Reduced Feedback Schemes Research Articles

Cognitive Sub-Band Scheduling and Link Adaptation for 5G URLLC

Link adaptation for downlink 5G ultra-reliable low latency communication (URLLC) is quite challenging. While reduced feedback schemes for link adaptation are crucial to achieve high spectral efficiency, accuracy of sub-band channel quality indicator (CQI) reports is more critical in 5G URLLC because of the ultra-high reliability and efficient resource utilization requirements. The existing differential feedback mechanism cannot reconstruct sub-band CQIs at the BS exactly. In this paper, existing differential feedback mechanism for link adaptation is first investigated using analytical and simulation models to verify its limitations in reconstructing the sub-band CQIs and to study its impact on URLLC reliability and resource utilization efficiency. Then a cognitive link adaptation using a controlled differential feedback scheme for sub-band CQIs is proposed based on the investigations. Extensive simulation results are provided to show the superior performance of the proposed cognitive solution and to illustrate the theoretical analysis.

User-Pair Scheduling and Mode Selection in Asymmetric Full-Duplex Systems With Limited Feedback: Algorithm and Scaling Laws

Determining which users to simultaneously schedule on the uplink and the downlink in a full-duplex (FD) system is crucial to control the inter-user interference between them and achieve a high spectral efficiency. The user-pair scheduling algorithm and the efficacy of FD is, thus, closely linked to the availability of channel state information and the feedback scheme that conveys it to the base station (BS). We consider a reduced feedback scheme in which a user that is scheduled feeds back only a limited number of quantized inter-user interferences that are below a pre-specified threshold. For it, we propose a novel user-pair scheduling and mode selection algorithm (UPSMA). We analyze the uplink and downlink spectral efficiencies of UPSMA for two channel models that highlight the different influences of small-scale fading and large-scale shadowing. We derive insightful asymptotic scaling laws that quantify the dependence of the threshold and the uplink and downlink spectral efficiencies on the number of users. UPSMA with limited feedback achieves a higher sum spectral efficiency than a half-duplex system and conventional FD resource allocation algorithms. Its performance is close to the exhaustive search algorithm in which the BS knows all the inter-user interferences.

Exploiting Correlation With Wideband CQI and Making Differential Feedback Overhead Flexible in 4G/5G OFDM Systems

Differential channel quality indicator (CQI) and wideband CQI are key components of the reduced feedback schemes employed in the 5G New Radio (NR) and 4G Long Term Evolution (LTE) standards. They enable a base station (BS) to acquire channel state information that is essential for rate adaptation and frequency-domain scheduling without overwhelming the uplink. We present a novel throughput-optimal rate adaptation rule that exploits the correlation between the differential and wideband CQIs to improve throughput without any additional feedback. It also shows that the prevalent conventional method that adds the two CQIs is sub-optimal. We then propose a novel flexible-overhead differential CQI feedback scheme, in which the number of bits for differential CQI can be different across the subbands. This provides a new flexibility to the BS to control the feedback overhead. It differs from the current rigid parameterization, in which a user always feeds back a 2-bit differential CQI for each subband. In various single-user and multi-user deployment scenarios involving small-scale fading, large-scale shadowing, and co-channel interference, the proposed approach achieves nearly the same throughput as the feedback scheme employed in 5G and LTE, but with much less overhead.

Modeling and Analysis of Differential CQI Feedback in 4G/5G OFDM Cellular Systems

Reduced feedback schemes are crucial in achieving the high data rates expected of orthogonal frequency-division multiplexing-based 4G and 5G cellular systems. They ensure that the feedback overhead required for acquiring the channel-state information to enable downlink scheduling and adaptive modulation and coding at the base station (BS) does not overwhelm the uplink. We present a novel modeling and analysis of the single-user and multi-user throughputs of the differential feedback scheme that is used in both 4G and 5G standards. In this feedback scheme, a user feeds back a 4-bit wideband channel quality indicator (CQI), which indicates the rate that the user can decode if the BS was to transmit to it over the entire system bandwidth, and a 2-bit differential CQI for each subband relative to it. Our analysis incorporates co-channel interference, different single-stream multi-antenna modes, and different schedulers, which cover a wide range of the tradeoff between the throughput and user fairness. It brings out several insights such as the increase in the throughput as the correlation between subbands increases and how differential feedback reduces the overhead significantly while only marginally reducing the throughput.

Base Station-Side Rate Estimation for Threshold-Based Feedback, and Design Implications in Multi-User OFDM Systems

Rate adaptation and scheduling are essential in ensuring that contemporary orthogonal frequency division multiplexing systems achieve high downlink spectral efficiencies. They depend upon reduced feedback schemes to efficiently feedback channel state information from the users to the base station (BS). In the popular threshold-based quantized feedback scheme, a user feeds back to the BS the quantized value of the signal-to-noise ratio for each subchannel. For this scheme, we derive a novel, throughput-optimal discrete rate adaptation (TORA) policy, which enables a system designer to reduce the feedback overhead. We present it in closed form for different multi-antenna diversity modes for the exponentially correlated subchannel gains model. We also develop a computationally simpler suboptimal variant of it. We derive an insightful lower bound for the fading- and user location-averaged throughput gain achieved by TORA over conventional rate adaptation for 1-bit feedback. We present extensive results to benchmark the system-level performance of TORA for different numbers of feedback bits and modulation and coding schemes available at the BS, and various schedulers, quantizers, and multi-antenna modes.

Throughput-Optimal Scheduling and Rate Adaptation for Reduced Feedback Best- $M$ Scheme in OFDM Systems

In orthogonal frequency division multiplexing systems, reduced feedback schemes provide essential channel state information from the users to the base station (BS) without overwhelming the uplink. For the practically important best- $M$ scheme, in which each user feeds back only its $M$ strongest subchannels and their indices to the BS, we derive a novel, throughput-optimal scheduling and rate adaptation policy that enables the BS to schedule the best user and its data rate for all the subchannels. The policy exploits the structure of the information fed back by the best- $M$ scheme and the correlation among subchannel gains. We present it in closed-form for the widely studied exponential correlation model. Using insights gleaned from the optimal policy, we propose a novel, low-complexity two subchannel reduction approach, which is seen empirically to be near-optimal and easily handles practically important general channel correlation models, quantized feedback, and co-channel interference in multi-cell scenarios. Compared with several ad hoc approaches, the proposed approaches improve the cell throughput without any additional feedback. A modified gradient-based opportunistic scheduler is also proposed to ensure user fairness.

Best-M Feedback in OFDM: Base-Station-Side Estimation and System Implications

Reduced feedback schemes play a critical role in orthogonal frequency division multiplexing-based cellular systems because they facilitate scheduling and rate adaptation by the base station (BS) while reducing the number of subchannels for which channel state information is fed back by the users. We address the problem of reliable transmission even on subchannels that are fed back by a few users due to feedback constraints. For the practically relevant best- $M$ feedback scheme, in which each user reports only its $M$ strongest subchannels to the BS, we derive a nonlinear constrained minimum mean square error estimator that enables the BS to estimate the signal-to-noise-ratios of all the subchannels of every user. We then propose two lower computational complexity approaches that incur a negligible loss in performance. The novelty of these approaches lies in their exploitation of the structure of the best- $M$ feedback information and the correlation among subchannel gains. Applications to general channel models and to quantized feedback are also shown. In terms of system-level impact, the proposed approaches improve the cell throughput compared to several conventional approaches – without requiring any additional feedback – for uncorrelated and correlated subchannels, and for various schedulers.

Optimal power policies and throughput scaling analyses in fading cognitive broadcast channels with primary outage probability constraint

Abstract This paper focuses on a spectrum-sharing-based fading cognitive radio broadcast channel (BC) with a single-antenna secondary base station (SBS) and M single-antenna secondary receivers (SRs) utilizing the same spectrum band with a delay-sensitive primary user (PU). The service-quality requirement for the primary user is set by an outage probability constraint (POC). We address the optimal power allocation problem for the SBS ergodic sum capacity (ESC) maximization in the secondary BC network subject to POC and a transmit power constraint at SBS specified by either a long-term or a short-term power constraint. The optimality conditions reveal that in each joint channel state, the SBS allocates transmission power to the only one selected SR with the highest value of a certain metric consisting of the ratio of the SR’s direct channel power gain and the sum of interference power and noise power at the SR. Then, the secondary network throughput scaling analysis as the number of SRs becomes large, is also investigated, showing that if PU applies a truncated channel inversion (TCI) power policy, the SBS ESC scales like ε p log(logM) where ε p is the PU outage probability threshold. To reduce the amount of channel side information (CSI) transferred between the two networks, we propose a suboptimal transmission scheme which requires only 1-bit feedback from the delay-sensitive PR (partial CSI). We show that the new power control policy is asymptotically optimal, i.e. the SBS ESC under this reduced feedback scheme still scales like ε p log(logM).

Analytical evaluation of hybrid feedback scheme for OFDMA system

This paper first introduces the performance analysis of two classical channel quality indicator (CQI) feedback schemes which are best-n feedback and the threshold based feedback and derives the mathematical expressions of average capacity which is described by Theorem 1 and 2. Then, a reduced feedback scheme is designed for multiple traffics and multi-channel. The novel scheme combines the best-n feedback and the threshold based feedback together to reduce the feedback overhead. The proposed scheme can not only guarantee the quality of service (QoS) requirement of real time (RT) traffic but also reduce feedback overhead at the cost of a marginal increased downlink overhead. Simulation results demonstrated the good performance of the proposed feedback scheme.

Novel Reduced-Feedback Wireless Communication Systems

Modern communication systems apply channel-aware adaptive transmission techniques and dynamic resource allocation in order to exploit the peak conditions of the fading wireless links and to enable significant performance gains. However, conveying the channel state information among the users’ mobile terminals into the access points of the network consumes a significant portion of the scarce air-link resources and depletes the battery resources of the mobile terminals rapidly. Despite its evident drawbacks, the channel information feedback cannot be eliminated in modern wireless networks because blind communication technologies cannot support the ever-increasing transmission rates and high quality of experience demands of current ubiquitous services. Developing new transmission technologies with reduced-feedback requirements is sought. Network operators will benefit from releasing the bandwidth resources reserved for the feedback communications and the clients will enjoy the extended battery life of their mobile devices. The main technical challenge is to preserve the prospected transmission rates over the network despite decreasing the channel information feedback significantly. This is a noteworthy research theme especially that there is no mature theory for feedback communication in the existing literature despite the growing number of publications about the topic in the last few years. More research efforts are needed to characterize the trade-off between the achievable rate and the required channel information and to design new reduced-feedback schemes that can be flexibly controlled based on the operator preferences. Such schemes can be then introduced into the standardization bodies for consideration in next generation broadband systems. We have recently contributed to this field and published several journal and conference papers. We are the pioneers to propose a novel reduced-feedback opportunistic scheduling scheme that combines many desired features including fairness in resources distribution across the active terminals and distributed processing at the MAC layer level. In addition our scheme operates close to the upper capacity limits of achievable transmission rates over wireless links. We have also proposed another hybrid scheme that enables adjusting the feedback load flexibly based on rates requirements. We are currently investigating other novel ideas to design reduced-feedback communication systems.

Sum Rate Analysis of a Reduced Feedback OFDMA Downlink System Employing Joint Scheduling and Diversity

We consider joint scheduling and diversity to enhance the benefits of multiuser diversity in an \OFDMA{} system. The \OFDMA{} spectrum is assumed to consist of $\Nrb$ resource blocks and the reduced feedback scheme consists of each user feeding back channel quality information (\CQI) for only the best-$\NFb$ resource blocks. Assuming largest normalized \CQI{} scheduling and a general value for $\NFb$, we develop a unified framework to analyze the sum rate of the system for both the quantized and non-quantized \CQI{} feedback schemes. Based on this framework, we provide closed-form expressions for the sum rate for three different multi-antenna transmitter schemes; Transmit antenna selection (\TAS), orthogonal space time block codes (\OSTBC) and cyclic delay diversity (\CDD). Furthermore, we approximate the sum rate expression and determine the feedback ratio $(\frac{\NFb}{\Nrb})$ required to achieve a sum rate comparable to the sum rate obtained by a full feedback scheme.

Reduced Feedback Schemes Using Random Beamforming in MIMO Broadcast Channels

A random beamforming scheme for the Gaussian MIMO broadcast channel with channel quality feedback is investigated and extended. Considering the case where the n receivers each have N receive antennas, the effects of feeding back various amounts of signal-to-interference-plus-noise ratio (SINR) information are analyzed. Using the results from order statistics of the ratio of a linear combination of exponential random variables, the distribution function of the maximum order statistic of the SINR observed at the receiver is found. The analysis from viewing each antenna as an individual user is extended to allow combining at the receivers, where it is known that the linear MMSE combiner is the optimal linear receiver and the CDF for the SINR after optimal combining is derived. Analytically, using the Delta Method, the asymptotic distribution of the maximum order statistic of the SINR with and without combining is shown to be, in the nomenclature of extreme order statistics, of type 3. The throughput of the feedback schemes are shown to exhibit optimal scaling asymptotically in the number of users. Finally, to further reduce the amount of feedback, a hard threshold is applied to the SINR feedback. The amount of feedback saved by implementing a hard threshold is determined and the effect on the system throughput is analyzed and bounded.

Calcium-Actin Waves and Oscillations of Cellular Membranes

We propose a mechanism for the formation of membrane oscillations and traveling waves, which arise due to the coupling between the actin cytoskeleton and the calcium flux through the membrane. In our model, the fluid cell membrane has a mobile but constant population of proteins with a convex spontaneous curvature, which act as nucleators of actin polymerization and adhesion. Such a continuum model couples the forces of cell-substrate adhesion, actin polymerization, membrane curvature, and the flux of calcium through the membrane. Linear stability analysis shows that sufficiently strong coupling among the calcium, membrane, and protein dynamics may induce robust traveling waves on the membrane. This result was checked for a reduced feedback scheme and is compared to the results without the effects of calcium, where permanent phase separation without waves or oscillations is obtained. The model results are compared to the published observations of calcium waves in cell membranes, and a number of testable predictions are proposed.

Performance analysis of downlink OFDMA resource allocation with limited feedback

Opportunistic resource allocation in orthogonal frequency division multiple access (OFDMA) with limited feedback improves the performance of wireless networks and it has been considered in several studies. However, the performances of reduced feedback schemes have not been characterized analytically. In this paper, the analysis of the resource allocation in downlink OFDMA with limited feedback is presented. OFDMA system with a practical resource fair round robin allocation and erroneous feedback channel is considered. The performances of the resource allocation based on three feedback methods are analyzed and compared. A resource-wise signal-to-noise ratio (SNR) quantization based feedback method is considered and two feedback strategies are based on the best-M method, where the M best resource blocks are informed to the transmitter. Analytical bit error probability (BEP) expressions are derived for binary phase-shift keying (BPSK) modulation for each feedback strategy. Furthermore, an asymptotic BEP performance for large SNR values is derived for the best-M feedback schemes. The outage capacity results are derived for the considered feedback schemes to illustrate performance with slow link adaptation. Numerical results based on the analysis are compared to those obtained by computer simulations to validate the correctness of the proposed analysis.