Joint diversity combining technique and adaptive modulation in wireless communications
Wireless communications has become a major economic sector with an unprecedented growth rate over the past decade. This phenomenal growth rate has increased even faster in the 21st century due to the success of wireless cellular systems and wireless local area networks. Furthermore, a variety of applications for high quality media content running on mobile devices have also fueled this phenomenon. In order to maintain the rapid growth rate and satisfy such a high demand from users, the next generation communication systems must achieve both reliability and high data rate using a limited spectrum, power, and complexity budget. Unfortunately, a harsh and unpredictable wireless radio propagation environment, with issues such as multipath, shadowing effects, and frequency selectivity, makes this goal very challenging. There are several techniques in wireless communication systems to combat, or even exploit, such a detrimental effect of fading channels. The most popular technique is the diversity combining technique, where multiple replicas of the same signal are used to reduce the amount of fading. By coherently combining these multiple copies of the transmitted signal, this technique provides reliability of the communication link and offers a higher dynamic range. Among other techniques is adaptive modulation, which attempts not to mitigate the fading effect but to take advantage of it by adaptively adjusting the modulation constellation to the instantaneous channel quality. Thus, this technique aims at achieving a high spectral efficiency given a certain level of bit error rate (BER). This dissertation examines diversity combining techniques and adaptive modulation with an emphasis on how these two different techniques can jointly operate in various wireless systems to achieve both reliability and high spectral efficiency. After a brief introduction to the conventional diversity combining schemes, the adaptive diversity combining schemes are first discussed including a performance analyis. Embedded with a target signal-to-noise ratio (SNR), which may be pre-determined based on the quality of service (QoS) for an application, the adaptive diversity combining schemes achieve a reduced complexity while satisfying the target performance. Second, a joint diversity combining and adaptive modulation technique in multi-carrier systems is proposed and presented with analytical results. Since the fourth generation (4G) wireless cellular system standards adopt orthogonal frequency division multiplexing (OFDM) as a basic transmission technology, techniques to improve the performance at the cell edge in such multi-carrier systems are becoming very important. Exploiting a diversity combining technique, the proposed scheme offers an improved spectral effi- ciency in the low SNR region. Finally, a simple and practical system based on a switched diversity scheme with adaptive modulation is presented. This system provides a reduced number of channel estimation while satisfying the optimum spectral efficiency compared to a selection diversity system. In addition, the switching threshold is easily manipulated so as to make an efficient use of the trade-off between spectral efficiency and the number of channel estimation. An extension of this scheme into a multiuser scenario is considered. This switch-based multiuser access scheme results in an average feedback load that is lower than using the optimal selection-based multiuser scheme. Numerical results show we can obtain a trade-off between spectral efficiency and the feedback load by choosing the switching threshold appropriately.