Performance Evaluation of WiMAX (802.16) Using Different Encoding Schemes

WiMAX stands for Worldwide Interoperability for Microwave Access. WiMAX is based on wireless Metropolitan Area Network (MAN) standards developed by IEEE 802.16 group. It operates in the licensed exempt and licensed spectrum between 2-11 GHz and 10-66 GHz frequency ranges respectively [1]. It was developed to connect the Internet and to provide a last mile wireless extension to cable and DSL broadband access. IEEE 802.16 provides up to 50 km of linear service area range and allows users connectivity without a direct Line of Sight (LOS) to a Base Station (BS). The technology also provides shared data rates up to 70 Mbps, which is enough bandwidth to simultaneously support many users.

In Space Time Block Codes (STBC), data stream is encoded in blocks and distributed among time and spaced antennas. At the transmitter side this encoded data is transmitted along multiple antennas whereas, at the receiver side mu ltiple antennas receive mu ltip le copies of the same signal and then extract the best possible out of it. It provides significant increase in throughput and range without any increase in the overall bandwidth and transmits power expenditure. It also increases the spectral efficiency (number of informat ion bits per hertz of bandwidth) of wireless system by using multiple antennas that are separated in space and time [2].

Turbo codes are the best approximation of the Shannon limit . In Turbo codes at the transmitter side a single bit is encoded into a combinat ion of bits depending on the architecture of the encoder. When this encoded data is transmitted over the channel the probability of error is reduced to a great extent. When the data reaches the receiver side the data is decoded back into the original bits that are understandable by the receiver.

A number of industry standards govern the design and performance of wireless broadband equipment. The standards that chiefly concern wireless broadband are 802.16 and its derivative 802.16a, both of wh ich were developed by the Institute of Electrical and Electronic Engineers (IEEE), a major industry standards body headquartered in the United States.

The IEEE 802.16 standard supports mult iple physical specifications due to its modular nature. The f irst version of the standard only supported single carrier modulation and after the passage of time and as technology grew; OFDM and scalable OFDMA were also used but only for operating in the Non Line of Sight (NLOS) environment and were meant to provide mobility. The standards were then further enhanced to work in the lower frequency range of 2-11GHz along with the previous 10-66GHz band.

The idea of OFDM co mes fro m Multi Carrier Modulation (M CM) transmission technique. MCM is based on the division of input bit stream into several parallel bit streams and then using them to modulate several sub carriers as shown in Fig 1 . Guard band is introduced in between each subcarrier so that they do not overlap with each other. This guard band also supports the band pass filter on the receiver side in identifying individual subcarriers. OFDM is a special form of spectrally efficient MCM technique. It differs fro m its predecessors in the way that it uses orthogonal subcarriers which also eliminate the use of a band pass filter fro m the receiver side. The orthogonal nature of the subcarriers also removes the Inter Carrier Interference (ICI) wh ich was a great problem p reviously. The guard band previously used is also removed in OFDM hence concluding in the reduction of bandwidth us age as shown in the Fig. 2. http://sites.google.com/site/ijcsis/ ISSN 1947-5500 2) Adaptive Modulation and Coding: AMC allows WiMAX systems to select the mos t appropriate Modulation and Coding Scheme (M CS) depending on the propagation conditions of the communication channel, e.g., if the propagation conditions are good, a higher order modulation scheme with a lo wer coding redundancy is used which also increase the data rate of the transmission, while on the other hand during a signal fade, the system selects a modulation scheme of a lower order to maintain both the connection quality and the lin k stability without an increase in the signal power [3]. For this purpose WiMAX uses four modulation schemes that are:

Binary Phase Shift Keying (BPSK) Quadrature Phase Shift Keying (QPSK) 16-Quadrature Amp litude Modulation (16-QAM) 64-Quadrature Amp litude Modulation (64-QAM)

The first step on the physical layer of WiMAX is the decision of the modulation scheme that has to be used as shown in Fig 3 . Init ially the transmitter transmits the bits using 16-QAM modulation. The modulated data is then mapped on to orthogonal channels using IFFT. In this mapped data, to avoid inter-carrier interference a guard band (of 1/8 o f the total number of bits) is added. The data is then transmitted over the channel and at the receiver end the guard band removed and the data decoded back into the original form. The receiver calcu lates the error in th

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