10.1109/TCOMM.2006.876828

A novel analytical method is introduced for exact statistical modeling of multiple access interference (MAI), in time hopping pulse position modulation and pulse amplitude modulation (TH-PPM and TH-PAM) ultra wideband (UWB) systems operating in additive white Gaussian noise (AWGN) channels. Based on this method, exact bit error rates (BER) are expressed in simple formulas. In a similar fashion, the exact BER of direct sequence (DS) UWB is also derived for PAM modulation. The proposed modeling of MAI considers complete asynchronism in user access, and is also suitable for accurately modeling the MAI components contributed by individual paths in channels with Poisson arrivals, based on the time variables. We further extend this method to derive general expressions for the BER performance in log-normal fading multi-path channels. In the course of these derivations, we also introduce a more accurate numerical approach to evaluate the characteristic function (CF) of a lognormal random variable

In this paper, the multiple access capacity of an Mary pulse position modulation (PPM) impulse radio (IR) system with antenna array is analyzed in dense multipath environments. An antenna array with Rake receivers is used to capture the signal energy from multipaths. Multiple access performance of the system is evaluated in terms of number of supported users for a given bit error rate and bit transmission rate with different number of antenna elements and selected paths. Numerical results show that the multiple access capacity of an M-ary IR system can be improved significantly by increasing the number of antenna elements and/or by adding more paths coherently at the receiver.

Ultra-wideband (UWB) technology is characterized by transmitting extremely short duration of radio impulses. To improve its multiple access capability, UWB technology can be combined with traditional spread-spectrum techniques. In this paper, we present the multiple access performance of direct-sequence (DS) UWB system with diversity reception. Numerical results show that the multiple access performance of a DS-UWB system can be improved significantly by increasing the number of antenna array elements and/or by adding more paths at the receiver

We present a statistical model for the characterization of ultra-wideband (UWB) channels for the outdoor office environment. The bandwidth is from 3-6 GHz and measurements are done for the line-of-sight (LOS) and non-line-of-sight (NLOS) cases. Small scale effects are modeled by the Saleh-Valenzuela (S-V) model with modifications on the ray arrival times and amplitude statistics to fit the empirical data. The ray arrival times are described by a mixture of Poisson processes while the amplitude statistics are follow a Nakagami distribution with the m parameter following a lognormal distribution. The parameters for the cluster and ray power-decay time constants, cluster and ray arrival times, mean excess delay and RMS excess delay are also extracted to define the channel model completely.

A new analytical method is introduced for the exact statistical modeling of multiple access interference (MAI), in time hopping pulse position modulation (TH-PPM) ultra wideband (UWB) systems. Based on this model the exact bit error rate (BER) is expressed in simple formulas. The proposed modeling of MAI considers complete asynchronism, and is also suitable for modeling the MAI components contributed by individual paths in channels with Poisson arrivals, based on the time variables.

In this paper, an iterative interference cancellation (IIC) scheme is proposed for space-time block coded (STBC) multirate multiuser systems. The proposed IIC scheme is based on minimum mean square estimation (MMSE) interference cancellation and maximum likelihood detection. In the proposed scheme, the high rate user data is decoded first and the decoded high rate user data is used to decode the low rate user data. Subsequently, low rate user data is used to re-decode the high rate user data. The bit error rate performance of the proposed IIC is better than the conventional non-iterative MMSE scheme. In addition, the proposed scheme performs well not only in multirate multiuser systems but also in equal rate multiuser systems. As expected, the performance margin diminishes with number of iterations.

In this paper, a new analytical method based on exact statistical modeling of multiple access interference (MAI) is proposed for exact bit error rate (BER) computation of time hopping pulse position modulation (TH-PPM) ultra wideband (UWB) systems. The proposed modeling considers full asynchronism. Furthermore, it is also suitable for modeling the MAI components contributed by individual paths in channels with Poisson arrivals, based on the time variables.

Impulse radio (IR) is based on the ultra-wideband (UWB) time-hopping spread spectrum technique in which subnanosecond pulses are modulated to convey information by precisely shifting the relative time position of the pulses. The influence of temporal and spatial diversities on the performance of ultra-wideband impulse radio systems is analyzed. We investigate how antenna diversity (uniform linear array and rectangular array) can be used to improve the bit error rate performance and to mitigate multiple user interference (MUI) of an IR system in multipath fading channels. Numerical results show that the bit error rate performance and multiple access capability of the IR system can be improved when the number of array elements and selected multipaths are increased.

In this paper, a computationally efficient adaptive channel estimation scheme is proposed for a space-time block coded (STBC) multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) system. This adaptive channel estimation scheme is based on recursive least square (RLS) algorithm. The proposed channel estimation scheme is less complex and bandwidth efficient than the conventional channel estimation schemes for MIMO-OFDM systems. Simulation results show that the proposed channel estimator performs quite close to the system where ideal channel state information is available. At the receiver, signals from different co-channel users are detected using minimum mean squared error (MMSK) interference cancellation (IC) method. It is shown that the signal detection with the proposed channel estimator could be used to increase the system capacity. Simulation results are compared with theoretical error bounds.

In this paper, an adaptive interference suppression scheme for space-time coded multiuser systems is presented. We consider M synchronous co-channel users, each is equipped with N/sub r/ transmitting antennas. The receiver is equipped with N/sub r/ (/spl ges/ M) receive antennas. It is shown that the proposed scheme estimates the weights adaptively and suppresses the interference without any explicit knowledge of the channel and the interference. In addition, this interference cancellation scheme has less complexity than the minimum mean squared error (MMSE) scheme, while maintaining the same decoding complexity with acceptable performance degradation due to weight estimation errors.