A COMPARISON OF pi/4-DQPSK MODEMS FOR USE IN A RURAL WIRELESS INTERNET ACCESS SYSTEM
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Currently, rural residents are not well served by existing Internet access options. Their primary option is the standard telephone modem, which does not allow for high speed access. A wireless Internet access solution has advantages over its wired counterpart, since it is cost-effective to deploy the system in sparsely populated areas. Telecommunications Research Laboratories has proposed a multipoint communication system (MCS) as a solution for rural network access. This thesis discusses radio modem solutions that are suitable for such a system. The pi/4-differential quadrature phase shift keying (pi/ 4-DQPSK) modulation scheme is used in the radio link of the system. This type of modulation allows three different types of detection: the baseband differential detector, the IF differential detector, and the frequency discriminator detector. These types of detectors have been assumed to have identical theoretical performance under ideal conditions in the existing literature. Little comparison has been performed that considers the effects of practical implementation issues on modem performance and implementation complexity. Practical impairments present in systems of this type are presented. These impairments include frequency offset, symbol timing offset, DC offset, phase imbalance, sub-optimal receiver filtering, hard-limiting effects, and frequency converter phase noise. Various detector implementations are simulated and the effect of the impairments on bit error rate (BER) performance for the different types of detectors is compared. Most impairments produce similar results in the detectors, but some discrepancies are noted. The detectors lend themselves to digital hardware implementation. Digital hardware realizations and their simulated BER performance are discussed for each detector type. The relative implementation complexity and the trade-offs between performance and complexity are discussed in terms of implementation in Altera Flex10K Complex Programmable Logic Devices (CPLD). The sensitivity of the BER performance to impairments is also discussed. For the proposed system, the baseband differential detector is the best solution since it results in the smallest hardware realization, while also achieving the best BER performance. The IF differential detector is similar in implementation size to the baseband differential detector, but has poorer BER performance. The frequency discriminator detector is not well suited to a digital realization, which results in poor BER performance and a greater implementation complexity. The most hardware intensive component of the detectors is the root raised cosine (RRC) filter. Replacing the RRC filters with analog Butterworth filters simplifies the detector realizations, but introduces BER performance degradation. A transmitter phase compensation technique is presented that improves the BER performance of a modem that uses sub-optimal detector filtering, while significantly reducing the detector complexity.