Multi-band splicing offers a promising solution to extend existing band-limited communication systems to support high-precision sensing applications. This technique involves performing narrow-band measurements at multiple center frequencies, which are then combined to effectively increase the bandwidth without changing the sampling rate. In this paper, we introduce a mmWave channel sounder based on multiband splicing, leveraging the sparse nature of wireless channels through compressed sensing and sparse recovery techniques for channel reconstruction. We focus on three sparse recovery methods: the widely used grid-based orthogonal matching pursuit (OMP) algorithm as a baseline, our newly developed two-stage mmSplicer algorithm, which extends the OMP method by introducing an additional stage for improving its performance for our application, and our adaptation of sparse reconstruction by separable approximation (SpaRSA), named Net-SpaRSA, optimized for wireless applications. All three algorithms are integrated into an experimental OFDM-based IEEE 802.11ac system. Our analysis centers on evaluating the performance of these algorithms under limited number of narrow-band measurements, demonstrating that accurate CIR estimation is achievable even using only 50% of the full wideband spectrum. Additionally, we analyze and compare the computational complexity of these algorithms to assess their practical feasibility

In this paper we study feedback mechanisms for unsourced random access (URA) communications. We propose an algorithm to construct feedback packets broadcasted to the users by the base station (BS) and a feedback packet format that allows the users to estimate their channels and infer positive or negative feedback based on the presented thresholding algorithms. We show that the proposed feedback technique leads to a substantial reduction in the packet error rates and signal-to-noise ratios (SNRs) required to support various numbers of active users. We also demonstrate that the proposed feedback imposes a much smaller complexity burden on the users compared to the feedback that acknowledges only successful or only all undecoded users. Finally, we present a theoretical analysis framework that closely matches the experimental results.

Non-orthogonal multiple access has been extensively discussed in New Radio (NR), the study item working on 5G air interface in 3GPP. Sparse code multiple access (SCMA) is one of the proposed MA schemes. In this paper, the basic concept of SCMA is introduced, including codebook mapping, multiple access procedure, and advanced receivers. Then, link level simulations are applied to verify the design of SCMA, which show that SCMA has many excellent properties: shaping and diversity gain by sparse codebooks, resilient to inter- user interference, and robust to codebook collision, thus it is a promising candidate MA scheme for 5G.

The fifth generation of cellular wireless networks known as 5G is based on user-centric non-cellular concept where the users are surrounded by many network nodes cooperating to serve the users providing a “cell-center” experience throughout the network. Traditional multi point cooperation techniques often rely on centralized coordination and require different and often stringent requirement on the central controller, backhaul capacity and overall network synchronization. A novel two tiered, open loop and distributed cooperation technique is proposed in this paper where the lower tier with fixed or slowly changing parameters and preferably using SCMA provide a ubiquitous performance to mobile users and users exposed to multiple network nodes, while the higher tier provides service to the users close to the network nodes and maintain the overall network throughput. Users scheduled by the lower tier signaling use joint detection techniques from multiple network nodes. Other users scheduled to the higher tier jointly decode the lower tier signal from one or multiple network nodes before proceeding with the detection of their intended signal. The proposed algorithm is based on distributed scheduling and imposes limited requirements on the central controller and backhaul capacity and does not require stringent network time and frequency synchronization among network nodes. Unlike traditional cooperation techniques, the proposed method is open loop and requires very limited feedback and signaling overhead. Performance evaluations show that the proposed technique provides significant network coverage enhancement especially for high speed mobile users.

Sparse code multiple access (SCMA) is a code domain non-orthogonal multiple-access technique introduced for future 5G wireless networks. This paper describes the basic ideas of SCMA, the SCMA codebook design, the encoder and decoder, as well as the SCMA enabled transmission schemes for different application scenarios, including uplink grant-free contention-based transmission, downlink multi-user superposition, and downlink open-loop CoMP for ultra-dense networks even with moving users. We shall show from design principles and application examples that SCMA can resolve some major issues of current wireless systems and establish itself as a strong candidate for 5G networks.

Sparse Code Multiple Access (SCMA), a non-orthogonal multiple access scheme, has been introduced as a key 5G technology to improve spectral efficiency. In this work, we propose SCMA to enable open-loop coordinated multipoint (CoMP) joint transmission (JT). The scheme combines CoMP techniques with multi-user SCMA (MU-SCMA) in downlink. This scheme provides open-loop user multiplexing and JT in power and code domains, with robustness to mobility and low overhead of channel state information (CSI) acquisition. The combined scheme is called MU-SCMA- CoMP, in which SCMA layers and transmit power of multiple transmit points (TPs) are shared among multiple users while a user may receive multiple SCMA layers from multiple TPs within a CoMP cluster. The benefits of the proposed scheme includes: i) drastic overhead reduction of CSI acquisition, ii) significant increase in throughput and coverage, and iii) robustness to channel aging. Various algorithms of MU-SCMA-CoMP are presented, including the detection strategy, power sharing optimization, and scheduling. System level evaluation shows that the proposed schemes provide significant throughput and coverage gains over OFDMA for both pedestrian and vehicular users.

Sparse code multiple access (SCMA) is a new frequency domain non-orthogonal multiple-access technique which can improve spectral efficiency of wireless radio access. With SCMA, different incoming data streams are directly mapped to codewords of different multi-dimensional cookbooks, where each codeword represents a spread transmission layer. Multiple SCMA layers share the same time-frequency resources of OFDMA. The sparsity of codewords makes the near-optimal detection feasible through iterative message passing algorithm (MPA). Such low complexity of multi-layer detection allows excessive codeword overloading in which the dimension of multiplexed layers exceeds the dimension of codewords. Optimization of overloading factor along with modulation-coding levels of layers provides a more flexible and efficient link-adaptation mechanism. On the other hand, the signal spreading feature of SCMA can improve link-adaptation as a result of less colored interference. In this paper a technique is developed to enable multi-user SCMA (MU-SCMA) for downlink wireless access. User pairing, power sharing, rate adjustment, and scheduling algorithms are designed to improve the downlink throughput of a heavily loaded network. The advantage of SCMA spreading for lightly loaded networks is also evaluated.

Multicarrier CDMA is a multiple access scheme in which modulated QAM symbols are spread over OFDMA tones by using a generally complex spreading sequence. Effectively, a QAM symbol is repeated over multiple tones. Low density signature (LDS) is a version of CDMA with low density spreading sequences allowing us to take advantage of a near optimal message passing algorithm (MPA) receiver with practically feasible complexity. Sparse code multiple access (SCMA) is a multi-dimensional codebook-based non-orthogonal spreading technique. In SCMA, the procedure of bit to QAM symbol mapping and spreading are combined together and incoming bits are directly mapped to multi-dimensional codewords of SCMA codebook sets. Each layer has its dedicated codebook. Shaping gain of a multi-dimensional constellation is one of the main sources of the performance improvement in comparison to the simple repetition of QAM symbols in LDS. Meanwhile, like LDS, SCMA enjoys the low complexity reception techniques due to the sparsity of SCMA codewords. In this paper a systematic approach is proposed to design SCMA codebooks mainly based on the design principles of lattice constellations. Simulation results are presented to show the performance gain of SCMA compared to LDS and OFDMA.

This paper investigates the benefits of amplify-and-forward (AF) relaying in the setup of multiantenna wireless networks. For this purpose, random sequential (RS) relaying is studied. It is shown that random unitary matrix multiplication at the relay nodes empowers the RS scheme to achieve a better diversity-multiplexing tradeoff (DMT) as compared to the traditional AF relaying. First, the RS scheme is proved to achieve the optimum DMT for a multiantenna full-duplex single-relay two-hop network. Applying this result, a new achievable DMT is derived for the case of multiantenna half-duplex parallel relay network. Interestingly, it turns out that the DMT of the RS scheme is optimum for the case of multiantenna two parallel noninterfering half-duplex relays. Furthermore, random unitary matrix multiplication is shown to also improve the DMT of the nonorthogonal AF relaying scheme for the case of a multiantenna single relay channel. Finally, the general case of multiantenna full-duplex relay networks is studied. First, a new lower-bound is derived on its DMT using the RS scheme. Furthermore, maximum multiplexing gain of the network is also shown to be achievable by traditional amplify-forward relaying. The gain value is equal to the minimum vertex cut-set of the underlying graph of the network, which can be computed in polynomial time in terms of the number of network nodes.

This paper deals with the delay-throughput analysis of a single-hop wireless network with n transmitter/receiver pairs. All channels are assumed to be block Rayleigh fading with shadowing, described by parameters (α, ω̅), where α denotes the probability of shadowing and ω̅ represents the average cross-link gains. The analysis relies on the distributed on-off power allocation strategy (i.e., links with a direct channel gain above a certain threshold transmit at full power and the rest remain silent) for the deterministic and stochastic packet arrival processes. It is also assumed that each transmitter has a buffer size of one packet and dropping occurs once a packet arrives in the buffer while the previous packet has not been served. In the first part of the paper, we define a new notion of performance in the network, called effective throughput, which captures the effect of arrival process in the network throughput, and maximize it for different cases of packet arrival process. It is proved that the effective throughput of the network asymptotically scales as (log n)/α̂, with α̂=△ αω̅, regardless of the packet arrival process. In the second part of the paper, we present the delay characteristics of the underlying network in terms of the packet dropping probability. We derive the sufficient conditions in the asymptotic case of n → ∞ such that the packet dropping probability tend to zero, while achieving the maximum effective throughput of the network. Finally, we study the trade-off between the effective throughput, delay, and packet dropping probability of the network for different packet arrival processes. In particular, we determine how much degradation will be enforced in the throughput by introducing the aforementioned constraints.

In this paper, we consider a downlink communication system in which a base station (BS) equipped with M antennas and power constraint P communicates with N users each equipped with K receive antennas. It is assumed that the users have perfect channel state information (CSI) of their own channels, while the BS only knows the partial CSI provided by the receivers via a feedback channel. We study the fundamental limits on the amount of feedback required at the BS to achieve the sum-rate capacity of the system (when BS has perfect CSI for all users) in the asymptotic case of N →∞, considering various signal to noise ratio (SNR) regimes. The main results of this paper can be expressed as follows. 1) In the fixed-SNR regime (where the SNR does not scale with N) and low-SNR regime (where the SNR is much smaller than 1/In(N) ), to achieve the (1 - ε)-portion of the sum-rate capacity, the total amount of feedback should scale at least with (ε-1). In the fixed-SNR regime, to reduce the gap between the achievable sum rate and the sum-rate capacity of the system (which is defined as the sum-rate gap) to zero, the amount of feedback should scale at least logarithmically with the sum-rate capacity, which is achievable by using the random beam-forming (RBF) scheme proposed by Sharif and Hassibi. In the low-SNR regime, we propose an opportunistic beam-forming (OBF) scheme, which is shown to be asymptotically feedback optimal. 2) In the high-SNR regime (where the SNR grows to infinity as N → ∞), the total amount of feedback depends on the number of receive antennas. In particular, to reduce the sum-rate gap to zero in the case of K; 1/M - 1, should scale at least logarithmically with the SNR. In the case of K ≥ M , the amount of feedback does not need to scale with the SNR. Moreover, we show that RBF is asymptotically feedback optimal in the high-SNR regime.

For a multicast network, in which a common source is transmitted to$N$users, the problem of maximizing the average rate subject to a coverage constraint (minimum quality of service) is studied. Considering such a network with single-antenna nodes and assuming that the channel state information is available only at the receiver side, the highest expected rate achievable by a random user in the network, called the expected typical rate, is derived in two scenarios: hard coverage constraint and soft coverage constraint. In the first case, the coverage is expressed in terms of the outage probability, while in the second case, the expected rate should satisfy a certain minimum requirement. It is shown that the optimum solution (achieving the highest expected typical rate for given coverage requirements) in both cases is achieved by an infinite layer superposition code for which the optimal power allocation among different layers is derived. The analysis is extended to a scenario with multiple transmit antennas. For the MISO case, a suboptimal coding scheme is proposed, which is shown to be asymptotically optimal, when the number of transmit antennas grows at least logarithmically with the number of users in the network.

This paper considers a two-hop network in which information is transmitted from a source via a relay to a destination. It is assumed that channels are quasi-static fading with additive white Gaussian noise and that all nodes are equipped with a single antenna. The channel state information (CSI) of each hop is available only at the corresponding receiver and relay is not capable of data buffering over multiple coding blocks. One commonly used design criterion in such configurations is the maximization of the average received rate at the destination. Considering infinite-layer coding at both the source and the relay, in conjunction with decode and forward strategy at the relay, we present a procedure to optimally distribute the available source and relay powers to different layers of their corresponding codes. Next, we demonstrate how this transmission technique can be generalized to a multihop setting. Assuming Rayleigh fading, the performance of the proposed coding scheme is evaluated for a two-hop network and compared with the performance of previously known strategies.

This paper studies the setup of a parallel MIMO relay network in which K relays, each equipped with N antennas, assist in transmitting data between a source and a destination, each equipped with M antennas (M ≤ N), in the half-duplex mode. It is assumed that there is no direct source-destination link and the communication is performed in two hops. An amplify-and-forward relaying scheme called Incremental Cooperative Beamforming Scheme (ICBS) is introduced and shown to achieve the capacity of the network in the asymptotic case of K → ∞ with a gap scaling of at most [(log(K))/({4}√{K})]. This result is shown to hold as long as the power of each relay is significantly larger than [(log3(K) log(log(K)))/(K)]. In addition, in the case that the source power is equal to the relays' power and both tend to infinity, the proposed scheme is shown to achieve the full multiplexing gain regardless of the number of relays. Simulation results confirm the validity of analytical arguments.

In this paper, we consider a decentralized wireless communication network with a fixed numberuof frequency subbands to be shared amongNtransmitter-receiver pairs. It is assumed that the number of active users is a realization of a random variable with a given probability mass function. Moreover, users are unaware of each other's codebooks and hence, no multiuser detection is possible. We propose a randomized frequency hopping (FH) scheme in which each transmitter randomly hops over a subset ofusubbands from transmission slot to transmission slot. Assuming all users transmit Gaussian signals, the distribution of the noise plus interference is mixed Gaussian, which makes calculation of the mutual information between the transmitted and received signals of each user intractable. We derive lower and upper bounds on the mutual information of each user and demonstrate that, for large signal-to-noise ratio (SNR) values, the two bounds coincide. This observation enables us to compute the sum multiplexing gain of the system and obtain the optimum hopping strategy for maximizing this quantity. We compare the performance of the FH system to that of the frequency division (FD) system in terms of the following performance measures: average sum multiplexing gain (η(1)) and average minimum multiplexing gain per user (η(2)). We show that (depending on the probability mass function of the number of active users) the FH system can offer a significant improvement in terms of η(1)and η(2)(implying a more efficient usage of the spectrum). In the sequel, we consider a scenario where the transmitters are unaware of the number of active users in the network as well as the channel gains. Developing a new upper bound on the differential entropy of a mixed Gaussian random vector and using entropy power inequality, we obtain lower bounds on the maximum transmission rate per user to ensure a specified outage probability at a given SNR level. We demonstrate that the so-called outage capacity can be considerably higher in the FH scheme than in the FD scenario for reasonable distributions on the number of active users. This guarantees a higher spectral efficiency in FH compared to FD.

In this paper, we consider a distributed one-hop wireless network with n pairs of transmitters and receivers. It is assumed that each transmitter/receiver node is only connected to k receiver/transmitter nodes which are defined as neighboring nodes. The channel between the neighboring nodes is assumed to be Rayleigh fading. The objective is to find the maximum achievable sum-rate of the network in the asymptotic case of n, k ¿ ¿. It is shown that the asymptotic throughput of the system scales as n log k/k. An opportunistic on-off scheduling is proposed and shown to be asymptotically throughput optimal.

This paper studies the setup of a multiple-relay network in which$K$half-duplex multiple-antenna relays assist in the transmission between either one or several multiple-antenna transmitter(s) and a multiple-antenna receiver. Each two nodes are assumed to be either connected through a quasi-static Rayleigh-fading channel, or disconnected. We propose a new scheme, which we callrandom sequential(RS), based on the amplify-and-forward relaying. We prove that for general multiple-antenna multiple-relay networks, the proposed scheme achieves the maximum diversity gain. Furthermore, we derive diversity–multiplexing tradeoff (DMT) of the proposed RS scheme for general single-antenna multiple-relay networks. It is shown that for single-antenna two-hop multiple-access multiple-relay$(K > 1)$networks (without direct link between the transmitter(s) and the receiver), the proposed RS scheme achieves the optimum DMT. However, for the case of multiple-access single-relay setup, we show that the RS scheme reduces to the naive amplify-and-forward (AF) relaying and is not optimum in terms of DMT, while the dynamic decode-and-forward (DF) scheme is shown to be optimum for this scenario.

In this paper, a downlink system in which a single-antenna base station communicates with k single antenna users over a time-correlated fading channel is considered. It is assumed that each receiver knows its own channel state, while the rate of the channel variation for all users and the corresponding initial fading gains are known to the base station. The average (per channel use) throughput of the system is studied by applying various adaptive signaling schemes. Assuming a large number of users in the system, it is shown that using a scheduling scheme in which the base station transmits to the user with the maximum initial fading gain, while using a fixed codeword length for all users, achieves the order of the maximum throughput. Moreover, an alternative scheduling scheme is proposed (by accounting for users' delays) and shown to achieve the optimum long-term fairness, while preserving the order of the maximum throughput.

In this paper, a downlink communication system, in which a base station (BS) equipped with M antennas communicates with N (N Gt 1) single-antenna users, in a Rician fading environment is considered. The asymptotic (in terms of the number of users) sum-rate capacity of the system, as well as the capacity-achieving strategies, are derived. The main results of the paper are as follows: i) in the region of K = o(log N), where K denotes the Rician factor, the sum-rate capacity scales as M log(1 + P/Meta), where P denotes the SNR and eta =Deltalog N/1+K, which is achieved by zero-forcing beam-forming (ZFBF) along with a low-complexity user selection algorithm that considers only the scattered component of the userspsila channels, ii) in the region K = omega(log N), in the case of co-located transmit antennas, the capacity scales as log(1+MP), which is achieved by time division multiple access (TDMA), iii) in the region K = omega(log N), in the case of isotropically-distributed specular components, the sum-rate capacity behaves as M log(1 + P), which is achieved by ZFBF, along with a user selection algorithm that considers only the specular component of the userspsila channels.

In this paper, a MIMO Broadcast Channel (MIMO-BC) with large (K) number of users is considered. It is assumed that all users have a hard delay constraint D. We propose a scheduling algorithm for maximizing the throughput of the system, while satisfying the delay constraint for all users. It is proved that by using the proposed algorithm, it is possible to achieve the maximum throughput and maximum fairness in the network, simultaneously, in the asymptotic case of K rarr infin. We introduce a new performance metric in the network, called "minimum average throughput", and prove that the proposed algorithm is capable of maximizing the minimum average throughput in a MIMO-BC, in the asymptotic case of K rarr infin. Finally, it is established that the proposed algorithm reaches the boundaries of the capacity region and stability region of the network, simultaneously, in the asymptotic case of K rarr infin.

We consider a wireless communication network with a fixed number of frequency sub-bands to be shared among several transmitter-receiver pairs. In traditional frequency division (FD) systems, the available sub-bands are partitioned into disjoint clusters (frequency bands) and assigned to different users (each user transmits only in its own band). If the number of users sharing the spectrum is random, this technique may lead to inefficient spectrum utilization (a considerable fraction of the bands may remain empty most of the time). In addition, this approach inherently requires either a central network controller for frequency allocation, or cognitive radios which sense and occupy the empty bands in a dynamic fashion. These shortcomings motivate us to look for a decentralized scheme (without using cognitive radios) which allows the users to coexist, while utilizing the spectrum efficiently. We consider a frequency hopping (FH) scheme (with iid Gaussian code-books) where each user transmits over a selection of sub-bands and hops to another selection (with the same cardinality) from transmission to transmission. We derive lower and upper bounds on the achievable rate of each user and demonstrate that for large signal-to-noise ratio (SNR) values, the two bounds coincide. This observation enables us to compute the sum-rate multiplexing gain (SMG) of the system. Subsequently, we show how each user can regulate its rate to guarantee fairness while maximizing SMG. We compare the FH and FD systems in terms of the following performance measures: average sum-rate multiplexing gain (eta1), average multiplexing gain per user (eta2), the minimum multiplexing gain per user (eta3) and service capability. We show that (depending on the probability mass function of the number of active users), the FH system can offer a significant improvement in terms of eta1and eta2(implying a more efficient usage of the spectrum). It is also shown that 1/epsi les eta3(FH)/eta3(FD)les 1, i.e., the loss incurred in eta3is not more than 1/epsi . Finally, computation of the so-called service capability shows that in FH systems any number of users can coexist fairly, while the maximum number of users in FD system is limited by the number of available bands.

In this paper, a downlink communication system, in which a base station (BS) equipped with antennas communicates with users each equipped with receive antennas, is considered. An efficient suboptimum algorithm is proposed for selecting a set of users in order to maximize the sum-rate throughput of the system, in a Rayleigh-fading environment. For the asymptotic case when tends to infinity, the necessary and sufficient conditions in order to achieve the maximum sum-rate throughput, such that the difference between the achievable sum-rate and the maximum value approaches zero, is derived. The complexity of our algorithm is investigated in terms of the required amount of feedback from the users to the BS, as well as the number of searches required for selecting the users. It is shown that the proposed method is capable of achieving a large portion of the sum-rate capacity, with a very low complexity.

A single-hop wireless network with K links is considered, where the links are partitioned into M clusters, each operating in a subchannel with bandwidth W/M. We assume that the links in each cluster perform the on-off power allocation strategy proposed in [1]. The problem is to analyze the average sum-rate of the network in terms of M and under the shadow- fading effect with probability a. It is demonstrated that for M ~ o(K) and 0 < alpha les 1, where alpha is a fixed parameter, the average sum-rate of the network scales as W/alpha log K. For M ~ Theta(K), we present an upper bound for the average sum-rate. It is proved that the maximum average sum-rate of the network for every value of 0 < alpha les 1 is achieved at M = 1. In fact, in the proposed model, partitioning the bandwidth W into M subchannels has no gain in terms of enhancing the throughput.

In this paper, an asymptotic analysis for the delay-throughput of a single-hop wireless network with n pairs of nodes is presented. The analysis relies on the decentralized on-off power allocation strategy, in which the on-off transmission policy for each link is based on comparing its direct channel gain with optimum threshold τn. We first provide a new definition of the transmission delay in a homogenous network. It is proved that the delay threshold level that results the dropping probability for each link tends to zero, while achieving the maximum average sum-rate scales as ω(n / log n). Also, the minimum delay in order to make the dropping probability for the whole network approach zero scales as ω(n / log n) + n. Furthermore, we drive lower and upper bounds for the link activation probability, q, such that the order of the average sum-rate is preserved. Based on the upper bound on q, an asymptotic analysis shows that the delay in each link and in the network improves without any significant impact on the the average sum-rate. Finally, we present a new definition of the throughput for the link in the cases of one and infinite buffer size. It is demonstrated that the maximum average throughput of the network with the decentralized on-off power allocation strategy is independent of the buffer size.

A wireless multicast network with a stringent decoding delay constraint and a minimum required multicast data rate is characterized. Assuming the channel state information is available only at the receiver sides, and a single antenna system, the optimal expected rate achievable by a random user in the network is derived in terms of the minimum multicast requirement in two scenarios: hard coverage constraint and soft coverage constraint. In the first case, the minimum multicast requirement is expressed by multicast outage capacity while in the second case, the expected multicast rate should satisfy the minimum requirements. Also, the optimum power allocation in an infinite layer superposition code, achieving the highest expected typical rate, is derived subject to the coverage constraints. For the MISO case, a suboptimal coding scheme is proposed, which is shown to be asymptotically optimal, when the number of transmit antennas grows at least logarithmically with the number of users in the network.

In this paper, the diversity-multiplexing trade-off is derived for a one-dimensional equally-spaced Rayleigh fading ad-hoc network. It is assumed that the interference from each link to the other links in the network declines exponentially with the distance, such that the attenuation between two neighbor links is rhoalpha0. For any given multiplexing gain r, the maximum diversity gain is achieved by utilizing a general time-sharing scheme. We obtain an explicit formula for the maximum diversity gain, and show that depending on the value of r and alpha0, there is an optimum time-sharing factor which yields the maximum diversity gain.

In this paper, a downlink communication system, in which a base station (BS) equipped with M antennas communicates with N users each equipped with K receive antennas is considered. We study the minimum required amount of feedback at the BS, in order to achieve the maximum sum-rate capacity. First, we define the amount of feedback as the average number of users who send information to the BS. In the asymptotic case of N rarr infin, we show that with finite amount of feedback, it is not possible to achieve the maximum sum-rate. Indeed, in order to reduce the gap between the achieve sum-rate and the optimum value to zero, a minimum feedback of ln ln ln N is asymptotically necessary. Then, we consider a practical scenario, in which the amount of feedback is defined as the average number of bits which is sent to the BS. We show that to achieve the maximum sum-rate, infinite amount of feedback is required. Moreover, the minimum amount of feedback, in order to reduce the gap to the optimum sum-rate to zero, scales as otimes(ln ln ln N), which is achievable by the random beam-forming scheme proposed in M. Sharif and B. Hassibi, (2005)

In this paper, a downlink scenario in which a single-antenna base station communicates with K single antenna users, over a time-correlated fading channel, is considered. It is assumed that channel state information is perfectly known at each receiver, while the statistical characteristics of the fading process and the fading gain at the beginning of each frame are known to the transmitter. By evaluating the random coding error exponent of the time-correlated fading channel, we show that there is an optimal codeword length which maximizes the throughput. We examine the throughput of the conventional scheduling that transmits to the user with the maximum signal to noise ratio using both fixed length codewords and variable length codewords. Although optimizing the codeword length improves the performance, it is shown that using the conventional scheduling, a gap of Omega(radiclog log log K) exists between the achievable throughput and the maximum possible throughput of the system. We propose a simple scheduling that considers both the signal to noise ratio and the channel time variation. We show that by using this scheduling, the gap between the achievable throughput and the maximum throughput of the system approaches zero

In this paper, we consider a downlink communication system, in which a base station (BS) equipped with M antennas communicates with N users each equipped with K receive antennas. We propose an efficient suboptimum algorithm for selecting a set of users in order to maximize the sum-rate throughput of the system. For the asymptotic case of N rarr infin, it is shown that by using a very simple preceding scheme of zero-forcing beam-forming, the optimum sum-rate which behaves like M log log N can be achieved. The complexity of our algorithm is investigated in terms of the required amount of feedback from the users to the base station, as well as the number of searches required for selecting the users. It is shown that the proposed method is capable of achieving a large portion of the sum-rate capacity, with a very low complexity

In this paper, we consider a low-complexity iterative receiver for decoding multi-user information data for the bandwidth efficient convolutionally coded UWB TH-CDMA system, in a multipath channel. The proposed receiver structure consists of a bank of soft interference cancellers (SIC), followed by a block of MMSE filters. The outputs of these filters, are fed to a bank of likelihood calculators (LC), each followed by a soft-input soft-output (SISO) convolutional decoder. Information is fed back from SISO decoders to SICs for interference cancellation. This receiver requires the knowledge of the attenuation and delay of the strongest path of each active user. We evaluate the performance of the proposed receiver in multipath channels. Our numerical results show that the new receiver significantly improves the system performance.