Opportunistic Feedback Research Articles

Secrecy Outage Analysis of Multiuser Downlink Wiretap Networks With Potential Eavesdroppers

We investigate the secrecy outage probability (SOP) of a downlink wiretap network consisting of a single legitimate base station (BS), multiple legitimate mobile stations (MSs), and multiple potential eavesdroppers (EVEs), where each EVE randomly attempts to overhear the data transmission with a certain probability. In particular, we consider an opportunistic feedback (OF) strategy in which each legitimate MS feeds their channel gain back to the BS for data reception only when its gain is greater than a certain threshold. We analyze a closed-form expression of the SOP under this strategy. As our main result, we demonstrate that the SOP of the OF strategy approaches that of the full feedback strategy as the number of legitimate MSs becomes large. It is worth noting that, for the first time, we mathematically characterize the SOP in this practical wiretap network setting having multiple potential EVEs.

A Pseudo-Random Beamforming Technique for Improving Physical-Layer Security of MIMO Cellular Networks

In this paper, we propose a pseudo-random beamforming (PRBF) technique for improving physical-layer security (PLS) in multiple input multiple output (MIMO) downlink cellular networks consisting of a legitimate base station (BS), multiple legitimate mobile stations (MSs) and potential eavesdroppers. The legitimate BS can obtain available potential eavesdroppers’ channel state information (CSI), which is registered in an adjacent cell. In the proposed PRBF technique, the legitimate BS pseudo-randomly generates multiple candidates of the transmit beamforming (BF) matrix, in which each transmit BF matrix consists of multiple orthonormal BF vectors and shares BF information with legitimate MSs before data transmission. Each legitimate MS generates receive BF vectors to maximize the receive signal-to-interference-plus-noise (SINR) for all pseudo-randomly generated transmit beams and calculates the corresponding SINR. Then, each legitimate MS sends a single beam index and the corresponding SINR value of the BF vector that maximizes the received SINR for each BF matrix since a single spatial stream is sent to each legitimate MS. Based on the feedback information from legitimate MSs and the CSI from the legitimate BS to eavesdroppers, the legitimate BS selects the optimal transmit BF matrix and the legitimate MSs that maximizes secrecy sum-rate. We also propose a codebook-based opportunistic feedback (CO-FB) strategy to reduce feedback overhead at legitimate MSs. Based on extensive computer simulations, the proposed PRBF with the proposed CO-FB significantly outperforms the conventional random beamforming (RBF) with the conventional opportunistic feedback (O-FB) strategies in terms of secrecy sum-rate and required feedback bits.

Throughput analysis using opportunistic feedback, capture effect and MRT in Nakagami-m fading

The downlink throughput of independent and correlated Nakagami-m fading transmission employing MRT is studied in this paper. Two typical correlation models are considered: (i) equi-correlation and (ii) exponential-correlation. Throughput for independent and identically distributed Nakagami-m fading is also obtained and employed to validate results for the two correlated cases. Compact expressions of the downlink throughput for three separate cases are derived which can be readily implemented in software packages such as MATLAB and Mathematica. Mathematical mistakes in the literature are identified and corrected. Numerical simulation is given for various scenarios.

Performance of Quantized Random Beamforming in Delay-Tolerant Machine-Type Communication

Machine-to-machine (M2M) communication represents a new paradigm for mobile cellular networks, where a massive number of low-cost devices request the transfer of small amounts of data without human intervention. One option to tackle this problem is obtained by combining random beamforming (RBF) with opportunistic scheduling. RBF can be used to induce larger channel fluctuations, and opportunistic scheduling can be used to select M2M devices when their overall channel quality is good. Traditional RBF does not fulfill M2M requirements, because overall channel quality needs to be tracked continuously. In order to tackle this limitation, a novel codebook-based RBF architecture that identifies in advance the time instants in which overall channel quality should be reported, within a coherence time window, is proposed. This opportunistic feedback mechanism reduces signaling overhead and enables energy saving at M2M devices. A simplified methodology is presented to evaluate the system mean data rate, using for this purpose closed form formulas derived from SNR distribution approximations. Results reveal that the performance loss that is experienced for introducing the proposed modifications to traditional RBF scheme is negligible. The concepts analyzed in this paper provide useful insights, and show that codebook-based RBF with simplified opportunistic scheduling algorithms is an excellent combination to provide wide-area M2M services with low-cost devices and limited signaling overhead.

Down-link throughput with opportunistic feedback, multi-user and diversities in Rayleigh fading environments

This paper studies throughput of wireless systems employing opportunistic feedback, multi-user, selection combining (SC) and maximal ratio combining (MRC) diversities for transmission in Rayleigh fading environments. The first contribution of this paper is the employment of an arbitrary average branch signal-to-noise ratio (SNR) Ω which is shown to improve system throughput under specific conditions. The effects of SC and MRC on system throughput are studied with new and detailed throughput expressions for both schemes are derived and validated. Performance comparisons between SC and MRC are also made under several scenarios by varying important parameters. It is also shown that (i) MRC outperforms SC under outage and normal operating conditions as expected, (ii) the proposed system outperforms a well-known system for large values of diversity order L and (iii) for L=1, the proposed system can be correctly reduced to other systems reported in the open literature which validates the current work.

Fundamental Limits of CDF-Based Scheduling: Throughput, Fairness, and Feedback Overhead

In this paper, we investigate fundamental performance limits of cumulative distribution function (CDF)-based scheduling (CS) in downlink cellular networks. CS is known as an efficient scheduling method that can assign different time fractions for users or, equivalently, satisfy different channel access ratio (CAR) requirements of users while exploiting multi-user diversity. We first mathematically analyze the throughput characteristics of CS in arbitrary fading statistics and data rate functions. It is shown that the throughput gain of CS increases as the CAR of a user decreases or the number of users in a cell increases. For Nakagami-m fading channels, we obtain the average throughput in closed-form and investigate the effects of the average signal-to-noise ratio, the shape parameter m, and the CAR on the throughput performance. In addition, we propose a threshold-based opportunistic feedback technique in order to reduce feedback overhead while satisfying the CAR requirements of users. We prove that the average feedback overhead of the proposed technique is upper bounded by -ln(p), where p is the probability that no user satisfies the threshold condition in a cell. Finally, we adopt a novel fairness criterion, called qualitative fairness, which considers not only the quantity of the allocated resources to users but also the quality of the resources. It is observed that CS provides a better qualitative fairness than other scheduling algorithms designed for controlling CARs of users.

Exact analysis of downlink capacity employing opportunistic feedback and maximal ratio transmission in Rayleigh fading environments

The downlink throughput of opportunistic-feedback multiuser wireless systems with capture effects employing maximal ratio transmission (MRT) in a Rayleigh fading environment is studied. To the best of the authors’ knowledge, MRT has not been employed to improve multiuser system throughput with capture effects. Employing MRT means that the channel fading gains between the base station (BS) and users are a chi-square distribution with 2L degrees of freedom, where L is the diversity order, as opposed to Rayleigh distribution which has been examined in detail in the literature. Thus, the main challenge is to obtain a new and compact channel capacity expression for the system in which the roles of BS and users are reversed to make it practically sound. The new downlink throughput expression is derived as a function of L, fading-gain threshold η, capture ratio z and other parameters. For large η, the capture effects are not as effective as when η is small. Effects of the capture ratio z on system throughput are also obtained. The new findings are validated using available results in the open literature.

Improved opportunistic feedback with multiuser diversity for wireless systems with finite queue

A contention-based opportunistic feedback protocol has been proposed in the literature, to achieve multiuser diversity for the downlink transmission of a multiuser wireless system, with the assumption that the incoming data for the active users are infinitely backlogged. However, this assumption is not valid if the authors consider the finite-length queuing effect in practical systems. By taking not only the channel states, but also the queuing states into account, the authors propose an improved opportunistic feedback protocol with multiuser diversity, and analyse its performance with adaptive modulation and coding. Additionally, instead of solving a system of equations directly, a simple iterative method is proposed to obtain the stationary distribution of the system, where the system behaviour is modelled by a two-dimensional finite-state Markov chain. Numerical results demonstrate that the authors proposed protocol is superior to the protocol in the literature in terms of the average throughput, the average spectral efficiency and the average packet delay, when considering the queuing dynamics.

Opportunistic Feedback Reduction for Multiuser MIMO Broadcast Channel with Orthogonal Beamforming

For the multiuser multiple-input multiple-output downlink channel, users feedback their channel state information to help base station schedule users and improve system sum rate. This incurs a large aggregate feedback bandwidth which grows linearly with the number of users. In this paper, we propose a novel scheme to reduce feedback load in a scheduled downlink orthogonal beamforming system by allowing users to opportunistically determine the number of feedback bits to use according to multiple decision thresholds. Through theoretical analysis, we show that, while keeping the aggregate feedback load of the entire system constant regardless of the number of users, the proposed scheme almost achieves the optimal asymptotic sum rate scaling with respect to the number of users (i.e., the multiuser diversity). Specifically, given the number of thresholds, the proposed scheme can achieve a constant portion of the optimal sum rate achievable by the system in which all the users always feedback, and the remaining portion (i.e., the sum rate loss) decreases exponentially to zero as the number of thresholds increases. We further consider feedback design by minimizing the number of thresholds and allocating quantization bits. Simulation shows that the proposed scheme can reduce feedback load and utilize limited feedback bandwidth more effectively than the existing feedback methods.

Reducing feedback load for OFDM-based wireless system

In this paper, the feedback load reduction problem in wireless systems based on orthogonal frequency division multiplexing (OFDM) is investigated and an opportunistic feedback scheme (OFS) is proposed. The key idea behind OFS is that only the key channel gains which can significantly affect the system throughput are fed back to the BS. Firstly, the key channel gains are proved to belong to a channel gain interval. Secondly, a statistical method is proposed to estimate the channel gain interval. Thirdly, the opportunistic feedback scheme is formulated and the feedback load of OFS is analyzed. The advantage of OFS is threefold: the first is OFS can work in both OFDM-based multicast system and OFDM-based unicast system. The second is the channel fading type of the BS-user link is not required, which is more realistic. The third is OFS can get better feedback load performance compared with other schemes, while achieving almost the same throughput performance compared with that of full feedback scheme.

Opportunistic Feedback and User Selection for Multiuser Two-Way Amplify-and-Forward Relay in Time-Varying Channels

Opportunistic Feedback and User Selection for Multiuser Two-Way Amplify-and-Forward Relay in Time-Varying Channels

Opportunistic Two-Stage Feedback and Scheduling for MIMO Downlink Systems

We propose an opportunistic two-stage feedback and scheduling algorithm that is based upon zero-forcing beamforming with semi-orthogonal user selection (ZFBF-SUS) to reduce its feedback load. In an SUS algorithm, a base station schedules semi-orthogonal users using the feedback of all users' channel information. However, such feedback overhead significantly increases with the number of users. To reduce the feedback load of ZFBF-SUS systems, the proposed two-stage feedback scheme opportunistically and separately exploits the signal-to-interference-plus-noise ratio and orthogonality in each stage such that only a fraction of users feed back their channel information. Based on this opportunistic feedback scheme, we decouple the entire process of the SUS into two sub-processes. The proposed two-stage feedback and scheduling scheme effectively exclude the inappropriate users for ZFBF transmission in both feedback and scheduling periods, and thus save the scarce resources consumed for the feedback process. We derive an analytical expression for the average number of feedback bits of the proposed system and optimize it in an averaged sense. Further, the sum-rate of the proposed system is theoretically investigated. Both analytical and simulation results show that the proposed algorithm achieves the performance of conventional ZFBF-SUS systems with a significantly reduced number of feedback bits.

Opportunistic Feedback and Scheduling to Reduce Packet Delays in Heterogeneous Wireless Systems

We consider an opportunistic feedback and scheduling scheme for multiuser wireless systems. In opportunistic feedback schemes, a set of thresholds is assigned to the users, and only users with channel quality above the threshold transmit the feedback on channel state information (CSI) to the base station. We propose an opportunistic feedback scheme that combines techniques from queue/weight-based opportunistic scheduling to improve users' delay performance. A set of queue-based weights designed to slowly track variations in users' backlog is computed; then, each user is assigned a threshold based on the weights. The proposed scheme effectively captures heterogeneity in the offered loads, which is crucial for delay performance. To that end, we formulate a nonconcave maximization problem and propose an approximation algorithm that is numerically shown to have high accuracy. By simulation, we show that our scheme substantially reduces the mean delay, compared with conventional schemes.

Sum Rate Analysis of One-Pico-Inside OFDMA Network With Opportunistic Scheduling and Selective Feedback

In this letter, we investigate the sum rate performance of a generic one-pico-inside OFDMA network with opportunistic scheduling policy and the best-M partial feedback design. By a detailed examination of the statistical property of the selected user's signal-to-interference-plus-noise ratio and its interplay with intercell interference, we derive the exact closed form expression for the sum rate under partial feedback. The general expression incorporates the special interference limited and noise limited scenarios as special cases. The result explains the exact interaction with partial feedback and the approach can be generalized and applied to many related problems.

CHANNEL FEEDBACK SCHEDULING FOR WIRELESS COMMUNICATIONS

Opportunistic scheduling can significantly improve wireless network performance by exploiting the underlying channel condition. There has been a lot of work on opportunistic scheduling, but the problem of finding the right feedback mechanism to convey channel information has largely been untouched. In emerging multichannel systems, the per-channel feedback induces a substantial amount of feedback overhead and requires high computational complexity. To reduce the feedback overhead, we consider an opportunistic feedback strategy that activates the channel feedback opportunistically according to the channel condition. Then, we combine the opportunistic feedback with the best-n channel feedback scheme where a mobile user chooses the best n channels and transfers this information to the base station. We analyze the throughput and the amount of channel feedback information for proportionally fair opportunistic scheduling under Rayleigh fading i.i.d. channels. The numerical results confirm that our partial feedback schemes achieve a remarkable reduction in the amount of feedback information without significant throughput degradation, thereby saving the scarce wireless bandwidth and limited battery power.

Throughput Analysis of Opportunistic Feedback for Downlink Multiuser Diversity with Capture Effect

An opportunistic feedback protocol has been proposed in the literature to achieve multiuser diversity for the downlink transmission of a multiuser wireless system. In this letter, through theoretical analysis and extensive simulations, it is shown that the capture effect can have a significant impact on system throughput. It is also shown that, to achieve optimal throughput, the number of minislots used for opportunistically finding a user in the protocol should not be too large due to the overhead induced by the control packets.

Two-Stage Opportunistic Feedback Over Multiuser Diversity Systems

In order to reduce feedback overload, a framework of contention-based feedback was established. However, since a user is randomly chosen, it cannot maximize capacity. Moreover, if a collision occurs, a base station cannot obtain any information to schedule a user; thus, a scheduling outage happens. In this paper, we propose a two-stage opportunistic feedback scheme to maximize capacity with the limited feedback. The proposed scheme guarantees selecting the user with the highest signal-to-noise ratio (SNR) channel state and considers the scheduling outage situation using previous feedback information. In addition, we define the effective capacity to show the relation between the capacity and the feedback overload. Simulation results show that the proposed scheme achieves higher effective capacity with limited feedback than existing contention-based feedback schemes do.

Net Throughput Maximization of Per-Chunk User Scheduling for MIMO-OFDM Downlink

Per-chunk user scheduling for multiple-input-multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) downlink is considered. By grouping adjacent subcarriers into chunks, the amount of required channel state information feedback is reduced. Based on the net throughput criterion, which accounts for the reduction in sum rate due to the feedback overhead, it is shown that there exists an optimal chunk size that maximizes the net throughput. To reduce the feedback requirement even further, an opportunistic feedback scheme is proposed, and a close approximation for its net throughput is derived. The net throughput of per-chunk user scheduling with optimized chunk size is compared to various other limited-feedback MIMO-OFDM downlink strategies. The results show that increasing the total number of users in the system results in the net throughput of most existing MIMO-OFDM downlink schemes decreasing to zero for moderate-size user pools, whereas the net throughput of per-chunk user scheduling with opportunistic feedback increases with the total number of users, even when that number is very large(>; 1000).

Comments on "Partial channel feedback schemes maximizing overall efficiency in wireless networks" [Apr 08 1306-1314

The proof of Proposition 2 in the above-mentioned paper was incorrect due to some flaws in the prerequisite analysis of the proposed hybrid feedback scheme. We fix the flaws and correct the proof in this comment. From the corrected proof, we arrive at a new observation that the hybrid feedback scheme has exactly the same feedback overhead as the pure opportunistic feedback scheme if the threshold is adaptively set for a target utilization. Simulation results demonstrate the correctness of our analysis.

Capacity and Fairness Trade-off in an Outage Situation over Multiuser Diversity Systems

Without an appropriate scheduling scheme for subscriber stations (SSs) in an outage situation, the opportunistic feedback protocol degrades the system performance and it does not achieve the desired capacity and fairness trade-off. In this paper, we propose a new downlink multiuser scheduling scheme to balance a trade-off between capacity and fairness with one-bit information. In particular, we propose a high successful-feedback SS selection (HSSS) scheme using one-bit information to provide the flexible trade-off in an outage situation. As a result, the proposed scheme not only reduces a significant feedback overload but also considers scheduling in the outage situation. Simulation results show that the proposed scheme achieves the flexible trade-off between capacity and fairness in the outage situation.