Rate adaptive spread spectrum OFDM system for tactical packet radio networks

Wireless communications systems in a tactical environment must be able to function effectively in a channel characterized by multipath fading and interference. Additionally, current and future communications requirements will necessitate the transmission of many different types of information with a wide range of data rates over these wireless links.; This thesis studies network access, modulation and interference mitigation techniques which are appropriate for a reconfigurable, variable Quality of Service (QoS) network in such a multipath and jamming environment. In contrast to the conventional static over-design toward the worst case approach, we propose a rate adaptive modulation scheme based on Orthogonal Frequency Division Multiplexing (OFDM) and Spread Spectrum concepts. The system is called Rate Adaptive OFDM (RA-OFDM). Our adaptive signaling is compared with other conventional adaptive modulation schemes and it is shown to be a simple and universal scheme applicable across large range of received signal quality.; In order to increase the RA-OFDM system dynamic range, adaptive frequency domain equalization techniques are proposed as an augmentation to the RA-OFDM system to provide interference immunity in excess of its maximum processing gain. All the equalizers evaluated are able to suppress the tone interference while combining the energy in the multiple paths. Furthermore, it is shown that the equalizer structures are compatible with the rate adaptive environment.; It is well known that both Forward Error Correction (FEC) coding and spread spectrum can all introduce error protection at the cost of bandwidth expansion. It is therefore relevant to evaluate the trade-off between FEC coding and spreading for a RA-OFDM system given a fixed RF bandwidth requirement. Typical FEC codes such as convolutional, Reed Solomon (RS) and concatenated codes are studied. The trade-off study is also extended into the CDMA environment.; The RA-OFDM scheme is further justified by analyzing the network throughput for a packet radio network, using multiple access schemes such as ALOHA, NP-CSMA, and CDMA. Implementation issues for the adaptive signaling scheme in a packet radio network environment are discussed and potential applications for Low Earth Orbit Satellite (LEOS) applications are presented.; The effectiveness of the RA-OFDM system to provide reliable communications over a broad range of channel conditions is demonstrated and an overall adaptation strategy of reasonable complexity is established.