Feedback Bits Research Articles

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.

Transmit Antenna Selection with Alamouti Coding and Power Allocation in MIMO Wiretap Channels

In this work, we propose a new transmit antenna selection (TAS) scheme which examines the trade-off between feedback overhead and secrecy performance in multiple-input multiple-output wiretap channels. Our new scheme is carried out in two steps. First, the transmitter selects the first two strongest antennas to maximize the instantaneous signal-to-noise ratio (SNR) of the transmitter-receiver channel. Second, Alamouti coding is employed at the selected antennas in order to perform secure data transmission. When equal power is applied to the selected antennas, we refer to our new scheme as TAS-Alamouti. To provide valuable insights into TAS-Alamouti, we derive new closed-form expressions for the secrecy performance metrics. In terms of these metrics, we show how in a Rayleigh fading channel that our TAS-Alamouti scheme outperforms the traditional single TAS scheme conditioned on the SNR of the transmitter-receiver channel being larger than a specific value. We show how in some antenna configurations no additional feedback, relative to single TAS, is required in order to realize such performance enhancements. Furthermore, we show how optimal power allocation (OPA) across the selected antennas at the transmitter leads to a new scheme, which we refer to as TAS-Alamouti-OPA, that outperforms single TAS unconditionally. Relative to TAS-Alamouti, TAS-Alamouti-OPA requires only one additional feedback bit.

Performance Analysis and Optimization for Interference Alignment Over MIMO Interference Channels With Limited Feedback

In this paper, we address the problem of interference alignment (IA) over MIMO interference channels with limited channel state information (CSI) feedback based on quantization codebooks. Due to limited feedback and hence imperfect IA, there are residual interferences across different links and different data streams. As a result, the performance of IA is greatly related to the CSI accuracy (namely number of feedback bits) and the number of data streams (namely transmission mode). In order to improve the performance of IA, it makes sense to optimize the system parameters according to the channel conditions. Motivated by this, we first give a quantitative performance analysis for IA under limited feedback, and derive a closed-form expression for the average transmission rate in terms of feedback bits and transmission mode. By maximizing the average transmission rate, we obtain an adaptive feedback allocation scheme, as well as a dynamic mode selection scheme. Furthermore, through asymptotic analysis, we obtain several clear insights on the system performance, and provide some guidelines on the system design. Finally, simulation results validate our theoretical claims, and show that obvious performance gain can be obtained by adjusting feedback bits dynamically or selecting transmission mode adaptively.

Adaptive Limited Feedback for MISO Wiretap Channels With Cooperative Jamming

This paper studies a multi-antenna wiretap channel with a passive eavesdropper and an external helper, where only quantized channel information regarding the legitimate receiver is available at the transmitter and helper due to finite-rate feedback. Given a fixed total bandwidth for the two feedback channels, the receiver must determine how to allocate its feedback bits to the transmitter and helper. Assuming zero-forcing transmission at the helper and random vector quantization of the channels, an analytic expression for the achievable ergodic secrecy rate due to the resulting quantization errors is derived. While direct optimization of the secrecy rate is difficult, an approximate upper bound for the mean loss in secrecy rate is derived and a feedback bit allocation method that minimizes the average upper bound on the secrecy rate loss is studied. A closed-form solution is shown to be possible if the integer constraint on the bit allocation is relaxed. Numerical simulations indicate the significant advantage that can be achieved by adaptively allocating the available feedback bits.

D-DB and D-RRDB Relaying with D-DQE Relay Network

SUMMARY In this study, a new method for Decode-Distributed Beamforming (D-DB) relaying is proposed. Each relay node decodes the source symbol by maximum likelihood detection. The detected symbol is entered into the stored Quantized Equal-gain (QE) codebook, where the label of the phase region is provided by a feedback link from the destination node. Therefore, the proposed relay network forms a Decode-Distributed QE (DDQE) relay network. The performances of the D-DQE codebooks are examined by Monte-Carlo simulations, in which the feedback links and channel estimations are assumed to be error-free. The simulation results reveal that the symbol error rates of the D-DQE relay system improve the error performance of the QE codebooks when relay nodes are close to the source node. When error-free feedback bits are provided, the performance of the proposed D-DQE is better than that of Alamouti’s Decode-Distributed Space-Time Coding (D-DSTC) relay network. The weakest relays are rejected to improve the performance of the D-DQE codebooks and reduce the number of feedback bits. This relay network is called Decode-Relay

Impact of store image on store loyalty and purchase intention: does it vary across gender?

Congestion control is one of the major issues in the communication networks. The approaches toward the congestion control are end-to-end congestion control and network assisted congested control. The Explicit Congestion Control Protocol uses ECN bits for feedback to control congestion. But it uses multiple bits in the header which faces certain obstacles in implementation. The proposed optimised congestion control protocol (OCCP) uses only two ECN bits for giving feedback about congestion. The reduced usage of ECN bits in the proposed system makes it easy to implement. The performance is also better than the existing one. The proposed system is implemented in a network simulator. For the implementation of the proposed system, NS2 discrete event simulation tool is used and experimented with a variety of parameter settings.

Active queue management based congestion control protocol for wireless networks

Congestion control is one of the major issues in the communication networks. The approaches toward the congestion control are end-to-end congestion control and network assisted congested control. The Explicit Congestion Control Protocol uses ECN bits for feedback to control congestion. But it uses multiple bits in the header which faces certain obstacles in implementation. The proposed optimised congestion control protocol (OCCP) uses only two ECN bits for giving feedback about congestion. The reduced usage of ECN bits in the proposed system makes it easy to implement. The performance is also better than the existing one. The proposed system is implemented in a network simulator. For the implementation of the proposed system, NS2 discrete event simulation tool is used and experimented with a variety of parameter settings.

Exploiting Multiuser Diversity with 1-bit Feedback for Spectrum Sharing

In this paper, we propose a limited feedback based underlay spectrum sharing scheme where a downlink secondary network shares the spectrum with a pair of primary users. The primary receiver and N secondary receivers each sends 1-bit feedback of channel quality information to their corresponding transmitters. By overhearing the primary feedback and receiving the secondary feedback, one of the secondary receivers is scheduled at the secondary transmitter in each fading block. The optimal channel quantization thresholds and power allocation are jointly determined by maximizing the average throughput of the secondary user under the average secondary power constraint and the average primary rate loss constraint. The average throughput of the secondary user grows as log log N.

A Study State Analysis of Mu Mimo Using Adaptive Algorithm

In conventional cellular systems, each base station (BS) transmits signals intended for a single user in a particular resource allocation. As bandwidth is a scarce resource, effective utilization of the available bandwidth in the system is essential in modern wireless systems especially for applications such as video streaming and voice over internet protocol (VoIP) which demands high data rate. Fortunately since the user's feedback the channel state information to the network, there is an opportunity for the BS to schedule more than one user's data in a single resource allocation by designing pre-coders which beam form the data to the intended user. This technique which is called multi-user multiple-input and multiple-output (MU-MIMO) is adopted to investigate an orthogonal frequency-division multiplexing (OFDM)-based downlink transmission scheme. We develop a analogous fast iterative truncation algorithm (FITRA) and display statistical results to exhibit tremendous PAR-reduction capabilities. The significantly condensed linearity necessities ultimately empower the routine of low-cost RF components for the large-scale MU-MIMO-OFDM downlink. We show that there is a strong preference for obtaining high-quality feedback, and that obtaining near-perfect channel information from as many receivers as possible provides a significantly larger sum rate than collecting a few feedback bits from a large number of users.

Ensemble Properties of RVQ-Based Limited-Feedback Beamforming Codebooks

The ensemble properties of random vector quantization (RVQ) codebooks for limited-feedback beamforming in multiinput multioutput (MIMO) systems are studied with the metrics of interest being the received \ssr SNR loss and mutual information loss, both relative to a perfect channel state information (CSI) benchmark. The simplest case of unskewed codebooks is first studied in the correlated MIMO setting and these loss metrics are computed as a function of the number of bits of feedback ( B), transmit antenna dimension ( Nt), and spatial correlation. In particular, it is established that: 1) the loss metrics are a product of two components-a quantization component and a channel-dependent component; 2) the quantization component, which is also common to analysis of channels with i.i.d. fading, decays as B increases at the rate 2-B/(Nt-1); 3) the channel-dependent component reflects the condition number of the channel. Further, the precise connection between the received \ssr SNR loss and the squared singular values of the channel is shown to be a Schur-convex majorization relationship. Finally, the ensemble properties of skewed codebooks that are generated by skewing RVQ codebooks with an appropriately designed fixed skewing matrix are studied. Based on an estimate of the loss expression for skewed codebooks, it is established that the structure of a good skewing matrix is critically dependent on the condition numbers of the effective channel (product of the true channel and the skewing matrix) and the skewing matrix.

Toward the Performance Versus Feedback Tradeoff for the Two-User MISO Broadcast Channel

For the two-user MISO broadcast channel with imperfect and delayed channel state information at the transmitter (CSIT), the work explores the tradeoff between performance on the one hand, and CSIT timeliness and accuracy on the other hand. This paper considers a broad setting where communication takes place in the presence of a random fading process, and in the presence of a feedback process that, at any point in time, may provide CSIT estimates-of some arbitrary accuracy - for any past, current or future channel realization. This feedback quality may fluctuate in time across all ranges of CSIT accuracy and timeliness, ranging from perfectly accurate and instantaneously available estimates, to delayed estimates of minimal accuracy. Under standard assumptions, the work derives the degrees-of-freedom (DoF) region, which is tight for a large range of CSIT quality. This derived DoF region concisely captures the effect of channel correlations, the accuracy of predicted, current, and delayed-CSIT, and generally captures the effect of the quality of CSIT offered at any time, about any channel. This paper also introduces novel schemes which-in the context of imperfect and delayed CSIT-employ encoding and decoding with a phase-Markov structure. The results hold for a large class of block and nonblock fading channel models, and they unify and extend many prior attempts to capture the effect of imperfect and delayed feedback. This generality also allows for consideration of novel pertinent settings, such as the new periodically evolving feedback setting, where a gradual accumulation of feedback bits progressively improves CSIT as time progresses across a finite coherence period.

Throughput analysis of transmit-nulling SDMA with limited feedback

We recently proposed a precoder codebook for a transmit-nulling space-division multiple access (TN-SDMA) to share spectrum with existing wireless services. Since a portion of the spatial subspaces of a multiantenna broadcast channel is used to eliminate the interference to coexisting systems, TN-SDMA could benefit the efficiency of spectrum usage from the coexistence of different systems in the same band while it always yields lower throughput per unit bandwidth than the orthogonal SDMA (called per user unitary and rate control (PU2RC)) that utilizes all the spatial subspaces for data transmission. This study aims to theoretically quantify the throughput loss of TN-SDMA relative to PU2RC and to analyze the effect of the main system parameters (signal-to-noise ratio (SNR) and the numbers of transmit antennas, users, and feedback bits) on the throughput loss. We derive the theoretical upper bound of the throughput loss of TN-SDMA relative to PU2RC, with the same feedback bits (codebook size). The throughput loss is lower with more transmit antennas, fewer users, lower SNR, or fewer feedback bits. It is interesting to note that the throughput loss converges to an upper limit with an increase in the SNR, which indicates that the SNR has a comparatively minor effect on the throughput loss in the high-SNR region. We also derive the required additional number of feedback bits for TN-SDMA to achieve the throughput of PU2RC (i.e., zero throughput loss). We find that the throughput achieved is feasible at the cost of a practically small number of additional feedback bits.

Adaptive orthogonal frequency division multiplexing underwater acoustic communications with limited feedback

In orthogonal frequency division multiplexing (OFDM) underwater acoustic (UWA) communications, some subcarriers may be subject to deep fading. If the channel state information (CSI) is available at the transmitter side, adaptive transmission techniques (e.g., power allocation) can be applied to mitigate the selective fading effect and increase the overall performance. Therefore, it is more valuable to analyze the UWA channel with limited CSI feedback. In this paper, we adopt two time-varying shallow water acoustic channels (slow-varying environment and fast-varying environment) as examples. Lloyd algorithm is employed to quantize the CSI at the receiver and construct the codebook, which is also known to the transmitter. Simulation results compare the performance between two different channels, and the performance between a few bits of feedback, perfect feedback, and non-feedback, respectively.

A Differential Feedback Scheme Exploiting the Temporal and Spectral Correlation

Channel state information (CSI) provided by a limited feedback channel can be utilized to increase system throughput. However, in multiple-input-multiple-output (MIMO) systems, the signaling overhead realizing this CSI feedback can be quite large, whereas the capacity of the uplink feedback channel is typically limited. Hence, it is crucial to reduce the amount of feedback bits. Prior work on limited feedback compression commonly adopted the block-fading channel model, where only temporal or spectral correlation in a wireless channel is considered. In this paper, we propose a differential feedback scheme with full use of the temporal and spectral correlations to reduce the feedback load. Then, the minimal differential feedback rate over a MIMO time-frequency (or doubly)-selective fading channel is investigated. Finally, the analysis is verified by simulation results.

Fairness, power allocation, and CSI quantization in block fading multiuser systems

This paper studies the fairness, power allocation, and channel state information (CSI) quantization in multiuser systems utilizing multiple feedback bits per user. For a given set of schedulers, we obtain the system throughput with different power allocation strategies and any combination of different fading distributions. Moreover, the system performance under user different outage probability constraints is investigated. Assuming homogenous users, as a special case, the throughput is determined for fixed and random request networks. Considering nonidentical fading channels between the transmitter and receivers, the throughput is found under the K-significant average feedback bit allocation technique, and two suboptimal fairness schemes are investigated which satisfy different quality-of-service requirements. The results show that using optimal power allocation, the first quantization region (QR) of each user is the only QR for which no power may be allocated. The system outage probability vanishes as the number of users goes to infinity. Users hard outage probability constraints can be satisfied at the cost of one more QR in the channel quantizer. Finally, the proposed fairness schemes are more flexible than the standard proportional fair (PF) scheduling in dealing with the throughput-fairness tradeoff. However, their superiority over PF scheduling depends on the fairness constraint.

Relay Subset Selection in Cooperative Systems with Beamforming and Limited Feedback

We consider a cooperative wireless network with a single source, a single destination and M cooperative relays and show that with limited feedback bits from the destination, using all the relays to perform beamforming may not be optimal. We further assume that the destination chooses N relays with power gains exceeding a threshold to do transmit beamforming and feeds back B bits to specify a precoding vector from a randomly generated precoder codebook. For the given average feedback bits per relay, B = B/M, we find the optimal threshold that maximizes the capacity of a decode-and-forward (DF) system when the number of relays is asymptotically large. We show that the system using this asymptotically optimal threshold has a larger capacity than a system using all relays for finite B, especially for small values of B. We also obtain numerically the optimal thresholds for systems with a finite number of relays. A simplified approach that uses a fixed number of relays is also proposed and leads to significant improvements. Given a fixed power budget, the power allocation between two phases is also examined in networks with asymptotically large numbers of relays, and the optimal power allocation fraction can be found numerically.

Two-Stage Channel Quantization for Scheduling and Beamforming in Network MIMO Systems: Feedback Design and Scaling Laws

This paper proposes an efficient two-stage channel quantization and feedback scheme for the downlink limited-feedback network multiple-input multiple-output (MIMO) system. In the first stage, the users report their best set of base-station antenna and physical resource block combinations, and the base-stations schedule the best user for each antenna in each resource block. The scheduled users are then polled in the second stage to feedback their quantized channel vectors. This paper proposes an analytical framework to show that, under a total feedback budget of B bits, the number of bits assigned to the second feedback stage should scale as log B, and in quantizing channel vectors from different base-stations, each user should allocate feedback bits in proportion to the channel magnitudes in dB scale. Under these optimized bit allocations, the overall sum rate of the system is shown to scale double-logarithmically with B, linearly with the total number of antennas, and logarithmically with transmit power, thus achieving both multiuser diversity and spatial multiplexing gains under limited feedback. Finally, realistic wireless propagation model of an urban small-cell deployment is used to show that the proposed scheme can approach the performance of a network MIMO system with full channel state information with only modest amount of channel feedback.

Full-Band CQI Feedback by Huffman Compression in 3GPP LTE Systems

GPP LTE system exhibits a vital feature of Frequency Selective Scheduling(FSS). Frequency scheduling relies on Channel Quality Indicator(CQI) report by the user equipment (UE). The main challenge in CQI reporting consists of reduction in number of reporting bits maintaining the performance of the system. This paper recommends a new CQI reporting approach which uses Huffman compression for reduction of overhead bits for full-band CQI report. Full-band feedback technique transmits entire CQI report instead of reporting limited bands as done in Best-M reporting techniques. The constraint of Huffman coding that the coding table must be known to both transmitter and receiver prior to transmission, is efficiently explored in this technique. Results indicate appreciable reduction in feedback bits as compared to uncompressed full-band reports. Also, full-band Huffman feedback technique shows profound rise in throughput of the system over sub-band feedback techniques. General Terms Wireless communication.

Adaptive Channel Feedback for Coordinated Beamforming in Heterogeneous Networks

In this paper, a channel direction information (CDI) feedback strategy is proposed to assist coordinated multi-point beamforming (CoMP-CB) between a macro base station (MBS) and multiple pico BSs (PBSs) in heterogeneous networks, where the numbers of bits for feedback are adaptive to the average channel gains of multiple users. To maximize the average sum rate achieved by the users with given feedback resource, we optimize the overall number of feedback bits for each user and the codebook sizes for quantizing the per-cell CDIs of each user. To provide feasible feedback scheme for practice use, a low complexity two-step solution is developed from deriving the lower bounds of the per-user average rate. In the first step, we optimize the number of overall feedback bits for each user, which provides a tradeoff between the accuracy of CDI quantization and the reliability of feedback transmission. In the second step, we optimize the bit allocation for quantizing the per-cell CDIs, which accommodates the difference in transmit powers and antennas of the MBS and PBSs and in the channels of macro and pico cells. Two adaptive feedback schemes are developed from two lower bounds. By characterizing the degree of freedom (DoF) achieved by each user, we show that the systems using the proposed adaptive feedback schemes are no longer interference-limited. The maximum per-user DoFs achieved by the proposed schemes depend on the per-antenna feedback resource constraint for each user. Simulation results show significant performance gain of the proposed scheme over NonCoMP and CoMP-CB with fixed codebook size.

Beamformer Design for the MIMO Interference Channels under Limited Channel Feedback

In this paper, beamforming design is considered for a K-user multiple-input-multiple-output interference channel (MIMO-IC) under limited channel feedback budget at the users. Using a geometrical framework, an analytical expression for the upper bound on the total rate loss due to limited channel feedback is derived. The rate loss is a function of the total feedback budget available to the receiver, the strength of the desired and the interfering links at the users and the network parameters. This rate loss upper bound metric is then minimized at each receiver locally by allocating the feedback bits optimally among the links observed by it. We obtain an interesting result that under limited channel feedback budget at the users, interference alignment (IA) may not be the appropriate beamforming strategy. Based on the feedback bit allocation policy to minimize the rate loss upper bound at the receiver and the corresponding channel quantization error statistics at the transmitter, an improved beamformer design is proposed. The proposed beamformer shows a higher throughput gain compared to all other existing schemes.