Antenna diversity gain in the wireless local area network standards Hiperlan/2 and IEEE 802.11a

The purpose of this thesis is to investigate the benefits present when antenna diversity is used in the two Wireless Local Area Network (WLAN) standards Hiperlan/2 and IEEE 802.11a. The standards represent the next generation of high-speed extensions to a wired network by providing bit rates up to 54 Mbps under optimal conditions. Typical application areas may be airports, offices and exhibition halls. Data transmission is done in the license free 5 GHz band, and the transmission technique used is Orthogonal Frequency Division Multiplexing (OFDM). This Multi Carrier Modulation technique was chosen due to its satisfactory performance in highly dispersive environments. The diversity technique investigated is called Antenna Diversity, which means that the receiver is equipped with multiple antennas. If the antennas are spaced apart at a properly chosen distance (or if different polarisation directions are used), almost uncorrelated versions of the signal can be obtained. These uncorrelated “branches” are combined in order to get a stronger signal. The combining methods used are versions of Switch Diversity, Selection Diversity and Maximum Ratio Combining (MRC). The diversity gain is evaluated for various data rates that are common in the two standards. The number of receiver antennas is varied between one and eight. The physical layers defined in Hiperlan/2 and IEEE 802.11a are almost identical. Only minor differences occur, resulting in variations in the data rates provided. A physical layer software simulator was implemented to facilitate data rate simulations common to the two standards. The simulator takes random data bits (which represent information sent from the MAC layer), applies the physical layer functions and performs a base band transmission over a time dispersive Rayleigh fading channel. A receiver, equipped with multiple antennas, was also implemented. The output of the receiver is an estimate of the transmitted bits after diversity algorithms have been used. The results of the simulations demonstrate that the coverage can be significantly improved with the methods tested. Due to its low implementation costs, Switch Diversity is the technique most likely used in a real system. When this method is used with eight antennas, a gain of up to 2 dB can be achieved (this corresponds to an improvement of 20% of the system range). This seems to be a small improvement, but the fact that the switch algorithm only chooses between antennas, not sub carriers, has to be considered. For Selection Diversity, the gain lies between 7-9 dB when eight antennas are used (which means that the system range is improved with about 50%). The detection is done on sub carrier level rather than on antenna level, and hence frequency selective dips can be avoided. This is also the case in MRC, but this method performs even better. The measured gain for eight antennas is 12-16 dB depending on the data rate chosen. This means that the system range is improved with up to 100%.Validerat; 20101217 (root