In this paper, a new soft interference cancellation (SIC) receiver is proposed for the single-carrier frequency-division multiple access (SC-FDMA) multiple-input multiple-output (MIMO) uplink in the Long Term Evolution (LTE) and LTE-Advanced (LTE-A) systems. The receiver works in an iterative fashion between the equalizer and decoder, by using the feedback from the decoder to cancel interfering spatial streams. In addition, the receiver also cancels inter-symbol interference of the desired stream caused by frequency selective fading channel, which reduces the well known “noise enhancement” effect in SC-FDMA. Two types of structures, parallel SIC (P-SIC) and serial SIC (S-SIC) are presented, and an iterative channel estimation method is introduced to further improve the performance of the SIC receiver. Feedback reliability metrics are defined and applied during equalization to avoid catastrophic error propagation. Simulation results demonstrate that with proper reliability metrics, hard slicing on feedback symbol generation can be used to save computational complexity with very small performance loss compared to soft slicing.

Two techniques for channel estimation and co-channel interference rejection are presented for the multiple-input, multiple-output (MIMO) uplink in the Long Term Evolution-Advanced (LTE-Advanced) system. The first technique consists of estimating the desired users' channels in the frequency domain (FD) before estimating interference covariance matrices for interference rejection. In the second technique, channel and interference covariance matrices are estimated jointly via a time domain (TD) approach. Interference rejection is achieved by using the channel and interference covariance matrix estimates in the MIMO equalizer. Significant interference rejection gains are observed.

The throughput performance is studied for a two-user wireless broadcast channel with hybrid automatic repeat request (HARQ) and user cooperation. Two transmission schemes are considered: superposition coding (SC) and frequency-division multiplexing (FDM). During the initial HARQ rounds, the access point (AP) transmits messages to the users using either SC or FDM. If a user is successful, the AP uses all resources to transmit to the other user in subsequent HARQ rounds. Meanwhile, the successful user attempts to decode the other user's message and cooperate simultaneously with the AP, as long as the maximum number of HARQ rounds L has not been reached. The cooperative transmissions use distributed space-time coding to avoid the half-duplex penalty. HARQ transmissions continue until success for both users or until L HARQ rounds have elapsed. Cooperation provides throughput gains subject to quality-of-service (QoS) constraints, even when the path loss between users is greater than that between each user and the AP.

Due to the limited energy supplies of nodes in many applications such as wireless sensor networks, energy efficiency is crucial for extending the lifetime of these networks. This paper addresses the cooperative transmission for outage-restricted multihop wireless ad hoc networks. The source node wants to transmit messages to a single destination. Other nodes in the network may operate as relay nodes. In this paper, new multihop cooperative protocol is proposed using the space-time codes for the purpose of energy savings, subject to a required outage probability at the destination. We restrict the cooperation to nodes along a chosen route. Two efficient power allocation schemes are derived, which depend only on the statistics of the channels. Furthermore, three efficient cooperative multihop transmissions are proposed when arbitrary distributed space-time codes are used. The proposed cooperative protocols offer different degrees of energy efficiency, spectral efficiency, complexity, and signalling overhead. Compared to non-cooperative multihop routing, an energy saving of up to 72% is achievable in line networks with 3 relays and an outage probability constraint of 10-3at the destination.

A two-user uplink interference channel with hybrid automatic retransmission request (HARQ) and base station cooperation is analyzed. Two forms of base station cooperation are studied: wireless and wireline cooperation. In wireless cooperation, the first base station that decodes its desired user's message acts as a relay for the co-channel user via space-time coding during the remaining HARQ rounds. This cooperative HARQ protocol is spectrally efficient despite the half-duplex constraint imposed on wireless transceivers. In wireline cooperation, the base stations cooperate via a wireline link with a fixed capacity. Individual user outage probabilities at each HARQ round are computed for both forms of base station cooperation. Based on these outage probabilities, an upper bound of the total user throughput is determined subject to individual user outage probability constraints after a maximum number of L HARQ rounds. For interference-limited scenarios, base station cooperation provides throughput gains up to a factor of 5.

Due to the limited energy supplies of nodes in many applications like wireless sensor networks, energy efficiency is crucial for extending the lifetime of these networks. This paper addresses the routing problem for outage-restricted multihop wireless ad hoc networks based on cooperative transmission. The source node wants to transmit messages to a single destination. Other nodes in the network may operate as relay nodes. In this paper, a new cooperative routing protocol is introduced using the Alamouti space-time code for the purpose of energy savings, given a required outage probability at the destination. Two efficient power allocation schemes are derived, which depend only on the statistics of the channels. In the first scheme, each node needs to know only the local channel statistics, and can be implemented in a distributed manner. In the second scheme, a centralized power control strategy is proposed, which has a higher energy efficiency, at the expense of more complexity and signalling overhead. Compared to non-cooperative multihop routing, an energy saving of 80% is achievable in line networks with 3 relays and an outage probability constraint of 10-3at the destination.

The throughput-delay performance of a half-duplex relay channel with hybrid-automatic retransmission request (HARQ) is analyzed in this paper. The protocol uses a form of incremental redundancy HARQ transmission with assistance from the relay via space-time coding if the relay decodes the message before the destination. The packet delay constraint is represented by L, the maximum number of HARQ rounds. An outage is declared if the packet is unsuccessful after L HARQ rounds. A delay-limited throughput is defined in this paper to explicitly account for finite delay constraints and associated non-zero packet outage probabilities. For small outage probabilities, this delay-limited throughput is greater than the conventional long-term average throughput. An exact expression is obtained for the outage probability to compare the throughput-delay performance of this half-duplex relay protocol with direct transmission. The delay-limited throughput of the relay channel is significantly larger than that of direct transmission for a wide range of signal-to-noise ratios (SNRs), target outage probabilities, delay constraints and relay positions.

This paper considers a two-user, quasi-static multiple access channel with hybrid-automatic repeat request (HARQ) and user cooperation. The protocol uses incremental redundancy HARQ packet transmission from both users until at least one userpsilas packet is successfully decoded. If one userpsilas packet is successful before the other, the successful user begins to act as a relay for the other user until packet success or until a maximum number of HARQ rounds L, where L represents the packet delay constraint. The performance of this cooperative multiple access channel is evaluated using the delay-limited throughput, defined as a measure of the throughput of an arbitrary packet, as well as the conventional long-term average throughput. The throughput metrics depend on each userpsilas individual outage probability, which is approximately equal to the userpsilas packet error probability as a function of the number of HARQ rounds. Compared to no user cooperation, this cooperative protocol provides significantly higher throughputs, with increasing gains for more stringent quality-of-service (QoS) constraints.

In this paper, a novel method of reducing the peak- to-average-power ratio (PAR) of multi-antenna OFDM systems is presented. The PAR is reduced by the addition of a PAR reduction signal to the transmitted signal. This PAR reduction signal is constrained to lie within the null space of the channel. Hence, only the original transmitted signal is received at the receiver because the channel nulls out the reduction signal. In this scheme, with the help of partial channel state information, the need for sending side information or allocating reserved tones (required for several existing PAR reduction schemes) is eliminated. We also present simulation results showing the performance improvement in the bit-error-rate curves while using the spatial nulling technique.

Lower bounds are computed on the throughput of packet-switched space-time coded systems with delay constraints. The delay constraints are accounted for by defining a delay-limited throughput as the instantaneous throughput of each packet that can be maintained with a certain probability. The analysis assumes quasi-static and independent fading across packets, packet retransmissions until success, and negligible overhead for acknowledgment packets and guard times. Upper bounds on the outage probabilities of orthogonal space-time block codes (OSTBC) and spatial multiplexing with horizontal encoding (SM-HE) are computed in the presence of channel estimation errors to obtain lower bounds on the delay-limited throughput. The target physical-layer data rate is selected at each signal-to-noise ratio (SNR) to maximize the lower bounds on the throughput. The results indicate that higher throughput is achieved using OSTBC at low SNR and SM-HE at high SNR. Furthermore, the loss of throughput due to channel estimation errors is quantified.

In this paper, outage regions are computed for the quasi-static fading multiple access channel (MAC) with constraints on the individual user outage probabilities and no channel state information at the transmitters (CSIT). An outage rate region is defined as the set of rate vectors for which the user outage probability constraints are satisfied simultaneously. Outage probability is typically defined in terms of a common outage event, defined as the event that a target user rate vector lies outside the achievable region of the MAC, conditioned on the fading state. In contrast, rate regions without CSIT are computed in this paper based on individual outage events and individual probability constraints that are satisfied simultaneously. An individual outage event for a given user occurs if the user's message is not correctly decoded, irrespective of the decoding success of the messages for other users. Individual outage rate regions are useful for the MAC with heterogeneous user channels and quality-of-service (QoS) requirements. Explicit outage rate regions and total throughputs are computed for the two-user fading MAC. It is shown that rate regions and throughputs are significantly larger using individual outage probabilities than using the common outage probability.

This paper addresses cross-layer resource allocation in quasi-static fading multiple access channels (MAC) with no channel state information at the transmitters (CSIT). Under the cross-layer approach without CSIT, the power and rate are determined based on an outage rate region as well as the current queue state information (QSI). For heterogeneous quality-of- service (QoS) requirements, an individual outage rate region results in better performance compared to the common outage rate region that is widely considered in the literature. This paper first reviews the recent notion of an individual outage rate region for the quasi-static fading MAC without CSIT. Individual outage rate regions are then used in two major types of scheduling policies: maximum weight matching scheduling (MWMS) and queue proportional scheduling (QPS). It is shown that MWMS and similar techniques based on weighted-sum-rate maximization require exponential complexity in the number of users because of the non-convex nature of the relevant optimization problems. By contrast, QPS can be very efficiently solved by using a successive feasibility check. Stochastic simulations show that compared to MWMS and the common outage rate region, significant throughput increase and delay reduction are possible by using the QPS scheduling policy with the individual outage rate region.

This paper presents power-efficient transmission schemes for the multiple-input multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) block-fading channel under the assumption that the channel during each fading block is known perfectly at the receiver, but is unavailable at the transmitter. Based on the well-known vertical Bell Labs layered space-time (V-BLAST) architecture that employs independent encoding for each transmit antenna and successive decoding at the receiver, this paper presents a per-antenna-based power and rate feedback scheme, termed the "closed-loop" V- BLAST, for which the receiver jointly optimizes the power and rate assignments for all transmit antennas, and then returns them to the transmitter via a low-rate feedback channel. The power and rate optimization minimizes the total transmit power for support of an aggregate transmission rate during each fading block. Convex optimization techniques are used to design efficient algorithms for optimal power and rate allocation. The proposed algorithms are also modified to incorporate practical system constraints on feedback complexity and on modulation and coding. Furthermore, this paper shows that the per-antenna-based power and rate control can be readily modified to combine with the conventional linear MIMO transmit preceding technique as an efficient and capacity-approaching partial-channel-feedback scheme. Simulation results show that the closed-loop V-BLAST is able to approach closely the MIMO-OFDM channel capacity assuming availability of perfect channel knowledge at both the transmitter and the receiver.

In this paper, multiple access channels are considered in which user data rates increase with the signal-to-noise ratio (SNR). To account for channel estimation errors, a finite- SNR diversity-multiplexing tradeoff framework is formulated. The multiplexing gain is defined for each user as the ratio of the user's data rate to the capacity of an additive white Gaussian noise (AWGN) channel. For a set of user multiplexing gains, a finite-SNR diversity gain is defined by the slope of the common outage probability as a function of SNR for quasi-static fading. These definitions have been used in previous work to characterize a finite-SNR diversity-multiplexing tradeoff in multiple access channels and point-to-point multiantenna channels. Inner and outer bounds on the feasible rate region of a multiple access channel with channel estimation errors are used to obtain estimates of the common outage probability. The outage probability is then used to estimate the diversity gain as a function of user multiplexing gains and SNR. It is shown that a finite-SNR formulation is essential to characterizing the impact of channel estimation errors on the diversity-multiplexing tradeoff.

In a Gaussian MIMO broadcast channel, serving more than one user at a time is generally more beneficial in terms of average sum-rate. Under partial channel state information (CSI) feedback constraints, however, serving only one user at a time can be a better multiple access method when the number of active users in the system is smaller than a certain threshold. This paper uses single user beam-forming (SUBF) to refer to the transmitting scheme that serves only one user at a time and multi-user beam-forming (MUBF) to refer to the scheme without such a constraint, i.e., a scheme that is allowed to serve more than one user at a time. It turns out that the aforementioned user threshold varies with other system parameters, especially, it increases as the number of transmit antennas increases. Monte Carlo simulations and a statistical analysis are provided to verify the results

Queue Proportional Scheduling (QPS) assigns each user a data rate proportional to the number of packets (or bits) in that user's queue. This paper analyzes stability, delay and fairness properties of QPS in a Gaussian broadcast channel (BC). QPS is shown to achieve throughput optimality, and guarantee fairness as well as different priorities among users in terms of average queuing delay. One well known throughput optimal policy for broadcast channels is Maximum Weight Matching Scheduling (MWMS) that maximizes the inner product of the queue state vector and the achievable rate vector. Simulation results with Poisson packet arrivals and exponentially distributed packet lengths demonstrate that QPS provides a significant decrease in average queuing delay compared to MWMS in a Gaussian BC.

In this paper, bounds on the outage probability and estimates of the diversity gain at finite signal-to-noise ratios (SNRs) are obtained for quasi-static fading multiple access channels in which user data rates increase with SNR. The data rate of each user is equal to the multiplexing gain multiplied by the capacity of an additive white Gaussian noise (AWGN) channel. Given the user multiplexing gains, the finite-SNR diversity gain is defined in terms of the slope of the common outage probability versus SNR at a particular point. This nonasymptotic framework represents an extension of previous work for point-to-point multiantenna channels to rate-adaptive multiple access channels. In contrast to asymptotic diversity-multiplexing tradeoffs as the SNR tends to infinity, finite-SNR analysis provides useful insight into achievable multiple access performance with different transmit power, path loss, and shadowing among users

In a fading broadcast channel (BC), Queue Proportional Scheduling (QPS) is presented via geometric programming (GP). Given the current queue state, QPS allocates a data rate vector such that the expected rate vector averaged over all fading states is proportional to the current queue state vector as well as on the boundary of the ergodic BC capacity region. One well known throughput optimal policy for a fading BC is Maximum Weight Matching Scheduling (MWMS), which maximizes the inner product of the queue state vector and achievable rate vector. Simulation results for Poisson packet arrivals and exponentially distributed packet lengths demonstrate that QPS provides a significant decrease in average queuing delay compared to MWMS. In addition, QPS is shown to guarantee fairness among users in terms of average queuing delay.

For fading broadcast channels (BC), a throughput optimal scheduling policy called queue proportional scheduling (QPS) is presented via geometric programming (GP). QPS finds a data rate vector such that the expected rate vector over all fading states is proportional to the current queue state vector and is on the boundary of the ergodic capacity region of a fading BC. Utilizing the degradedness of BC for each fading state, QPS is formulated as a geometric program that can be solved with efficient algorithms. The GP formulation of QPS is also extended to orthogonal frequency-division multiplexing (OFDM) systems in a fading BC. The throughput optimality of QPS is proved, and it is shown that QPS can arbitrarily scale the ratio of each user's average queueing delay. Throughput, delay, and fairness properties of QPS are numerically evaluated in a fading BC and compared with other scheduling policies such as the well-known maximum weight matching scheduling (MWMS). Simulation results for Poisson packet arrivals and exponentially distributed packet lengths demonstrate that compared with MWMS, QPS provides a significant decrease in average queueing delay and has more desirable fairness properties.

This correspondence studies the statistical distribution of the signal-to-interference-plus-noise ratio (SINR) for the minimum mean-square error (MMSE) receiver in multiple-input multiple-output (MIMO) wireless communications. The channel model is assumed to be (transmit) correlated Rayleigh flat-fading with unequal powers. The SINR can be decomposed into two independent random variables: SINR=SINR/sup ZF/+T, where SINR/sup ZF/ corresponds to the SINR for a zero-forcing (ZF) receiver and has an exact Gamma distribution. This correspondence focuses on characterizing the statistical properties of T using the results from random matrix theory. First three asymptotic moments of T are derived for uncorrelated channels and channels with equicorrelations. For general correlated channels, some limiting upper bounds for the first three moments are also provided. For uncorrelated channels and correlated channels satisfying certain conditions, it is proved that T converges to a Normal random variable. A Gamma distribution and a generalized Gamma distribution are proposed as approximations to the finite sample distribution of T. Simulations suggest that these approximate distributions can be used to estimate accurately the probability of errors even for very small dimensions (e.g., two transmit antennas).

A nonasymptotic framework is presented to analyze the diversity-multiplexing tradeoff of a multiple-input-multiple-output (MIMO) wireless system at finite signal-to-noise ratios (SNRs). The target data rate at each SNR is proportional to the capacity of an additive white Gaussian noise (AWGN) channel with an array gain. The proportionality constant, which can be interpreted as a finite-SNR spatial multiplexing gain, dictates the sensitivity of the rate adaptation policy to SNR. The diversity gain as a function of SNR for a fixed multiplexing gain is defined by the negative slope of the outage probability versus SNR curve on a log-log scale. The finite-SNR diversity gain provides an estimate of the additional power required to decrease the outage probability by a target amount. For general MIMO systems, lower bounds on the outage probabilities in correlated Rayleigh fading and Rician fading are used to estimate the diversity gain as a function of multiplexing gain and SNR. In addition, exact diversity gain expressions are determined for orthogonal space-time block codes (OSTBC). Spatial correlation significantly lowers the achievable diversity gain at finite SNR when compared to high-SNR asymptotic values. The presence of line-of-sight (LOS) components in Rician fading yields diversity gains higher than high-SNR asymptotic values at some SNRs and multiplexing gains while resulting in diversity gains near zero for multiplexing gains larger than unity. Furthermore, as the multiplexing gain approaches zero, the normalized limiting diversity gain, which can be interpreted in terms of the wideband slope and the high-SNR slope of spectral efficiency, exhibits slow convergence with SNR to the high-SNR asymptotic value. This finite-SNR framework for the diversity-multiplexing tradeoff is useful in MIMO system design for realistic SNRs and propagation environments

Two new algorithms are presented to select the number and subset of active transmit antennas among the users in a correlated multiple-input multiple-output (MIMO) multiple access channel. The algorithms are based on the outage probability of a multiuser spatial multiplexing system using independent decoding with linear and successive interference cancellation receivers at the access point. A key advantage of these algorithms is the avoidance of tracking the fading channel by using channel covariance matrices rather than instantaneous channel realizations. The first algorithm selects the active transmit antennas to minimize the total transmit power among the users given the outage probability, the data rate of each user and estimates of the channel covariance matrices. The second algorithm selects the active transmit antennas to maximize the sum rate of the multiple access channel given the outage probability, the transmit power constraint of each user and the channel covariance matrices. In comparison to a system using all available transmit antennas, both selection algorithms provide significant performance gains in correlated fading channels for linear and successive interference cancellation receivers. In addition, the algorithms exhibit gains for linear receivers in independent fading

The diversity performance of a multiple-input multiple-output (MIMO) wireless system is analyzed at finite signal-to-noise ratios (SNRs) when the data rate increases with SNR. The target data rate at each SNR is proportional to the capacity of an additive white Gaussian noise (AWGN) channel with an array gain. The proportionality constant, which can be interpreted as a finite-SNR spatial multiplexing gain, dictates the sensitivity of the rate adaptation policy to SNR. The diversity gain as a function of SNR for a fixed multiplexing gain is defined by the negative slope of the outage probability versus SNR curve. The finite-SNR diversity gain provides an estimate of the additional power required to decrease the outage probability by a target amount. A lower bound on the outage probability of the MIMO system is used to characterize the diversity performance. It is seen that the achievable diversity gains at finite SNRs are significantly lower than asymptotic values given in the literature. This diversity characterization is useful in determining suitable rate adaptation strategies at realistic SNRs.

This paper proposes a new transmitting scheme for a multi-antenna (MIMO) Gaussian broadcast channel. The transmitter exploits multiuser diversity using a small amount of feedback information from the receivers. The feedback information consists of the maximum achievable rate for each user and the necessary power distribution profile across the spatial dimensions that the transmitter should use to achieve that rate. The receivers use the iterative water-filling algorithm to maximize the achievable rate. The new scheme serves a single user at each transmission, hence the name single user random beamforming (SUBF) scheme. The proposed scheme maximizes the average sum rate under a TDMA environment (i.e., supporting a single user at a time) with the partial channel state information (CSI) feedback constraint.

A novel framework is presented to characterize the tradeoff between diversity and multiplexing of space-time codes at finite signal-to-noise ratios (SNR). The diversity gain of a space-time code is defined by the slope at a particular SNR of the outage probability versus SNR curve for a multiplexing gain defined by the ratio of the system spectral efficiency to the capacity of an additive white Gaussian noise (AWGN) channel. The finite-SNR diversity-multiplexing tradeoff is evaluated for orthogonal space-time block codes and spatial multiplexing with horizontal encoding. The tradeoff curves provide a characterization of achievable diversity and multiplexing gains for a given space-time code at SNR encountered in practice. It is seen that the achievable diversity gains at finite SNR are significantly lower than the asymptotic values given in the literature.

This paper proposes a transmit power optimization algorithm for a Gaussian vector broadcast channel. The optimal power allocation minimizes the sum trace of input covariance matrices of all the users in the Gaussian vector broadcast channel. By duality the transmit power optimization can be transformed into an equivalent convex optimization in a multiple access channel. Recursive equations are derived as sufficient conditions for the minimum transmit power and an iterative algorithm is proposed to solve the equations. Numerical examples show that the iterative algorithm converges very fast and efficiently determines the input covariance matrices for the optimum power allocation.

The paper presents a simulation-based study of a MAC protocol, named contention free bursting (also referred to as enhanced distributed coordination function, EDCF, bursting) described as an optional mode of operation in 802.11e WLANs. A simple recovery mechanism is proposed upon transmission failure during the burst. The new scheme aims to provide better quality of service when high throughput applications operate in the presence of low throughput, latency-sensitive applications.

In this letter, expressions are given for the symbol-error rate (SER) of the Vertical Bell Laboratories Layered Space-Time (V-BLAST) system, taking into account error propagation due to channel-estimation errors. In addition to error propagation, suboptimal substream ordering due to imperfect channel estimates is accounted for. First, the conditional SER is determined using the distribution of the signal-to-interference-plus-noise ratio of each substream, conditioned on the channel estimate. Then, the average SER as a function of the channel estimation error-to-signal ratio (ESR) is upper bounded by averaging over the distribution of the channel estimates. The upper bound on the SER is tighter than previous bounds in the literature. Comparisons with exact simulations demonstrate the accuracy of the SER expressions for a large range of ESRs.

A novel algorithm is presented to select the transmit antennas, constellations and distribution of total power of the users in a correlated multiple-input multiple-output (MIMO) multiple access channel. Given a target probability of symbol error, the spectral efficiency for each user and estimates of the channel correlation matrices, the transmit antennas, constellations and user power distribution are determined such that the required signal-to-noise ratio (SNR) is minimized for linear and vertical bell laboratories layered space-time (V-BLAST) receivers. This selection algorithm results in significant performance gains not only for correlated channels but also for independently fading channels.

We present a novel channel partitioning and modulation technique for linear time-varying (LTV) channels using adaptive bases of localized complex exponentials. We show that localized complex exponentials are approximate eigenfunctions of underspread LTV channels. A basis of localized complex exponentials that approximately diagonalizes the LTV channel is selected adaptively by the receiver during a training period. The basis selection process is equivalent to matching the support intervals of the localized complex exponentials to the rate of the channel time variation. The receiver sends information regarding the selected basis to the transmitter which modulates the subsequent data stream in this basis. The adaptive modulation technique performs significantly better than conventional orthogonal frequency-division multiplexing systems for rapidly varying LTV channels such as time-frequency-selective mobile radio channels.

We provide expressions for the bit error rate of various transmit and receive diversity schemes for orthogonal frequency-division multiplexing (OFDM) systems in the presence of frequency offset, phase noise and channel estimation errors. The derivations are also applicable for a general multiplicative distortion of the received signal. We compare two transmit diversity schemes (space-time and space-frequency diversity) with maximal ratio combining (MRC) receive diversity for frequency-selective Rayleigh fading channels. Our results show that with perfect channel estimates, practical values of the phase noise do not significantly degrade the performance of the various diversity methods for BPSK modulation. In contrast, the transmit diversity schemes for OFDM are much more sensitive to channel estimation errors than MRC receive diversity.

We provide expressions for the bit error rate of various transmit and receive diversity schemes for orthogonal frequency division multiplexing (OFDM) systems in the presence of frequency offset, phase noise, and channel estimation errors. The derivations are also applicable for a general multiplicative distortion of the received signal. Our results show that with perfect channel estimates, practical values of the phase noise do not significantly degrade the performance of the various diversity methods for binary phase-shift keying modulation. In contrast, the transmit diversity schemes for OFDM are much more sensitive to channel estimation errors than maximal ratio combining receive diversity.

A new non-parametric method is presented for the estimation of linear time-varying (LTV) channels. A basis of smooth local exponentials, which are approximate eigenfunctions of underspread LTV channels, is chosen to select time intervals during which the time variation of the channel is small. The time-varying transfer function is expanded in the basis of smooth local exponentials to obtain an estimate of the time-varying impulse response. Simulations for mobile radio channels demonstrate robust tracking of the time-varying channel for a large range of mobile speeds.

A new method is presented for estimating the mobile speed and the average received power in general wireless propagation environments. The locally stationary received signal is expanded in a local exponential basis using best basis methods of wavelet analysis. An estimate of the time-varying Doppler power spectrum is obtained together with an estimate of the maximum Doppler frequency, which is proportional to the mobile speed. The average received power is then estimated by integrating the time-varying spectrum. Simulations demonstrate good tracking of variable mobile speed and average received power for a wide range of angular distributions of incident power. The estimator is shown to perform significantly better than an adaptive averaging method described in the literature. The speed and average power estimates are also used to detect the corner effect in urban cellular systems to improve handoff performance and reduce the call dropping rate.

A new technique is described for estimating the nonstationary mean signal received at a mobile station in a Rayleigh fading environment. The estimate is based on samples taken at the midpoints between the local minima of the received envelope. The continuous wavelet transform is used to estimate the local minima. An estimate of the mean signal is obtained using a fixed number of local minima. This technique requires neither an estimate of the mobile speed nor an adaptive temporal averaging window in contrast to other estimators. Simulations show that the mean signal is estimated well in a nonstationary environment with variable mobile speed.

This paper presents a new technique for estimating the speed of a mobile station in a wireless system. The mobile speed maps the characteristic spatial scale of the received signal into a characteristic temporal scale. The continuous wavelet transform tracks changes in the temporal scale to estimate the mobile speed as a function of time. This technique requires neither knowledge of the average received power of the nonstationary signal nor adaptation of a temporal observation window, in contrast to other speed estimators given in the literature. Simulation results indicate the ability to track a variable speed profile.

A new technique is described for estimating the speed of a mobile station in a wireless system. The mobile speed maps the characteristic spatial scale of the received signal into a characteristic temporal scale. The continuous wavelet transform tracks changes in the temporal scale to estimate the mobile speed as a function of time. This technique requires neither knowledge of the average received power of the nonstationary signal nor adaptation of a temporal observation window, in contrast to other speed estimators given in the literature. Simulations show the tracking of a variable speed profile.

A new handoff algorithm based on recognition of patterns in the received signal power is presented. The handoff algorithm uses the constancy of the large scale signal variation with respect to a base station to improve handoff performance in comparison with the conventional hysteresis rule. A probabilistic neural network (PNN) is used for a pattern classifier. For a given environment, a training run is performed. A set of training patterns consists of averaged signal power samples (in dB) from nearby base stations within adjacent spatial windows. A probability of failure based on signal strength is defined and determines the possible serving base station(s) for each training class. A handoff is performed if a nearby class in the sequence of classes requires a handoff. Performance comparison of this handoff algorithm and the hysteresis rule is made for environments with four and five base stations. Simulation results indicate that for a fixed probability of failure, the pattern recognition-based handoff algorithm results in considerably fewer handoffs in comparison to the hysteresis rule. This reduction of unnecessary handoffs decreases the signalling load.