Channel Related Optimization of Wireless Communication Systems

Abstract: This thesis deals with different optimization problems in the design of wireless communication systems. It is mainly directed to the design of systems based on multicarrier techniques and orthogonal frequency division multiplex, OFDM, but some of the problems apply to single carrier systems as well.

The influence of different pilot patterns is analyzed when pilot symbol assisted modulation, PSAM, is used in OFDM systems. It is desirable to decrease the number of required pilot symbols and it is shown that the pilot pattern used plays a major role to enable reliable channel estimates from a small amount of pilot symbols. Rearrangement of the pilot pattern enables a reduction in the number of needed pilot symbols up to a factor 10, still retaining the same bit error performance.

The effect of the number of sub-channels used in an OFDM system is analyzed with respect to resulting bit error rate. An analytical expression for the bit error rate on Rayleigh fading channels when interchannel interference, ICI, caused by channel changes during a symbol and energy loss due to the cyclic prefix are regarded. This expression is used to optimize the number of sub-channels, and thereby the sub-channel bandwidth (sub-channel spacing) in the system. It is argued that the system can be optimized neglecting the effect of imperfect channel estimation and on a worst case assumption for the Doppler frequency and signal to noise ratio.

The benefits of using pre-compensation (precoding) in wireless time division duplex, TDD, systems are also investigated. The uplink channel estimate is used to compensate the channel impact on the downlink symbols. This enables less complex receiver structures in the mobile terminal since channel equalization is performed in the base station. Three different methods where amplitude and/or phase are adjusted are analyzed in terms of performance limits. Closed-form expressions for the QPSK bit error rate are given assuming a fully known channel. It is shown that pre-compensation is an attractive alternative to differential decoding. Phase-only compensation is preferred at low signal to noise ratios, while at high signal to noise ratios an order of magnitude improvement in the bit error rate can be obtained by including amplitude pre-compensation.

All the analyses and optimizations are general and can be applied to any OFDM system.