Channel Information Feedback Research Articles

Open‐loop precoder for spatial multiplexing in MIMO channels with phase correlation

An open-loop precoder, which does not require any feedback channel information, is proposed to enhance the performance of spatial multiplexing in transmit-correlated multi-input–multi-output (MIMO) channels. Different from previous open-loop precoders based only on the amplitude parameter of channel correlation, the proposed precoder, designed with both amplitude and phase parameters, is more suitable for practical environments. The proposed scheme performs better than previous schemes in MIMO channels with random phase correlation.

IEEE 802.11n WLAN에서 블록 승인 기반의 동적 A-MPDU 집적 방법

By adopting the MAC(Media Access Control) protocol which enables to pack multiple MPDUs(MAC-level Protocol Data Units) into a single PPDU(Physical Layer Protocol Data Unit), IEEE 802.11n WLAN supports high throughput. Up to now, there have been a lot of existing channel information-based A-MPDU schemes dynamically determining the number of MSDUs according to the wireless channel condition information which is sent from the receiver to sender. However, the channel information-based scheme has a serious drawback having a high system overhead due to the frequent channel feedback information. To reduce the system overhead, the proposed BA-base dynamic A-MPDU scheme simply chooses the number of MSDUs to be retransmitted by not the frequent channel feedback information but the BA signal representing whether MPSUs belonging to the A-MPDU are received or not. Through NS-2(Network Simulator-2), we found that the proposed scheme had higher throughput and lower packet error rate than the existing fixed A-MPDU scheme.

Design of double codebook based on 3D dual-polarized channel for multiuser MIMO system

Abstract This paper proposes a precoding method based on double codebook for multiuser three-dimensional multiple-input multiple-output (3D MIMO) system deployed with dual-polarized antenna array. Considering the block diagonal feature of the transmit correlation matrix of 3D dual-polarized channel, we adopt the double codebook structure, where the precoding matrix is composed of an inner long-term matrix and an outer short-term matrix. Then, we introduce a set of 3D beams which possess good discrimination in horizontal and vertical domains to be the basic beam set and adopt a topology describing the 3D beam set to illustrate the angle information in an intuitive way. For the purpose of multiuser transmission, we also propose a corresponding limited channel information feedback scheme and a user scheduling criteria based on this double codebook structure. Simulation results demonstrate that the precoding method based on double codebook for the dual-polarized 3D MIMO system effectively improves system throughput compared with 2D transmission scheme.

Smart color channel allocation for visible light communication cell ID

In wireless communication network, multi-channel communication technology is a promising solution for throughput performance enhancement and collision avoidance. Similarly with visible light communication, multi-channel approach will show more advances due to the limitation of color channel and co-channel interference at the photo detector. In this research, we proposed a new color channel allocation for visible light communication Cell ID based on IEEE 802.15.7 specification. The proposed scheme is based on feedback channel information, channel priority and cognitive technique. We hope that with the flexibility, efficiency, and reliable scheduling of cognitive issue, it proves to a promising technology for future wireless network generally and especially visible light communication. The numerical results and experimental implementation show the enhancement of signal-to-noise ratio, blocking probability, and CCA time.

Adaptive Generator Sequence Selection in Multilevel Space–Time Trellis Codes

It has been shown that multilevel space–time trellis codes (MLSTTCs) designed by combining multilevel coding (MLC) with space–time trellis codes (STTCs) can provide improvement in diversity gain and coding gain of the STTCs. MLSTTCs assume perfect channel state information (CSI) at receiver and no knowledge of CSI at transmitter. Weighted multilevel space–time trellis codes (WMLSTTCs), designed by combining MLSTTCs and perfect CSI at transmitter are capable of providing improvement in coding gain of MLSTTCs. In this paper, we present improvement in performance of MLSTTCs by using channel feedback information from the receiver for adaptive selection of generator sequences. The selected generator sequences are used for encoding the component STTCs. The receiver compares current channel profile at receiver with a set of predetermined channel profiles, and sends an index of a predefined channel profile closest to the current channel profile to the transmitter. The transmitter selects a code set that matches best with the current channel profile at receiver using the index. The selected code set having different sets of generator sequences is used by STTC encoders to generate dynamic space–time trellis codes (DSTTCs). The DSTTCs act as component codes in multilevel coding for generating new codes henceforth referred to as multilevel dynamic space–time trellis codes (MLDSTTCs). Analysis and simulation results show that MLDSTTCs provide improvement in performance over MLSTTCs.

ADAM: An Adaptive Beamforming System for Multicasting in Wireless LANs

We present the design and implementation of ADAM, the first adaptive beamforming-based multicast system and experimental framework for indoor wireless environments. ADAM addresses the joint problem of adaptive beamformer design at the PHY layer and client scheduling at the MAC layer by proposing efficient algorithms that are amenable to practical implementation. ADAM is implemented on a field programmable gate array (FPGA) platform, and its performance is compared against that of omnidirectional and switched beamforming based multicast. Our experimental results reveal that: 1) switched multicast beamforming has limited gains in indoor multipath environments, whose deficiencies can be effectively overcome by ADAM to yield an average gain of threefold; 2) the higher the dynamic range of the discrete transmission rates employed by the MAC hardware, the higher the gains in ADAM's performance, yielding up to ninefold improvement over omni with the 802.11 rate table; and 3) finally, ADAM's performance is susceptible to channel variations due to user mobility and infrequent channel information feedback. However, we show that training ADAM's signal-to-noise ratio (SNR)-rate mapping to incorporate feedback rate and coherence time significantly increases its robustness to channel dynamics.

Dual-Mode Low-Complexity Codebook Searching Algorithm and VLSI Architecture for LTE/LTE-Advanced Systems

Codebook is a useful precoding technique for bandwidth limited systems because only few bits are required to feedback channel information. Many researchers propose several codebook selection criteria for linear receivers. However, these selection criteria are very computationally intensive, and most of them are not feasible for aperiodic feedback reporting in both LTE and LTE-Advanced (LTE-A) systems. Under the limited processing hardware and feedback delay constraints, low-complexity receivers with efficient codebook selection scheme are desirable. In this work, a low-complexity codebook searching engine is proposed to support both LTE and LTE-A operations. Also, it can be operated for both aperiodic and periodic feedback reporting. The properties and mutual correlations of the LTE and LTE-A codebooks are firstly analyzed. Then, a low-complexity grouping FFT-based codebook searching algorithm is proposed, which can be shared by LTE and LTE-A systems. Since the proposed algorithm is an algorithmic transformation, there is no performance loss. The proposed schemes have significant effect of reducing the number of multiplications by 56% compared with referenced works. Finally, a dual-mode low-complexity codebook searching engine with TSMC 90 nm is implemented. The IP size is 2.2 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and the equivalent gate count is 547.6 K. The operating frequency is 125 MHz.

Efficient Power Allocation in Turbo Decoded OFDM Systems Using Logarithmic Maximum A-Posteriori Algorithm and Soft Output Viterbi Algorithm Under Various Fading Environments

Orthogonal frequency division multiplexing (OFDM) is a multicarrier modulation technique which is gaining wide popularity in Wireless cellular networks as it is used to mitigate the multipath effects and to achieve high data rate. The Coded Orthogonal Frequency Division Multiplex (COFDM) technique is an extension of the conventional OFDM systems as they provide reliable reception of signals affected by strong distortions. The increase spectral efficiency in turn increases power. Therefore power optimization has to be considered, when coding is incorporated into the existing system. In this paper, the conventional Viterbi decoder is used along with soft coding technique which provides better convergence property. These results are compared to the Soft Output Viterbi Algorithm (SOVA) and Logarithmic Maximum A-posteriori Decoder (Log-Map) which give better performance than conventional Viterbi algorithm. The power optimization is done in all the cases and the advantages of multicarrier modulation are thus reassured without much channel information feedback from receiver to transmitter.

Robust Multi-Antenna Multi-User Precoding Based on Generalized Multi-Unitary Decomposition With Partial CSI Feedback

In this paper, we present a novel matrix decomposition method, called generalized multi-unitary decomposition (GMUD), and use it to achieve robust multi-antenna multi-user multiple-input-multiple-output (MIMO) precoding with limited and partial channel information feedback. GMUD transforms complex matrix {H} into H = P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">θ</sub> , <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">θ,r</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</sup> , where R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> is a lower triangular matrix, and P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">θ, r</sub> and Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">θ, r</sub> are unitary matrices. A unique feature of GMUD is that it gives multiple solutions of the unitary matrices P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">θ, r</sub> and Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">θ, r</sub> based on two parameters, namely the direction parameter θ and magnitude parameter r. This property enables the GMUD precoder to steer the precoding vectors of different users to jointly optimize their performance. Compared with regularized-inverse precoding (an existing multi-user MIMO precoding technique that we have extended to handle multiple receive antennas), the advantages of GMUD precoding are that it requires only partial channel state information (CSI) from the users, and its performance is robust to CSI quantization.

A New Precoding Scheme for the Codebook Based MISO Beamforming

Feedback of channel information can be used in MISO precoding to increase link reliability. To reduce the overhead in feedback signaling channel, codebook based precoding scheme is adopted in practical systems including LTE. However, the size-limited codebook has obvious quantization error which will incur some performance loss compared to the optimal precoding. In this paper, we propose a new precoding scheme for the codebook based MISO system. The proposed scheme utilized the PMI report from UE to modify the precoding vector towards the optimal beamforming vector on per slot basis. Simulation results indicate that the proposed scheme has obvious SNR gain over the conventional precoding scheme under same feedback overhead. 

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.

Mean Value-Based Power Allocation without Instantaneous CSI Feedback in Spectrum Sharing Systems

In this paper, we investigate a power allocation method without feedback of the instantaneous channel gain between a secondary transmitter (ST) and a primary receiver (PR) in spectrum sharing systems where channel information between the ST and the PR is outdated. We propose a mean value-based power allocation scheme (MVPA), which uses only the mean value of the channel gain of the ST-PR link. The proposed MVPA satisfies the interference outage constraint at the PR without feedback of the instantaneous channel information, which enables MVPA to reduce feedback burden. We also derive the ergodic capacity based on MVPA in closed-form and compare the ergodic capacity based on MVPA with that of a power allocation method using the outdated channel information instantaneously. Through the comparison, we convincingly demonstrate that MVPA without feedback of instantaneous channel information provides more reliable and superior capacity performance by reducing capacity loss from which the power allocation method using the outdated channel information severely suffers.

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.

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.

Feedback Reduction for CDI-Based Beamforming in the MIMO Broadcast Channel

Recent research has demonstrated that for the multi-antenna broadcast channel, applying transmit precoding based on the channel spatial covariance matrices leads to temporal stability of the achievable network sum rate. This approach allows reduction in the frequency of channel information feedback events at the expense of increased feedback complexity. While recent work has demonstrated the possibility of feedback complexity reduction through matrix decompositions, this paper demonstrates significant additional feedback reduction through compression of the covariance matrices using the Karhunen-Loève Transform.

MIMO Precoding Using Rotating Codebooks

Next-generation wireless communications rely on multiple-input-multiple-output (MIMO) techniques to achieve high data rates. Feedback of channel information can be used in MIMO precoding to fully activate the strongest channel modes and improve MIMO performance. Unfortunately, the bandwidth of the control channel by which the feedback is conveyed is severely limited. An important issue is how to improve the MIMO precoding performance with minimal feedback. In this paper, we present a method that uses a rotating codebook technique to effectively improve the precoding performance without the need to increase feedback overhead. The basic idea of the rotating codebook precoding is to expend the effective precoding codebook size via rotating multiple codebooks so that the number of feedback bits remains unchanged. Simulation results are presented to show the performance gain of the proposed rotating codebook precoding over conventional precoding.

Performance of Adaptive Modulation with STBC and Imperfect Feedback Information over Rayleigh Fading Channel

Based on imperfect channel information feedback, t he performance of space-time coded multi-antenna systems with adaptive modulation and feedback delay is investigated in Rayleigh fading channel , and the corresponding theoretical analysis for spectrum efficiency (SE) and bit error rate (BER) is presented . T he fading gain value is partitioned into a number of regions by which the modulation is adapted in terms of the region the fading gain falls in . The fading gain switching thresholds for attaining maximum spectrum efficiency under a target BER and fixed power constraint are achieved. Based on the se results, we derive the theoretical calculation formulae for the system spectrum efficiency and average BER in detail. As a result, the closed-form expressions for the spectrum efficiency and average BER are respectively obtained for perfect feedback . Besides, the system performance with delayed feedback information is analyzed, and t he accurate and approximate BER expressions of the systems with imperfect feedback are derived, respectively. The obtained theoretical formulae will have more accuracy than the existing formulae. Simulation results show that the theoretical SE and BER are in good agreement with the corresponding simulated values, and the system has slight BER performance degradation for the normalized time delay less than 0.01.

Rate Scaling Laws in Multicell Networks Under Distributed Power Control and User Scheduling

We analyze the sum rate performance in multicell single-hop networks where access points are allowed to cooperate in terms of a joint resource allocation. The resource allocation policies considered here combine power control and user scheduling. Although promising from a conceptual point of view, the optimization of the sum of per-link rates hinges on tough issues such as computational complexity and the requirement for heavy receiver-to-transmitter and cell-to-cell channel information feedback. In this paper, however, we show that simple distributed algorithms can scale optimally in terms of rates, when the number of users per cell U is allowed to grow large. We use extreme value theory to provide scaling laws for upper and lower bounds for the network sum-rate (sum of single user rates over all cells), corresponding to zero-interference and worst-case interference scenarios. We show that the scaling is either dominated by path loss statistics or by small-scale fading, depending on the regime and user location scenario. A surprising result is that the well known log log U rate behavior exhibited in i.i.d. fading channels with maximum rate schedulers is transformed into a log U behavior when path loss is accounted for. Additionally, by showing that upper and lower rate bounds behave in fact identically, asymptotically, our results suggest, remarkably, that the impact of multicell interference on the rate (in terms of scaling) actually vanishes asymptotically, when appropriate resource allocation policies are used.

Opportunistic Multicasting Scheduling Using Erasure-Correction Coding over Wireless Channels

This paper proposes an opportunistic multicast scheduling scheme using erasure-correction coding to jointly explore the multicast gain and multiuser diversity. For each transmission, the proposed scheme sends only one copy to all users in the multicast group at a transmission rate determined by a SNR threshold. Analytical framework is developed to establish the optimum selection of the SNR threshold and coding rate for given channel conditions to achieve the best throughput in both cases of full channel knowledge and only partial channel knowledge of the average SNR and fading type. Numerical results show that the proposed scheme outperforms both the worst-user and best-user schemes for a wide range of average SNR and multicast group size. Our study indicates that full channel knowledge is only significantly beneficial at smallmulticast group size. For a large multicast group, partial channel knowledge is sufficient to closely approach the achievable throughput in the case of full channel knowledge while it can significantly reduce the overhead required for channel information feedback. Further extension of the proposed scheme applied to OFDM system to exploit frequency diversity in a frequency-selective fading environment illustrates that a considerable delay reduction can be achieved with negligible degradation in multicast throughput.

Asymptotic Achievable Rate Analysis for Selection Strategies in Amplify-and-Forward MIMO Two-Hop Networks With Feedback

This paper investigates relay-selection strategies in multiple-input-multiple-output (MIMO) relay networks, where joint source and relay preceding are utilized, and a single MIMO relay is adaptively selected for forwarding signals during each transmission period. Assuming that an optimal relay-selection scheme is implemented with full channel-information feedback, we study the optimal selection performance by characterizing its achievable rate with two derived bounds. Based on the rate bounds, we show that the corresponding achievable rate scales as (M/2) log log K for a fixed number of antennas M and an increasing number of relay nodes K → ∞. Furthermore, for feedback reduction, we propose a relay-selection scheme with limited feedback while still achieving the optimal achievable rate-scaling law for a large K. Numerical results verify the effectiveness of the limited feedback-selection strategy under practical values of relay number K.