldoul Inltrle...,
QPSK
Modul.r I_b.nd
OFDM
Subsytttm
AWeN
En. Rite
Ch,nn,'
C,'ouldon
r'1 AWGN
0"'.,
En,RN
C"oulltion
-
VitI.1 De.de,
Bit to Inte", Conveltlr1
Tu.o
Fri.."
VitI.i
Decoder Comllion4
De.de,
Comldo",' Delnterle. .,
QPSK
Demoduillor I_b,nd
OFO" Demod
Su-,m
Fig 4.5 WiMax Transceiver System employing a CODEC using RS+ Turbo
REFERENCES 1.
Amalia Roca, "Implementation of WiMax Simulator in Simulink",
Vienna, Feb 2007 2.
Claudio Sacchi, Olga Zlydareva, "Objeet-Oriented Model fo SDR Library for WiMax/ UMTS System Baseband Level", Technical
Report, University of Trento, Department of Information and Communication Technology, April 2007
3.
Rajeshree Raut & Kishor Kulat, "Optimal Coder-Decoder Design for Mobile Communication System", International Journal IJCSCT
4.
Rajeshree Raut & Kishor Kulat, "Application based Criterion for Selection of Error Control Code", International Conference ICE1T,
Vol-I, pp 20-28
Jan 10-12, 2007
5.
Rajeshree Raut & Kishor Kulat, "Turbo Codes in Mobile Communication", Conference on Recent Trends in Mechatrinics, Nanotechnology & Robotics, April 15-16, 2006, NIIT Rourkela,pp.
6.
Yinhva Wang & et aI," QPSK Modulation and Demodulation", ELE 791 Software Radio Design, Dec 14, 2004.
53.
7. 8. 9.
Christopher Lamont Taylor, "Puncutured Convolutional Coding Scheme for Multi-Carrier Multi-Antenna Wireless Systems", a Research project. Donaggio Piero & et aI, "Implementation of WiMax IEEE 802.16 Receiver". Louay M.A. Jalloul and Sam P. Alex, " Methodology and Performance of IEEE 802.16e System", IEEE Communications & Signal Processing
Societies Orange County Joint Chapter(Com Sig), Dec 7, 2006 10.
Luis Abraham Sanchez and et aI, "IEEE 802.16e design and transceiver th architecture selection for Mobile WiMax systems", 18 International Conference on Electronics, Communication & Computers, 20 Feb2008,
IEEE Computers Society.pp. 41-44
Author Information The author of the paper Prof. Rajeshree D. Raut is working as Assistant Professor in Electronics & Communication Department of Shri Ramdeobaba Kamla Nehru Engineering College, Nagpur. She is pursuing her Ph.D. in Error Control Coding and has published seven papers in related research topics. Amongst this one paper is published in the international Journal IJCSCT.
