Adaptive interference suppression in wireless systems

Interference in wireless networks can be caused by other users in the same network, from other networks, by intentional jammers and spoofers, or other devices operating in that environment. Interference cancellation algorithms use single or multiple antenna elements. Multiple antennas increase the range of the device and also reduce power consumption compared to a single antenna, because the antenna can be focused in a desired direction.; The multistage constant modulus (CM) array proposed earlier to capture multiple received signals consists of cascade of CM array combined with an adaptive signal canceler to remove the signal captured by the CM array from each stage. When the received sources are correlated, a source captured by the cascade CM array is not completely canceled, and versions of the parallel CM array proposed earlier do not guarantee that different sources will be captured across the stages. In this paper, we present a hybrid implementation of the multistage CM array for separating correlated signals. The canceler weights in the cascade structure provide estimates of the direction vectors of the captured signals across the stages, which are used in a parallel implementation of the linearly constrained CM (LCCM) array leading to a hybrid structure. Since the direction vectors are obtained directly from the canceler weights, the hybrid implementation does not require prior knowledge of the array response matrix and is independent of the type of antennas used in the receiver.; Orthogonal frequency-division multiplexing (OFDM) is a multicarrier communications technique where the information signal is transmitted on several orthogonal subcarriers. Although OFDM is relatively resistant to multipath and intersymbol interference (ISI), its performance is diminished by cochannel interference. In this dissertation, we describe blind time- and frequency-domain adaptive antenna systems for a multiuser OFDM network that beamform in the direction of a desired node and null interference from cochannel nodes. The subcarriers of each antenna element are assigned independent adaptive weights so that nulls can be placed in the directions of the interferers affecting only those subcarriers, and without any training sequences. To avoid this problem, we describe a constrained receiver, which derives the directional constraint by extending the subcarrier adaptive beamformer to include a bank of signal cancelers.; Although a code-division multiple-access (CDMA) system is spectrally efficient and provides some immunity against intentional reception, its capacity and performance are generally limited by multiple access interference (MAI) caused by other users. In this paper, we present a cyclodespreader with a parallel architecture that converts an aperiodic CDMA sequence into a piecewise periodic sequence in each arm of the receiver. The users are then detected using a blind interference suppression algorithm. Since the received signal is a mixture of users with different power levels, a multistage interference cancellation scheme is implemented where the stronger users are captured and canceled from the received signal, thus improving the signal-to-interference-plus-noise ratio (SINR) of the weaker users.