Time hopping is the strategy employed in impulse radio systems to increase the communication reliability by spreading the multiple access interference (MAI) over numerous time intervals. This results in a non-stationary interference that makes the error probability performance to be dominated by the worst-case. In this paper we employ a collision model analysis for this time-varying interference that is ruled by the system loading in term of the corresponding MAI collision probability. The model proves to be simple and accurate to characterize the decision variable conditioned to the collision events. This provides a reliable framework for evaluating the error probability in multipath faded channels with RAKE receiver either for coded and uncoded systems. In this paper we derive closed form approximations of the bit-error-rate for multipath channels with exponential power delay profile and Nakagami-m fading. Validations by numerical analysis show that the collision model provides an accurate framework to evaluate also performance of realistic channel models.

This paper focuses on the uplink of multicell IEEE 802.16-d WiMax systems with OFDM modulation and antenna array at the base stations. We propose an analytical framework to assess the average error probability of all the different transmission modes over space-time dispersive Rayleigh fading channels. The proposed method takes into account the effects of the multilevel modulation, the error correction capability of the concatenated code, the interleaving scheme, the power-angle structure of the inter-cell interference and the array processing at the base station. Simulation results corroborate the proposed analysis for a IEEE 802.16-d cellular system over different propagation scenarios.

In this paper we consider a multicell and multi- antenna E-UTRA system as conforming to 3GPP recommendation. Focusing on the downlink, the system performance is severely hampered by the intercell interference due to the frequency reuse. For this reason, 3GPP is currently studying some policies to mitigate the interference impairments. We compare here two main approaches: interference coordination and interference randomization. A collision based model permits to model the intercell interference and to analytically assess the bit-error-rate for both the techniques. The analysis and the simulation results show that interference coordination is effective for moderate system load, whereas interference randomization achieves a definite performance improvement in case of heavy loaded systems due to the frequency diversity gain.

Orthogonal random beamforming (ORB) constitutes a mean to exploit spatial multiplexing and multi-user diversity (MUD) gains in multi-antenna broadcast channels. To do so, as many random beamformers as transmit antennas (M) are generated and on each beam the user experiencing the most favorable channel conditions is scheduled. Whereas for a large number of users the sum-rate of ORB exhibits an identical growth rate as that of dirty paper coding, performance in sparse networks (or in networks with an uneven spatial distribution of users) is known to be severely impaired. To circumvent that, in this paper we modify the scheduling process in ORB in order to select a subset out of the M available beams. We propose several beam selection algorithms and assess their performance in terms of sum-rate and aggregated throughput (i.e., rate achieved with practical modulation and coding schemes), along with an analysis of their computational complexity. Since ORB schemes require partial channel state information (CSI) to be fed back to the transmitter, we finally investigate the impact of CSI quantization on system performance. More specifically, we prove that most of the MUD can be still exploited with very few quantization bits and we derive a beam selection approach trading-off system performance vs. feedback channel requirements.

The performance of time hopping impulse radio systems is severely affected by asynchronous multiple access interference (MAI). The transmission reliability can be enhanced by using correction coding provided that some knowledge on the interference scenario is made available at the receiver. In this paper we design an efficient decoding and interference estimation receiver that embed interference estimation into maximum likelihood detector. The collision model approach permits to simplify the MAI estimation to the number of colliding users in each frame. We also analytically assess the error rate performance in the limiting case of perfect MAI estimation (lower bound on bit-error-rate) and no-estimation (upper bound). Simulation results validate the proposed scheme and the analytical analysis.

The capacity region of gaussian multi-input- multiple-output (MIMO) broadcast channels has been shown to be achieved by dirty paper coding (DPC). However, due to the high complexity of DPC and the infeasibility of the channel state information (CSI) at the transmitter, in this paper we focus on random beamforming with multiple beams (RB- MUX). The main advantages of RB-MUX are multiuser diversity and spatial multiplexing gain at limited feedback load. In this paper we analytically assess the ergodic sum-rate of RB-MUX scheme with multiple antennas at the receiver by considering three combining techniques: maximum ratio combiner (MRC), optimum combiner (OC) and antenna selection (AS). The analysis relies on the assumption of large average SNR, so that the additive noise can be assumed negligible as compared to the beams mutual interference. Simulation results corroborate our analysis.

A multicell system which supports multiple antennas and OFDMA modulation/access is considered in this paper as conforming IEEE 802.16-2005 standard. Focusing on the uplink, the system performance is severely hampered by the out-of-cell interference due to the frequency reuse. Here we analytically assess the error probability by employing a collision model to describe the non-stationarity of the out-of-cell interference due to random subcarriers permutation as prescribed by PUSC mode. The collision model permits an accurate analysis both of uncoded and coded transmissions. Two different decoding strategies are investigated according to the amount of information available at the receiver. Simulation results corroborate the proposed analysis.

A multicell WiMax system which supports orthogonal frequency division multiplexing (OFDM) and antenna arrays at the base stations is considered in this paper as conforming to the IEEE 802.16-2004 standard. Focusing on the uplink, we propose an analytical framework to assess the average error probability of the system over time-dispersive (or, equivalently, frequency selective) and space-dispersive (due to the antenna array) Rayleigh fading channels. The proposed method takes into account the effects of the correlation of the channel gains over the space-frequency domain, the power-angle structure of the inter-cell interference, the array processing at the base station and the interleaving scheme. Simulation results corroborate the proposed analysis for a IEEE 802.16-2004 cellular system over different propagation scenarios.

In multi-antenna downlink systems the optimization of linear preceding is severely hampered by the amount of feedback. An efficient solution consists in opportunistic schemes, which generate M random beams and schedule the users with the highest signal-to-noise-plus-interference ratios (SINRs), which can be made available to the transmitter with very little feedback. Although this technique has been shown to be optimal for asymptotically large number of users, severe performance degradation occurs in practical system where the number of users is limited. In this paper we propose an enhancement of this strategy based on an adaptive beam selection procedure. Instead of transmitting all the generated beams, the scheduler picks the optimum subset of beams that maximizes the system sum-rate according to the feedback information. We propose and compare several beam selection algorithms according to different complexity requirements. In particular, we show that the proposed approaches give substantial gains with respect to conventional opportunistic schemes

Opportunistic schemes employing multiple beams have recently attracted significant interest due to the capability to achieve both multiuser diversity and spatial multiplexing gain at limited feedback load. In this paper we explore the effect of the feedback quantization on the performance of these schemes. We derive a closed-form expression of the system throughput with and without feedback quantization. By doing so, we analytically assess the impact of the number of terminals and the restriction in the feedback bandwidth. It is shown that most of the throughput is preserved by using a few quantization bits

The cross-layer design is a promising strategy to maximize the spectral efficiency in communication systems. In this paper we focus on packet-oriented systems and we propose to combine adaptive modulation and coding (AMC) at physical layer and hybrid automatic repeat request (ARQ) at the data link layer. In the proposed multilevel hybrid ARQ (M-HARQ) strategy, the transmitted symbols are decomposed into a set of parallel sub-channels with an independent ARQ protocol over each sub-stream. It follows a layered structure where each sub-channel can be decoded only once the lower sub-channels have been resolved, thus subchannels are sequentially detected. The number of parallel streams is adaptively arranged according to the channel state information (CSI). Considerable throughput gain is achieved using the same encoder/decoder structure for all the sub-channels. Moreover, M-HARQ permits an adaptive and flexible implementation even when CSI is not available. The drawback is an increase in the decoding delay, which might be tolerable in some applications

This paper is focused on the performance of convolutional codes with bit interleaving in presence of non-stationary noise due to time-varying interference scenario. Two different decoding schemes are considered based on the knowledge of the instantaneous or the average noise power at the receiver. Analytical performance on average error probability has been derived under the assumption that interference can be modelled as Gaussian noise with random power. More specifically, the closed-form is for central /spl Xscr/E2 noise power with any degree of freedom, while upper bounds are derived for either static or fading channels. Simulations validate the analytical relationships and show that there is a definite advantage in exploiting the instantaneous noise power when the non-stationary interference can be modelled as time-varying power with heavily skewed distributions (e.g., lognormal).

In mobile communications the movement of the users makes the propagation channel to be time-varying. Algorithms that track channel variations have to trade between complexity and accuracy. Since the second-order statistic of time-varying channels is stationary, estimation of the channel can be reduced to track a set of r uncorrelated parameters. Based on this decomposition, in this paper we propose to simplify the optimum Kalman filter (KF) by tracking the r channel modes separately and by using the steady-state solution of the KF gain.

In the downlink of a multi-user MIMO system over a fading channel, the base station can assign the available spatial streams to different users by capitalizing on multiuser diversity or enforcing fairness constraints. Assuming linear precoding (beamforming) at the base station, the problem amounts to the joint design of precoding matrices and channel aware scheduling, according to the cross-layer paradigm. In this paper we formulate the joint optimization of scheduling and linear precoding within a physical layer-oriented framework, where the performance metric is the transmission rate. Moreover, we constraint the precoding scheme to ensure interference-free reception of spatial streams (orthogonal space division multiple access, OSDMA). Fairness constraints are either inspired by the max-min or the proportional fair criterion. Performance of different schemes is evaluated by numerical simulations allowing to assess the tradeoffs between sum-rate (multi-user diversity) and fairness