Cooperative Multiple Access Channel Research Articles

Cooperative Multiple-Access Channels With Distributed State Information

This paper studies a memoryless state-dependent multiple access channel (MAC) where two transmitters wish to convey a message to a receiver under the assumption of causal and imperfect channel state information at transmitters (CSIT) and imperfect channel state information at receiver (CSIR). In order to emphasize the limitation of transmitter cooperation between physically distributed nodes, we focus on the so-called distributed CSIT assumption, i.e., where each transmitter has its individual channel knowledge, while the message can be assumed to be partially or entirely shared a priori between transmitters by exploiting some on-board memory. Under this setup, the first part of the paper characterizes the common message capacity of the channel at hand for arbitrary CSIT and CSIR structure. The optimal scheme builds on Shannon strategies, i.e., optimal codes are constructed by letting the channel inputs be a function of current CSIT only. For a special case when CSIT is a deterministic function of CSIR, the considered scheme also achieves the capacity region of a common message and two private messages. The second part addresses an important instance of the previous general result in a context of a cooperative multi-antenna Gaussian channel under i.i.d. fading operating in frequency-division duplex mode, such that CSIT is acquired via an explicit feedback of perfect CSIR. The capacity of the channel at hand is achieved by distributed linear precoding applied to Gaussian codes. Surprisingly, we demonstrate that it is suboptimal to send a number of data streams bounded by the number of transmit antennas as typically considered in a centralized CSIT setup. Finally, numerical examples are provided to evaluate the sum capacity of the binary MAC with binary states as well as the Gaussian MAC with i.i.d. fading.

Multiple Access Channels With Full-Duplex Amplify-and-Forward Transmitter Cooperations

In this paper, a full-duplex (FD) transmitter cooperative scheme is proposed to improve the achievable rate for multiple access channels (MACs). In the proposed scheme, by leveraging the FD amplify-and-forward (AF) protocol, two FD transmitters send their own signals and forward the received signals from their counterparts to the receiver, which adopts both the forward and backward decoding schemes to decode the source messages. First, the analyses of the equivalent channel model and the statistics of the accumulated additive white Gaussian noise and residual self-interference (ANRI) caused by imperfect self-interference cancellation are performed. Then, the achievable rate regions are obtained by utilizing both the forward and backward decoding schemes, and a two-stage iterative algorithm is proposed to characterize these regions: in each iteration, the covariance matrix of the ANRI is first updated; then, for the fixed covariance matrix of the ANRI, the transmission parameters of the two transmitters are calculated by a two-step iterative algorithm based on the parametric Dinkelbach algorithm, sequential convex programming (SCP), and semidefinite relaxation (SDR). The numerical results show that the proposed scheme outperforms the half-duplex cooperative scheme and conventional MAC scheme without cooperations when the channels between the two transmitters are in good condition.

Energy and Data Cooperative Multiple Access Channel With Intermittent Data Arrivals

We consider an energy harvesting two user cooperative Gaussian multiple access channel, where both of the users harvest energy from nature. The users cooperate at the physical layer (data cooperation) by establishing common messages through overheard signals and then cooperatively sending them. We study two scenarios within this model. In the first scenario, the data packets arrive intermittently over time. We find the optimal offline transmit power and rate allocation policy that maximize the departure region. We first show that there exists an optimal policy, in which the single user rate constraints in each time slot are tight, yielding a one-to-one relation between the powers and rates. Then, we formulate the departure region maximization problem as a weighted sum departure maximization in terms of rates only. Next, we propose a sequential convex approximation method to approximate the problem at each step and show that it converges to the optimal solution. We solve the approximate problems using an inner–outer decomposition method. In the second scenario, the data packets are available at the beginning of the transmission, but the users now have the ability to cooperate at the battery level (energy cooperation), in addition to data cooperation. The energy cooperation is facilitated by wireless energy transfer and is bidirectional. For this scenario, we find the jointly optimal offline transmit power and rate allocation policy together with the energy transfer policy that maximize the departure region. We provide necessary conditions for energy transfer and prove some properties of the optimal transmit policy, thereby shedding some light on the interplay between energy and data cooperation.

Cooperative Diamond Channel With Energy Harvesting Nodes

We consider the energy harvesting diamond channel, where the source and two relays harvest energy from nature and the physical layer is modeled as a concatenation of a broadcast and a multiple access channel. Since the broadcast channel is degraded, one of the relays has the message of the other relay and the multiple access channel can be modeled as a cooperative multiple access channel with common data. We find the optimal offline transmit power and rate allocations that maximize the end-to-end throughput. For the broadcast side, we show that there exists an optimal source power allocation, which is equal to the single-user optimal power allocation for the source energy arrivals. We then show that the fraction of the power spent on each broadcast link depends on the energy arrivals for the relays. For the multiple access side with no co-operation, with fixed source rates, we show that the problem can be cast as a multiple access channel with both data and energy arrivals and can be formulated in terms of data transmission rates only. We use a dual decomposition method to solve the overall problem efficiently. Finally, we focus on the diamond channel with co-operative multiple access capacity region and find the optimal rates and powers using a decomposition into inner and outer maximization problems.

Cognitive Cooperative MAC With One Primary and Two Secondary Users: Achievable Rates and Optimal Power Control

We consider a three-user fading cognitive cooperative multiple access channel (MAC) with one primary and two secondary transmitters. We propose two encoding/decoding strategies with varying levels of cooperation, based on block Markov superposition encoding and backward decoding. The first is an overlay model, where the secondary users (SUs) aid the transmission of the primary user (PU) by causally decoding the PU message and forwarding it while also cooperating among each other. The second is an underlay model, where the SUs cooperate by decoding and forwarding each other's messages while treating the signal received from the PU as noise. In either case, the PU is guaranteed to operate at its maximum achievable single user rate. We characterize the achievable SU rate region for both models and maximize this region as a function of the transmit powers. The simulation results show that the SU rate region can be significantly enlarged, particularly using the overlay model.

Alternative relaying for cooperative multiple-access channels in wireless vehicular networks

In this paper, a novel spectrally efficient half-duplex cooperative transmission protocol is proposed for cooperative multiple-access channels in wireless vehicular networks, where multiple sources (vehicles) deliver messages to a common destination (roadside base station or roadside infrastructure) with the help of multiple decode-and-forward relays (roadside stations). The basic idea is to apply superposition coding at each transmitter in order to achieve the full diversity gain, where the linear zero-forcing detection is used at each relay to combat inter-relay interference. Compared to existing uplink cooperative protocols, the proposed scheme can exploit the cooperation involving both the relays and the sources. An achievable diversity-multiplexing trade-off is developed for the proposed transmission protocol. Even with strong inter-relay interference which has been ignored by many existing works, the proposed scheme can still approach the optimal multiple-input single-output upper bound. Numerical results have also been provided to demonstrate the performance of the proposed protocol.

On Cooperative Multiple Access Channels With Delayed CSI at Transmitters

We consider a cooperative two-user multiaccess channel in which the transmission is controlled by a random state. Both encoders transmit a common message and, one of the encoders also transmits an individual message. We study the capacity region of this communication model for different degrees of availability of the states at the encoders, causally or strictly causally. In the case in which the states are revealed causally to both encoders but not to the decoder we find an explicit characterization of the capacity region in the discrete memoryless case. In the case in which the states are revealed only strictly causally to both encoders, we establish inner and outer bounds on the capacity region. The outer bound is nontrivial, and has a relatively simple form. It has the advantage of incorporating only one auxiliary random variable. In particular, it suggests that there is none, or at best only little, to gain from having the encoder that transmits both messages also sending an individual description of the state to the receiver, in addition to the compressed version that is sent cooperatively with the other encoder. We then introduce a class of cooperative multiaccess channels with states known strictly causally at both encoders for which the inner and outer bounds agree, and so we characterize the capacity region for this class. In this class of channels, the state can be obtained as a deterministic function of the channel inputs and output. We also study the model in which the states are revealed, strictly causally, in an asymmetric manner, to only one encoder. Throughout this paper, we discuss a number of examples, and compute the capacity region of some of these examples. The results shed more light on the utility of delayed channel state information for increasing the capacity region of state-dependent cooperative multiaccess channels, and tie with recent progress in this framework.

Capacity Region of Cooperative Multiple-Access Channel With States

We consider a two-user state-dependent multiaccess channel in which the states of the channel are known noncausally to one of the encoders and only strictly causally to the other encoder. Both encoders transmit a common message and, in addition, the encoder that knows the states noncausally transmits an individual message. We find explicit characterizations of the capacity region of this communication model in both discrete memoryless and memoryless Gaussian cases. In particular, the capacity region analysis demonstrates the utility of the knowledge of the states only strictly causally at the encoder that sends only the common message in general. More specifically, in the discrete memoryless setting, we show that such a knowledge is beneficial and increases the capacity region in general. In the Gaussian setting, we show that such a knowledge does not help, and the capacity is same as if the states were completely unknown at the encoder that sends only the common message. Furthermore, we also study the special case in which the two encoders transmit only the common message and show that the knowledge of the states only strictly causally at the encoder that sends only the common message is not beneficial in this case, in both discrete memoryless and memoryless Gaussian settings. The analysis also reveals optimal ways of exploiting the knowledge of the state only strictly causally at the encoder that sends only the common message when such a knowledge is beneficial. The encoders collaborate to convey to the decoder a lossy version of the state, in addition to transmitting the information messages through a generalized Gel'fand–Pinsker binning. Particularly important in this problem are the questions of 1) optimal ways of performing the state compression and 2) whether or not the compression indices should be decoded uniquely. By developing two optimal coding schemes that perform this state compression differently, we show that when used as parts of appropriately tuned encoding and decoding processes, both compression à-la noisy network coding by Lim <etal xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"/> or the quantize-map-and-forward by Avestimeher <etal xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"/> , i.e., with no binning, and compression using Wyner–Ziv binning are optimal. The scheme that uses Wyner–Ziv binning shares elements with Cover and El Gamal original compress-and-forward, but differs from it mainly in that backward decoding is employed instead of forward decoding and the compression indices are not decoded uniquely. Finally, by exploring the properties of our outer bound, we show that, although not required in general, the compression indices can in fact be decoded uniquely essentially without altering the capacity region, but at the expense of larger alphabets sizes for the auxiliary random variables.

Fairness-oriented physical layer network coding in cooperative multiple access channels due to MRC and SIC

This paper investigates the impacts of network coding (NC) on user fairness from the network perspective in multiple access channels. Firstly, simultaneous outage is exploited as the metric to evaluate user fairness. To optimize it, a fairness-oriented physical layer NC scheme is proposed, in which users' signals are superimposed in specific calculated proportion at relay and then decoded by maximal ratio combination (MRC) and serial interference cancellation (SIC) at destination. Furthermore, the influence of power allocation coefficient error on simultaneous outage is also analyzed to reveal its correlation with user fairness. Simulation results indicate that the proposed scheme can supply approximately equal performance promotion for each cooperation-participated user and outperform those without fairness consideration in terms of the employed metric.

Network coding with diversity and outdated channel state information

Physical-layer network coding (PNC) has the potential to significantly improve the throughput of wireless networks where the channels can be modeled as additive white Gaussian noise (AWGN) channel. As extending to multiple channels, this technique requires both amplitude and phase compensation at each transmitter and will lead to inefficient systems yielding no diversity even with perfect channel state information (CSI). In order to avoid these limitations, we apply network coding with diversity (NCD) to achieve a form of selection diversity and extend NCD to cooperative multiple access channels in this paper. However, in practical wireless communication systems, the CSI could become outdated due to the difference between the CSI used in the relay selection and data transmission phases. Hence, the selected relay may not be the best one during data transmission phase due to the dynamic change in the wireless channels. Therefore, we first explore the relation between the present and past CSIs. Exploiting this relationship, the NCD scheme with outdated CSI is investigated based on the past CSI. To evaluate the performance of this scheme, an information-theoretic metric, namely the outage capacity, is studied under this condition.

Performance analysis of user fair-based relay adaptive power allocation in physical-layer network coding cooperation

Network Coding (NC) is an effective technology to enhance the cooperative system spectral efficiency. However, since it is network-oriented, the existing performance metric of single-user outage can not comprehensively evaluate its gain and the impact to the entire network, which affect the user fairness. This paper proposes two novel user fair-based adaptive relay power allocation algorithms in single-relay NC cooperative multiple access channels. Firstly, common outage probability is employed as the performance metric, and to minimize it, a specific condition is deduced. On this basis, the instantaneous channel information-based adaptive relay power allocation scheme and the channel statistic information-based one with lower complexity are designed respectively, which make users’ signals superimposed at accurately calculated proportion to maintain fairness. Simulation results show that compared with other existing schemes, the proposed schemes can best maintain user fairness, and effectively improve the common outage performance of the whole system, at the expense of small spectral efficiency.

OFDM Modulated Cooperative Multiple-Access Channel with Network-Channel Coding

In this paper, we consider the half-duplex cooperative multiple-access channel (CMAC) with frequency-selective block-fading. Each link employs an orthogonal frequency division multiplexing (OFDM) system, where modulated symbols are drawn from a finite constellation set. We first obtain the diversity order of the CMAC, as a function of the time sharing variables of the users and the rates of the codes. To achieve this rate-diversity tradeoff, we use the principle of network coding where messages of the two sources are jointly encoded. Both separate and joint network-channel coding approaches are considered. Specifically, we design multiple turbo codes that minimize the outage probabilities of these approaches. We also give a code structure for the multiple turbo codes to achieve full diversity of the system. The codes are optimized using the extrinsic information transfer (EXIT) chart analysis with iterative decoding tailored for OFDM modulated CMAC. Numerical examples show that with our proposed design technique, the achieved frame error rate is within 0.5dB from the information outage. Without network-channel coding, the outage probability of distributed coding cannot achieve the diversity order given in the rate-diversity tradeoff.

Generalized Sequential Slotted Amplify and Forward Strategy in Cooperative Communications

This paper proposes a generalized sequential slotted amplify and forward (GSSAF) strategy for single-antenna wireless cooperative networks. The diversity and multiplexing tradeoff (DMT) is analyzed. Applications to cooperative multiple relay channels, cooperative broadcast channels (CBC) and cooperative multiple access channels are considered and the DMT upper bound is proven to be achievable for each case. Other than proposing the best known near-optimal strategy for CBC, another important contribution is to show that GSSAF can be used as a unified strategy to achieve DMT optimality in wireless cooperative networks with unit multiplexing gains.

Cooperative multiple access channels: Achievable rates and optimal resource allocation

We investigate the multiple access channels (MAC) where sources can cooperate via half-duplex relaying and refer to it as cooperative MAC channels (CMAC). Assuming perfect channel state information (CSI) at the transmitters and the receivers, we determine the bounds on the achievable rate region of a Gaussian CMAC channel and an inner bound on the outage capacity region of a fading CMAC channel. Based on superposition modulation, a half-duplex cooperative relay scheme with optimal resource allocation is proposed to achieve the bounds of capacity region. Analytical results and simulation results show that the achievable rate region of a Gaussian CMAC channel is larger than that of a Gaussian MAC channel with direct transmission (DT) schemes. But they have the same achievable sum rate. Moreover, the proposed scheme can provide higher outage capacity region than DT schemes in a fading MAC channel due to the fact that sources can share the resources with each other to reduce outages.

An achievable rate region of cooperative multiple-access channels with hybrid CF and DF cooperation

In this letter, we derive an achievable rate region of a two-user cooperative multiple-access (CMA) channel where one user performs decode-and-forward (DF) cooperation and the other user performs Wyner-Ziv-type compress-and-forward (CF) cooperation. The proposed rate region is achieved using a communication scheme that applies block Markov coding at each user and backward decoding at destination. Numerical results show that for some channel scenarios, the proposed hybrid CF and DF cooperation scheme enlarges the rate region achieved by the pure DF-based cooperation scheme.

On the study of network coding with diversity

Recently proposed physical-layer network coding (PNC) has demonstrated the promise to significantly improve the throughput of wireless networks whose links can be modeled as additive white Gaussian noise (AWGN) channels. However, the extension to multipath channels is problematic, since the technique would then require both amplitude and phase compensation at each transmitter. Phase compensation requires accurate distributed phase tracking, whereas the required amplitude compensation is even more troubling, as it leads to an inefficient system that yields no diversity even in the presence of perfect channel estimates. Here, a system that avoids these limitations is obtained by reaching up one level higher in the network hierarchy and performing distributed relay selection with cognizance of the PNC technique that we will employ at the physical layer. Since the resulting scheme will achieve a form of selection diversity, we term it ldquonetwork coding with diversityrdquo (NCD). To facilitate performance evaluation, two information-theoretic metrics, the outage and ergodic capacity, are studied. Our analytical and simulation results show that the proposed protocol achieves more robust performance and higher system throughput than comparable schemes. Finally, the proposed network coding is extended to the context of cooperative multiple access channels, which yields a new cooperative protocol with larger outage and ergodic capacity compared with existing transmission schemes.

Scaling Laws for Dense Gaussian Sensor Networks and the Order Optimality of Separation

We investigate the optimal performance of dense sensor networks by studying the joint source-channel coding problem. There are N uniformly spaced sensor nodes sampling noiselessly a one-dimensional spatial random process over an interval [0, U0]. The overall goal of the sensor network is for the sensor nodes to code and transmit the measurement samples to a collector node over a cooperative multiple-access channel with noisy feedback, and for the collector node to reconstruct the entire random process with minimum expected distortion. We provide separation-based lower and upper bounds for the minimum achievable expected distortion when the underlying random process is Gaussian. When the Gaussian random process satisfies some general conditions, such as the eigenvalues of its Karhunen-Loeve expansion decrease roughly inverse polynomially in order x, i.e., the kth eigenvalue is roughly k-x, we evaluate the lower and upper bounds explicitly, and show that they are of the same order for a wide range of power constraints. Thus, for these random processes, under these power constraints, we show that the minimum achievable expected distortion decreases as (log NP(N)1-x, where P(N) is the sum power constraint on the sensor nodes. Further, we show that the achievability scheme that achieves the lower bound on the distortion is a separation-based scheme that is composed of multiterminal rate-distortion coding and amplify-and-forward channel coding. Therefore, we conclude that separation is order-optimal for the dense Gaussian sensor network scenario under consideration, when the underlying random process satisfies some general conditions.

On the Diversity-Multiplexing Tradeoff for Wireless Cooperative Multiple Access Systems

A spectrally efficient strategy is proposed for cooperative multiple access (CMA) channels in a centralized communication environment with N users. By applying superposition coding, each user will transmit a mixture containing its own information as well as the other users', which means that each user shares parts of its power with the others. The use of superposition coding in cooperative networks was first proposed in [E. G. Larsson and B. R. Vojcic, Cooperative transmit diversity based on superposition modulation, IEEE Commun. Lett., vol. 9, pp. 778 780, Sep. 2005. ] , which will be generalized to a multiple-user scenario in this paper. Since the proposed CMA system can be seen as a precoded point-to-point multiple-antenna system, its performance can be best evaluated using the diversity-multiplexing tradeoff. By carefully categorizing the outage events, the diversity-multiplexing tradeoff can be obtained, which shows that the proposed cooperative strategy can achieve larger diversity/multiplexing gain than the compared transmission schemes at any diversity/multiplexing gain. Furthermore, it is demonstrated that the proposed strategy can achieve optimal tradeoff for multiplexing gains 0 les tau les1 whereas the compared cooperative scheme is only optimal for 0 les tau les (1/N) . As discussed in the paper, such superiority of the proposed CMA system is due to the fact that the relaying transmission does not consume extra channel use and, hence, the deteriorating effect of cooperative communication on the data rate is effectively limited.

Power Control for Fading Cooperative Multiple Access Channels

For a fading Gaussian multiple access channel with user cooperation, we obtain the power allocation policies that maximize the average rates achievable by block Markov superposition coding, subject to average power constraints. The optimal policies result in a coding scheme that is simpler than the one for a general multiple access channel with generalized feedback. This simpler coding scheme also leads to the possibility of formulating an otherwise non-concave optimization problem as a concave one. Using the perfect channel state information available at the transmitters to adapt the powers, we demonstrate gains over the achievable rates for existing cooperative systems.

Cooperative lattice coding and decoding in half-duplex channels

We propose novel lattice coding/decoding schemes for half-duplex outage-limited cooperative channels. These schemes are inspired by the cooperation protocols of Azarian et al. and enjoy an excellent performance-complexity tradeoff. More specifically, for the. relay channel, we first use our lattice coding framework to generalize Yang and Belfiore implementation of the non-orthogonal amplify and forward cooperation protocol. This generalization is shown to offer significant performance gains while keeping the decoding complexity manageable. We then devise a novel variant of the dynamic decode and forward protocol, along with a lattice-coded implementation, which enjoys a near-optimal diversity-multiplexing tradeoff with a low encoding/decoding complexity. Finally, for the cooperative multiple-access channel, we present a lattice-coded implementation of the non-orthogonal amplify and forward protocol and demonstrate its excellent performance-complexity tradeoff. Throughout the paper, we establish the performance gains of our proposed protocols via a comprehensive simulation study.