Abstract
Abstract In modern broadband wireless access systems such as mobile worldwide interoperability for microwave access (WiMAX) and others, repetition coding is recommended for the lowest modulation level, in addition to the mandatory concatenated Reed-Solomon and convolutional code data coding, to protect vital control information from deep fades. This paper considers repetition coding as a time-diversity technique using maximum ratio combining (MRC) and proposes techniques to define and to calculate the repetition coding gain G r and its effect on bit error rate (BER) under the two fading conditions: correlated lognormal shadowing and composite Rayleigh-lognormal fading also known as Suzuki fading. A variable-rate, variable-power 10-state finite-state Markov channel (FSMC) model is proposed for the implementation of the adaptive modulation and coding (AMC) scheme in mobile WiMAX to maximize its spectral efficiency under constant power constraints in the two fading mechanisms. Apart from the proposed FSMC model, the paper also presents two other significant contributions: one is an innovative technique for accurate matching of moment generating functions, necessary for the estimation of the probability density function of the combiner's output signal-to-noise ratio, and the other is efficient and fast expressions using Gauss-Hermite quadrature approximation for the calculation of BER of QPSK signal using MRC diversity reception.
Highlights
In modern wireless communication networks such as 3G long-term evolution and Worldwide interoperability for microwave access (WiMAX), modulation and coding are adapted to the fading condition of the channel, typically to the received signal-to-noise ratio (SNR) fed back to the base station by the subscriber station
Since in the Adaptive modulation and coding (AMC) scheme in mobile WiMAX, and repetition coding of 6, 4, and 2 times is recommended only for rate 1⁄2 Quadrature phase-shift keying (QPSK) modulation and coding, it is important that we first derive accurate closed-form formulas for bit error rate (BER) of QPSK signals from an maximum ratio combining (MRC) combiner and the corresponding RC gain when the wireless system operates in lognormal shadowing and in composite Rayleigh-lognormal fading environments
By partitioning the range of the received SNR into a finite number of intervals to match the discrete set of modulation and coding, a finite-state Markov channel (FSMC) model can be constructed for the implementation of the AMC scheme in fading wireless channels
Summary
In modern wireless communication networks such as 3G long-term evolution and WiMAX, modulation and coding are adapted to the fading condition of the channel, typically to the received signal-to-noise ratio (SNR) fed back to the base station by the subscriber station. One of the significant findings in this paper is that the channel fading correlation, while significantly degrading the BER performance, practically does not affect the proposed variable power control algorithm and its resulting 10-state FSMC model for mobile WiMAX. This is because repetition coding is applied only to the first three states, but the total power in these states is too small to affect the overall variable power control scheme. The third is the definition of the repetition coding gain Gr and its incorporation into the design of the transmit power control policy of a 10-state FSMC model for the AMC scheme in mobile WiMAX using repetition coding for QPSK signal. The time separation, the correlation coefficient between any two diversity subchannels, depends on the size of the FEC-encoded data blocks to be repeated as well as the speed of the mobile receiver
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