The autocorrelation receiver (AcR) is a suboptimum, low-complexity receiver architecture particularly suited to ultra-wideband (UWB) communication systems. As the bit rate increases, interference among pulses due to multipath propagation causes serious impairments of the AcR's performance. To mitigate this effect, we propose an appropriate design of the chip code and of the delay hopping (DH) code. We provide conditions to be satisfied by the DH code in order to reduce the nonlinear intersymbol interference (ISI) and the bias term, which are peculiar nuisance parameters of autocorrelation receivers. By extending the length of the chip code, we show that N/sub p/ transmitted pulses per symbol can be employed to suppress the average linear ISI of N/sub p/-1 previous symbols. Simulated results confirm the performance improvement in terms of bit-error rate. However, in previous work it has been shown that the noise power linearly increases with N/sub p/. Although a large number of pulses per symbol is favorable for ISI mitigation, we show that the transmission of a single pulse minimizes the probability of error, for bit rates lower than an upper bound depending on the channel root mean-square delay spread and on the noise power.

The performance of a conventional threshold detector for a frame differential autocorrelation receiver in IR UWB systems in a dense multipath channel are improved by means of appropriate chip code and delay hopping code design. We point out which conditions must be satisfied by the delay hopping code in order to reduce the nonlinear ISI and the bias term, which are peculiar nuisance parameters of autocorrelation receivers. Moreover, by extending the periodicity of the chip code, we show how to suppress linear ISI. Simulated results confirm the performance improvement in terms of BER. An analytical expression relating the noise power to the system parameters is also presented. These results can be extended to the well known transmitted reference systems.

A discrete-time equivalent system model is derived for differential and transmitted reference (TR) ultra-wideband (UWB) impulse radio (IR) systems, operating under heavy intersymbol-interference (ISI) caused by multipath propagation. In the systems discussed, data is transmitted using differential modulation on a frame-level, i.e., among UWB pulses. Multiple pulses (frames) are used to convey a single bit. Time hopping and amplitude codes are applied for multi user communications, employing a receiver front-end that consists of a bank of pulse-pair correlators. It is shown that these UWB systems are accurately modeled by second-order discrete-time Volterra systems. This proposed nonlinear equivalent system model is the basis for developing optimal and suboptimal receivers for differential UWB communications systems under ISI. As an example, we describe a maximum likelihood sequence detector with decision feedback, to be applied at the output of the receiver front-end sampled at symbol rate, and an adaptive inverse modeling equalizer. Both methods significantly increase the robustness in presence of multipath interference at tractable complexity.

A discrete-time equivalent system model is derived for the intersymbol interference in a differential ultra wideband (UWB) impulse radio (IR) system operating in a multipath propagation environment. Data is transmitted using differential binary modulation on a frame-level (i.e., among UWB pulses), which is a variant of the well-known transmitted reference UWB schemes, proposed for higher data rates. It is shown that the UWB system is accurately modeled as a second-order discrete time Volterra system. Equations are presented to determine the coefficients of this non-linear model. The impact of noise is characterized. The proposed model can be used to develop signal processing algorithms for UWB-receivers in order to enhance the data rate or multiuser capabilities, for instance.

In this paper we propose two innovative detection schemes for IR-UWB communication systems based on the autocorrelation receiver and differential detection. The proposed schemes avoid the need for a complex analog multiplier by employing: (i) a limiter in the reference branch only; or (ii) employing also a limiter in the signal branch. Analytical models for the statistics of the decision variables are presented for the receivers, and the analytical and simulated BER exhibit an excellent agreement. We point out the dependence of the performance on some important system parameters, that shows the system design trade-off between receiver complexity and performance.

This paper presents a new uplink beamforming technique applied to the WCDMA system, here called partial-adaptive beamforming. Instead of concentrating upon the performance of every single user, the radiation pattern aims to maximize the performance of a group of users and only a limited number of beams has to be formed. We present two algorithms that group effectively the users (grouping algorithm) and shape the radiation pattern (shaping algorithm) in order to enhance the performance of all users being part of the same group. The required hardware and the computational complexity of the proposed algorithms compared to the fully-adaptive beamforming technique is drastically reduced. Simulation results show a considerable gain respect to the fixed beam approach in terms of increased capacity and enlarged coverage. Furthermore the algorithms converge at a speed suitable to implement them following the dynamic evolution of the traffic within the sector.