The co-channel interference (CCI) has become a major throughput-limiting factor for cell-edge users in the modern cellular systems such as the Long Term Evolution (LTE) network. Spatial interference whitening (IW) is an effective low-complexity linear method to suppress colored spatial interference in a MIMO setup. However, for CCIs using Alamouti's type of SFBC (space-frequency block coding) transmission, the conventional per-subcarrier IW fails to exploit the SFBC structure and thus is suboptimal. To address that, we propose an optimal method called enhanced interference whitening (eIW). eIW works on a properly combined augmented MIMO model for two subcarriers involved in the same SFBC encoding block and thus can fully exploit the interference structure. Simulations show that if the interfering cell uses SFBC significant performance gains can be achieved by eIW no matter whether or not SFBC is used for the serving cell.

In this paper, we develop a random access scheme which combines the widely used binary exponential backoff (BEB) algorithm with a cross-layer tree algorithm (TA) that relies on successive interference cancellation (SIC) with first success (FS). BEB and SICTA/FS complement each other nicely in enabling the novel protocol to attain a maximum stable throughput (MST) as high as 0.6 without packet loss. Although BEB-SICTA/FS avoids the deadlock problem caused by the error propagation commonly present in successive interference cancellation (SIC) algorithms, it may still suffer from deadlock effects induced by the "level skipping" caused by harsh wireless fading effects. We further develop a novel BEB-SICTA/F1 protocol, which is a modified version of BEB-SICTA/FS. Analysis and simulations demonstrate that this simple modification leads to high-throughput random access while completely avoiding deadlock problems.

We consider joint congestion and contention control for multihop wireless ad hoc networks, where the goal is to find optimal end-to-end source rates at the transport layer and per-link persistence probabilities at the medium access control (MAC) layer to maximize the aggregate source utility. The primal formulation of this problem is non-convex and non-separable. Under certain conditions, by applying appropriate transformations and introducing new variables, we obtain a decoupled and dual-decomposable convex formulation. For general non-logarithmic concave utilities, we develop a novel dual-based distributed algorithm using the subgradient method. In this algorithm, sources at the transport layer adjust their log rates to maximize their net benefits, while links at the MAC layer select transmission probabilities proportional to their conceived contribution to the system reward. The two layers are connected and coordinated by link prices. Our solutions enjoy the benefits of cross-layer optimization while maintaining the simplicity and modularity of the traditional layered architecture.

We consider joint congestion control and multiuser scheduling in a hybrid wireline and wireless network, where the air interface of wireless links is based on orthogonal frequency division multiplexing (OFDM). For static channels, we formulate this cross-layer design as a network utility maximization (NUM) problem with both wireline and wireless link constraints. The convexity of the problem enables a well-established dual-based approach to decompose it into two subproblems, the transport layer source rate adaptation and the medium access control (MAC) layer multiuser OFDM scheduling, which are connected and coordinated by link prices. While the rate and link price adjustments follow the same fashion as the conventional utility-based congestion control for wireline networks, the key difference is the multiuser OFDM scheduling performed at the wireless access point (AP). Independent from specific utilities used by each source, this scheduling problem always maximizes a wireless link-price-weighted sum throughput (LPWST), which can be solved efficiently by a block-coordinate descent method, resulting in optimal subcarrier assignment and power allocation at the AP. Convergence of the dual-based algorithm is established using the convex optimization theory. To extend our results to dynamic wireless channels, we provide a NUM formulation with long-term average feasible rate region and develop a gradient scheduling algorithm to handle channel variations. Our work represents a systematic cross-design framework for distributed fair resource allocation in a hybrid network with both static and dynamic wireless channels.

A novel random access protocol combining a tree algorithm (TA) with successive interference cancellation (SIC) has been introduced recently. To mitigate the deadlock problem of SICTA arising in error-prone wireless networks, we put forth a SICTA with first success (SICTA/FS) protocol, which is capable of high throughput while requiring limited-sensing and gaining robustness to errors relative to SICTA.

Well appreciated at the physical layer, user cooperation is introduced here as a diversity enabler for wireless random access (RA) at the medium access control sublayer. This is accomplished through a two-phase protocol in which active users start with a low power transmission attempting to reach nearby users and follow up with a high power transmission in cooperation with the users recruited in the first phase. We show that such a cooperative protocol yields a significant increase in throughput. Specifically, we prove that for networks with a large number of users, the throughput of a cooperative wireless RA network operating over Rayleigh-fading links approaches the throughput of an RA network operating over additive white Gaussian noise links—thus justifying the title of the paper. The message borne out of this result is that user cooperation offers a viable choice for migrating diversity benefits to the wireless RA regime, thus bridging the gap to wireline RA networks, without incurring a bandwidth or energy penalty.

Random access is well motivated and has been widely applied when the network traffic is bursty and the expected throughput is not high. The main reason behind relatively low-throughput expectations is that collided packets are typically discarded. In this paper, we develop a novel protocol exploiting successive interference cancellation (SIC) in a tree algorithm (TA), where collided packets are reserved for reuse. Our SICTA protocol can achieve markedly higher maximum stable throughput relative to existing alternatives. Throughput performance is analyzed for general$d$-ary SICTA with both gated and window access. It is shown that the throughput for$d$-ary SICTA with gated access is about$(\ln d)/(d-1)$, and can reach$0.693$for$d=2$. This represents a 40% increase over the renowned first-come-first-serve (FCFS)$0.487$tree algorithm. Delay performance is also analyzed for SICTA with gated access, and numerical results are provided.

In this correspondence, we deal with noncoherent communications over multiple-input-multiple-output (MIMO) wireless links. For a Rayleigh flat block-fading channel with M transmit- and N receive-antennas and a channel coherence interval of length T, it is well known that for TGtM, or, at high signal-to-noise-ratio (SNR) rhoGt1 and Mlesmin{N,lfloorT/2rfloor}, unitary space-time modulation (USTM) is capacity-achieving, but incurs exponential demodulation complexity in T. On the other hand, conventional training-based schemes that rely on known pilot symbols for channel estimation simplify the receiver design, but they induce certain SNR loss. To achieve desirable tradeoffs between performance and complexity, we propose a novel training approach where USTM symbols over a short length Ttau(tauis a small fraction of T, and recovers part of the SNR loss experienced by the conventional training-based schemes. When rhorarrinfin and TgesTtauges2M=2Nrarrinfin, but the ratios alpha=M/T, alpha1=Ttau/T are fixed, we obtain analytical expressions of the asymptotic SNR loss for both the conventional and new training-based approaches, serving as a guideline for practical designs

We investigate the potential for high throughput when combining random backoff schemes with a robust cross-layer tree algorithm (TA) for wireless random access. We first develop a BEB-SICTA/FS protocol which combines the binary exponential backoff (BEB) algorithm with a recently proposed SICTA/FS protocol saturation throughput analysis of BEB-SICTA/FS motivates the combined protocol herein because: 1) by using the practically feasible SICTA/FS to resolve collisions in a conventional BEB based protocol for wireless random access, we can achieve high throughput; and 2) BEB can sufficiently reduce the collision size and thus enhance the efficiency of SICTA/FS, since SICTA/FS is more efficient when the number of initially collided packets is small. Guided by our analysis, we further put forth a GBEB-SICTA/FS protocol which is capable of higher and more steadfast saturation throughput than BEB-SICTA/FS. Finally, we tailor our protocols for the IEEE 802.16 broadband wireless access (BWA) networks and test their performance through simulations

We analyze the stability properties of slotted Aloha with capture for random access over fading channels with infinitely-many users. We assume that each user node knows only its own uplink channel gain, and uses this decentralized channel state information (CSI) to perform power control and/or probability control. The maximum stable throughput (MST) for a general capture model is obtained by means of drift analysis on the backlog Markov chain. We then specialize our general result to a signal-to-interference-plus-noise ratio (SINR) capture model. Our analysis shows that if the channels of all users are identical and independently distributed (i.i.d.) with finite means, the system is unstable under any kind of power and probability control mechanism that is based only on decentralized CSI.

Relative to a centralized operation, opportunistic medium access capitalizing on decentralized multiuser diversity in a channel-aware homogeneous slotted Aloha system with analog-amplitude channels has been shown to incur only partial loss in throughput due to contention. In this context, we provide sufficient conditions for stability as well as upper bounds on average queue sizes, and address three equally important questions. The first one is whether there exist decentralized scheduling algorithms for homogeneous users with higher throughputs than available ones. We prove that binary scheduling maximizes the sum-throughput. The second issue pertains to heterogeneous systems where users may have different channel statistics. Here we establish that binary scheduling not only maximizes the sum of the logs of the average throughputs, but also asymptotically guarantees fairness among users. The last issue we address is extending the results to finite state Markov chain (FSMC) channels. We provide a convex formulation of the corresponding throughput optimization problem, and derive a simple binary-like access strategy

A novel random access protocol combining a tree algorithm (TA) with successive interference cancellation (SIC) has been introduced recently. By migrating physical layer benefits to the medium access control (MAC) through a cross-layer approach, SICTA can afford stable throughput as high as 0.693. However, SICTA may lead to deadlocks caused by channel fading and error propagation in error-prone wireless networks. To mitigate such effects, we put forth a truncated version of SICTA that we term SICTA/FS (SICTA with first success). We establish using analysis and simulations that while providing high throughput, SICTA/FS is robust to errors, it is easy to implement, and can be readily incorporated to existing standards.

Cooperative wireless communication systems have attracted much attention in recent years, due to the diversity advantage they can afford. Existing cooperative transmission modalities have been developed in conjunction with fixed-rate multiplexing based on TDMA, CDMA or FDMA. We advocate user cooperation as the method of choice for enabling diversity in wireless random access networks. The specific protocol developed herein exploits the fact that user cooperation can be viewed as a form of multipath, and capitalizes on the suitability of long pseudo-noise (PN) spreading codes for dealing with multipath channels. Analysis and numerical results confirm that throughput increases considerably when random access via spread-spectrum slotted Aloha protocols is aided by user collaboration.

Random access Aloha protocols have well documented merits in terms of simplicity and favorable delay-throughput trade-off under moderate bursty traffic loads. Short spreading codes have been used in conjunction with random access to endow Aloha with benefits originating from spread-spectrum communications. Instead of short, symbol-periodic spreading, this paper considers long pseudo-random (PN) packet-periodic sequences in the context of spread-Aloha and establishes that long PN codes increase the maximum stable throughput by reducing the probability of collisions. Relying on a dominant system approach, we analyze the resultant throughput and demonstrate that increasing the PN code length quickly transforms the collision-limited channel to an interference-limited one. In particular, we investigate how throughput depends on user load and packet length. Finally, we discuss synchronization issues and provide corroborating numerical results.

Contention tree algorithms have provable stability properties, and are known to achieve stable throughput as high as 0.487 for the infinite population Poisson model. A common feature in all these random access protocols is that collided packets at the receive-node are always discarded. In this paper, we derive a novel tree algorithm (TA) that we naturally term SICTA because it relies on successive interference cancellation to resolve collided packets. Performance metrics including throughput and delay are analyzed to establish that SICTA outperforms existing contention tree algorithms reaching 0.693 in stable throughput.

We develop a model for decentralized opportunistic transmissions of a finite (n) user slotted ALOHA system that relies on decentralized channel state information (D-CSI). D-CSI refers to each user node having access only to its own uplink channel gain. Every user adapts both rate and re-transmission probability to D-CSI in every slot. Due to the variable transmission rate from slot to slot, incoming packets with fixed length are re-assembled to deliver as many bits as possible per slot. By modeling the number of bits in queues as a discrete Markov chain, we analyze the stability region and the maximum stable throughput (MST). In addition, we derive a decentralized opportunistic scheme and show that it achieves a fraction (1-1/n)/sup n-1/ of its centralized counterpart's throughput, where (1-1/n)/sup n-1/ is a factor due to the inherent contention in random access.

We analyze the stability properties of slotted Aloha with capture for random access over fading channels when the number of users is very large. We consider the most commonly used power capture model, namely the one based on signal-to-interference-plus-noise ratio (SINK). Existing works have shown that for this model the capture capability alone can stabilize the slotted Aloha when the distribution of the received user power satisfies certain conditions. However, our analysis shows that as long as the received powers of all the users are independent and identically distributed with finite mean, the system is unstable. We further establish that power or probability control based on decentralized channel state information (CSI) can not render an infinite-user system stable.