Multiple access techniques for broadband fixed wireless applications

OFDM is proposed as the basic modulation scheme for broadband fixed wireless networks, in order to take advantage of its robustness against multipath delay spread. Code division multiaccess (CDMA) is applied on top of multicarrier modulation to provide a simple and efficient multiple access method. Smart antenna is used to greatly increase the system capacity and/or to solve the inherent problems in a dynamic up/down boundary time division duplex (TDD) system.; The effects of nonlinear amplification in the proposed OFDM-CDMA systems caused by a high-power amplifier are investigated. An adaptive pre-distortion linearization technique based on look-up tables is implemented to reduce the nonlinear distortion of multicarrier signals.; The effects of time-selective multipath fading and carrier frequency offset on multicarrier systems are identified. A novel all-digital frequency offset correction scheme is proposed for OFDM wireless systems, based on an adaptive cyclic least mean square (AC-LMS) algorithm.; OFDM-CDMA is proposed as a multiaccess scheme for the fixed broadband wireless system. As an example, this multiaccess scheme is investigated in local multipoint distribution services (LMDS). A cellular architecture is used to provide service coverage. Cross polarization reuse and directive subscriber antennas are deployed to reduce the interference and increase the link power margin. Orthogonal-CDMA is used in both uplink and downlink to eliminate the intra-cell multiaccess interference (MAI). Two types of base antennas are investigated, one is an ideal four-sectored antenna, the other one is a smart antenna. The system performance is obtained through a simplified interference model and through Monte Carlo simulations. In general, the proposed multiaccess system can provide much larger capacity than the traditional type TDMA/FDMA systems, and it is much more robust against multipath fading. Using a 16-element smart antenna at the base for reception only, simulation results show that the same code word channel can be reused as many as 8 times in each cell. This means that it can provide an 8-fold capacity improvement.; After a vector channel model for multicarrier smart antenna systems is developed, it is discovered that multipath delay spread is the main reason causing the variation of the spatial signature of a smart antenna with frequency.; In OFDM systems, when the number of sub-carrier is large enough, each individual channel will only have frequency-flat fading. In a multicarrier smart antenna system, there are two ways to perform the beamforming. One way is to perform this in time-domain, which is very simple but there is performance degradation, especially in a frequency selective fading environment. The other one is to accomplish the work in frequency-domain, subchannel-by-subchannel, which is the better solution. However, the implementation complexity of the latter system is quite high. To make a trade-off between the system complexity and performance, a group-beamforming algorithm is proposed, which is capable of maintaining an optimum performance with a decreased implementation complexity.; Compared to a frequency division duplex system (FDD), a time division duplex system (TDD) can provide many benefits, such as more efficient usage of available bandwidth, low implementation costs, etc. The potential problems in a dynamic boundary TDD system are identified. To achieve one of the most significant benefits promised by TDD over FDD, which is the dynamic uplink/downlink traffic handling capability, smart antenna technique is absolutely the best choice to suppress the uplink base-to-base interference. Extensive computer simulations have verified this point.