Instantaneous Channel Realization Research Articles

DMT analysis and optimal scheduling for two-hop parallel relay networks

Two-hop parallel networks with half-duplex relays over quasi-static Rayleigh fading channels are considered. Assuming relays have only local channel state information, optimal communication schemes for these networks in terms of the diversity multiplexing trade-off (DMT) are obtained. It is shown that the dynamic quantize-map-and-forward (DQMF) and the static quantize-map-and-forward (SQMF) communication strategies achieve the optimal DMT at different ranges of the multiplexing gain. In the DQMF scheme, a relay sets its transmitting/listening times according to the instantaneous source-relay channel condition. However, in the SQMF scheme, a relay sets its transmitting/listening times to be a constant independent of the instantaneous source-relay channel gain. Essentially, we calculate the DMT of the network under the general setting for arbitrary average signal-to-noise ratios over links and it is shown that the DMT of the proposed schemes matches the upper bounds. Moreover, the optimal transmitting/listening times of the half-duplex relays as functions of the instantaneous channel realizations are derived for different communication strategies in the general setting. Finally, various simulation scenarios are considered and the DMT of the proposed schemes and the dynamic decode and forward communication strategy are compared.

End-to-End Distortion Analysis of Nonorthogonal Layered Coding Over Relay-Assisted Networks

This article investigates cross-layer design of nonorthogonal layered (NOL) coding schemes over relay-assisted downlink of cellular networks that incorporates with successive refinement (SR) source code for transmission of adaptive multiresolution multimedia content. Two novel-layered relaying strategies, namely NOL compress-forward (NOL-CF) and NOL decode-forward (NOL-DF) coding, concatenated to the SR source coding scheme are outlined. The network parameters are jointly optimized, to derive optimal end-to-end system performance. Taking the expected distortion (ED) as the end-to-end system performance metric, the explicit analytical solution achieving optimal ED under the proposed NOL-CF scheme with two layers is derived. For the NOL-DF coding, the numerical analysis demonstrates the merits of the proposed scheme, which dynamically adapts the transmission to the instantaneous channel realization in the absence of channel knowledge at the source node. The superiority of the proposed layered coding over several available nonorthogonal multiple access relaying schemes is demonstrated for Gaussian channels. Also, simulation results confirm the effectiveness of the proposed relay-assisted NOL codes to improve the ED over Rayleigh fading channel.

Secure Quadrature Channel Modulation

In this paper, a physical layer security (PLS) scheme based on quadrature channel modulation (QCM) called secure QCM (SQCM) is proposed for time division duplex (TDD) multi-input multi-output (MIMO) media based modulation (MBM) systems to resist eavesdropping. By exploiting the abundant legitimate channel realizations generated by the radio frequency (RF) mirrors, secure mappings between the original information bits and the transmit patterns are designed, where the transmit patterns include the transmit antenna activation pattern (AAP), mirror activation pattern (MAP) and symbol modulation pattern. The secure mappings used by the transmitter change for each transmission and are unavailable at the eavesdropper due to the independent fading of the legitimate link and the eavesdropping link. Based on the secure mappings, the proposed SQCM conveys the source information bits securely through the complex modulation symbol and instantaneous channel realizations by using a single RF chain. The ergodic secrecy rate is analyzed and a closed-form approximation is derived. Simulation results show that the closed-form approximation coincides with the experimental ergodic secrecy rate, and the achievable ergodic secrecy rate is not influenced by the number of eavesdropping antennas or the channel advantage of the eavesdropping link. Compared to the secure MBM scheme using the traditional artificial noise (AN) technique, the proposed SQCM scheme shows significant performance gain without consuming jamming power when the eavesdropper has more antennas than the legitimate receiver. Compared to the precoded MBM and probabilistic optimized MBM schemes exploiting the eavesdropper's CSI, the proposed SQCM scheme has better secrecy performance in all SNR region and medium-to-high SNR regions respectively without requiring the eavesdropper's CSI.

Constant Envelope Precoding With Adaptive Receiver Constellation in MISO Fading Channel

Constant envelope (CE) precoding is an appealing transmission technique which enables the realization of high power amplifier (PA) efficiency. For CE precoding in a single-user multiple-input single-output (MISO) channel, a desired constellation is feasible at the receiver if and only if it can be scaled to lie in an annulus, whose boundaries are characterized by the instantaneous channel realization. Therefore, if a fixed receiver constellation is used for CE precoding in a fading channel, where the annulus is time-varying, there is in general a non-zero probability of encountering a channel that makes CE precoding infeasible, thereby causing a high probability of error. To tackle this problem, this paper studies the adaptive receiver constellation design for CE precoding in a single-user MISO flat-fading channel with an arbitrary number of antennas at the transmitter. We first investigate the fixed-rate adaptive receiver constellation design to minimize the symbol error rate (SER). Specifically, an efficient algorithm is proposed to find the optimal amplitude-and-phase shift keying (APSK) constellation with two rings that is both feasible and of the maximum minimum Euclidean distance (MED), for any given constellation size and instantaneous channel realization. Numerical results show that by using the optimized fixed-rate adaptive receiver constellation, our proposed scheme achieves significantly improved SER performance over CE precoding with a fixed receiver constellation. Furthermore, based on the family of optimal fixed-rate adaptive two-ring APSK constellation sets, a variable-rate CE transmission scheme is proposed and numerically examined.

$(n,K)$-User Interference Channels: Degrees of Freedom

This paper analyzes the gains of opportunistic communication in multiuser interference channels. Consider a fully connected n-user Gaussian interference channel. At each time instance, only K≤n transmitters are allowed to be communicating with their respective receivers and the remaining (n-K) transmitter-receiver pairs remain inactive. For finite n, if the transmitters can acquire the instantaneous channel realizations and if all channel gains are bounded away from zero and infinity, the seminal results on interference alignment establish that for any K arbitrary active pairs the total number of spatial degrees of freedom per orthogonal time and frequency domain is K/2. In dense networks (n → ∞), however, as the size of the network increases, it becomes less likely to sustain the bounding conditions on the channel gains. By exploiting this fact, we show that when n obeys certain scaling laws, by opportunistically and dynamically selecting the K active pairs at each time instance, the number of degrees of freedom can exceed K/2 and in fact can be made arbitrarily close to K. More specifically, for single-antenna transmitters and receivers, the network size scaling as n ∈ ω(SNR <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d⌈d-1⌉</sup> ) when power allocation is allowed and scaling as n ∈ ω(SNR <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d(K-1)</sup> ) without power allocation are sufficient conditions for achieving d ∈ [1, K] degrees of freedom. Moreover, for achieving these degrees of freedom the transmitters do not require the knowledge of the instantaneous channel realizations. Hence, invoking opportunistic communication in the context of interference channels leads to achieving higher degrees of freedom that are not achievable otherwise. We extend the results for multi-antenna Gaussian interference channels.

Impact of CSI on Radio Resource Management Techniques for the OFDMA downlink

Adaptive resource allocation can drastically increase the throughput of an Orthogonal Frequency Division Multiple Access (OFDMA) system when the Channel State Information (CSI) is accurately known. Unfortunately, in practice, perfect CSI is rarely possible. In this paper, we consider adaptive sub-carrier assignment for downlink multiuser OFDMA systems, where the transmitter has no knowledge of the instantaneous channel realizations. The problem we address is maximizing the sum-capacity of the system subject to user Quality of Service (QoS) requirements. A heuristic algorithm presented in \cite{Zhang06} is modified in order to provide an enhanced sub-optimal solution. Numerical results show that resources can be adaptively allocated using statistical CSI (SCSI) and that such an approach allows for an important number of user QoS requirements to be met. Comparisons between the instantaneous CSI (ICSI) and SCSI based resource allocation schemes demonstrate that their performance difference is highly dependent on the number of active users present in the cell, the QoS constraint, and the transmit power.

A Rateless Coded Protocol for Half-Duplex Wireless Relay Channels

We propose a rateless coded protocol for a half-duplex wireless relay channel where all links experience independent quasi-static Rayleigh fading and the instantaneous channel realizations are unavailable at the transmitters. We assume that the network does not have a stringent delay constraint-thus the source and/or the relay continue transmitting until the destination acknowledges successful decoding. We identify rateless coded relaying as the natural choice, where each transmission from the source and/or the relay adds incremental redundancy to help the destination recover the original message. Our proposed protocol utilizes, in conjunction with rateless coding, a combination of the two popular relay cooperation schemes, namely decode-forward and compress-forward. Assuming very limited feedback from the destination, we derive the theoretical performance bounds specifically with binary phase-shift keying. We then implement the rateless coded relaying protocol using Raptor codes. The degree profiles for the Raptor codes are designed to maximize the average throughput-with the design formulated as a convex optimization problem. Using discretized density evolution for asymptotically large block lengths, the optimized codes lose approximately 5% in performance from the theoretical limit, whereas with practical finite block lengths, the performance loss is approximately 9%.

Dual-hop adaptive packet transmission systems with regenerative relaying

We propose and analyze two different approaches for performing adaptive modulation and coding (AMC) in dual-hop transmission systems, where a source node S communicates with a certain destination node D via an intermediate decode-and- forwarding relay station R. In this regard, we consider the most general case, where the AMC on the S-R and R-D links might be performed independently of each other, thus requiring a potential buffering of data packets at the relay station before they can be forwarded to the destination D. With our first approach, AMC is performed solely based on the instantaneous channel realizations, without considering the current buffer level at the relay station, what consequently might result in packet losses due to potential buffer overflows. Our second approach eliminates this problem by taking the current buffer level into account in the AMC transmission over the first hop and reducing the transmission rate accordingly if the buffer would overflow otherwise. Both schemes are analyzed analytically based on a finite-state Markov chain model of our system. Building upon this model, we derive analytical closed-form expressions for a variety of different key performance indicators, which are illustrated for various practical cases by means of selected numerical results.

CDIT-Based Constrained Resource Allocation for Mobile WiMAX Systems

Adaptive resource allocation has been shown to provide substantial performance gain in OFDMA-based wireless systems, such as WiMAX, when full channel state information (CSI) is available at the transmitter. However, in some fading environments (e.g., fast fading), there may not be a feedback link sufficiently fast to convey the CSI to the transmitter. In this paper, we consider resource allocation strategies for downlink multiuser mobile WiMAX systems, where the transmitter has only the channel distribution information (CDI), but no knowledge of the instantaneous channel realization. We address the problem of subchannel assignment and power allocation, to maximize the ergodic weighted-sum rate under long-term fairness, minimum data rate requirement and power budget constraints. This problem is formulated as a nonlinear stochastic constrained optimization problem. We provide an analytical solution based on the Lagrange dual decomposition framework. The proposed method has a complexity of (KM) for K users and M subchannels. Simulation results are provided to compare the performance of this method with other allocation schemes and to illustrate the tradeoff between maximized weighted-sum rate and the constraints.

Transmission Scheduling for Optimizing Sensor Network Lifetime: A Stochastic Shortest Path Approach

We present transmission scheduling algorithms for maximizing the lifetime of a sensor network. In each data collection, only one group of sensors are scheduled to transmit their measurements directly to an access point (AP) through a fading channel, causing a reduction in battery energy levels of these sensors. We formulate the problem of dynamically choosing which group of sensors should communicate with the AP to maximize network lifetime as a stochastic shortest path Markov decision process. We consider three types of channel information structure: global channel state information (CSI), channel statistics, and local CSI. For optimal scheduling using global CSI (i.e., the scheduler has the information of all sensors' instantaneous channel realizations), we propose an algorithm that obtains the optimal stationary scheduling policy with computational complexity reduced from exponential to polynomial in network size when the network is dense. Due to the large implementation overhead in acquiring global CSI, we consider scheduling algorithms that use channel statistics (i.e., each sensor only knows its own channel distribution) or local CSI (i.e., each sensor knows its own instantaneous channel realization). The optimal scheduling using channel statistics is formulated as a shortest path multiarmed bandit problem, which has an indexable optimal policy. We derive a closed-form expression for the Gittins index. We further show that the Gittins index is simply the residual energy when the channel fading is identically (but not necessarily independently) distributed across sensors, i.e., the optimal scheduling policy in this case is to choose the sensor with the most residual energy. Finally, we study an asymptotically optimal scheduling protocol using local CSI. This protocol has a distributed implementation yet achieves a comparable performance as the optimal scheduling using global CSI

Transmit antenna selection based on outage probability for correlated MIMO multiple access channels

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

On beamforming with finite rate feedback in multiple-antenna systems

We study a multiple-antenna system where the transmitter is equipped with quantized information about instantaneous channel realizations. Assuming that the transmitter uses the quantized information for beamforming, we derive a universal lower bound on the outage probability for any finite set of beamformers. The universal lower bound provides a concise characterization of the gain with each additional bit of feedback information regarding the channel. Using the bound, it is shown that finite information systems approach the perfect information case as (t-1)2/sup -B/t-1/, where B is the number of feedback bits and t is the number of transmit antennas. The geometrical bounding technique, used in the proof of the lower bound, also leads to a design criterion for good beamformers, whose outage performance approaches the lower bound. The design criterion minimizes the maximum inner product between any two beamforming vectors in the beamformer codebook, and is equivalent to the problem of designing unitary space-time codes under certain conditions. Finally, we show that good beamformers are good packings of two-dimensional subspaces in a 2t-dimensional real Grassmannian manifold with chordal distance as the metric.