Showing papers by "Chenyang Yang published in 2010"

A low-complexity subcarrier-power allocation scheme for frequency-division multiple-access systems

Abstract: This letter aims to design a low-complexity subcarrier-power allocation scheme to improve the communication reliability of various types of frequency-division multiple-access (FDMA) systems. Both uplink and downlink are considered. Specifically, a low-complexity worst subcarrier avoiding (WSA) subcarrier-allocation scheme is proposed, in order to avoid assigning users the subcarriers experiencing severe fading. After the subcarrier-allocation, channel-inversion assisted power-allocation is employed to assign the subcarriers the corresponding power. Our studies and simulation results show that the achievable error performance of the FDMA systems employing the proposed subcarrier-power allocation algorithm is independent of the multiplexing method. The proposed algorithm outperforms the existing subcarrier-power allocation algorithms that have a similar complexity as the proposed one.

System and Method for Cooperative Spectrum Sensing in Cognitive Radio Systems

Abstract: A system and method for cooperative spectrum sensing in cognitive radio (CR) systems is provided. A method for CR user operation includes coordinating with partner CR users to share spectrum sensing information, combining spectrum sensing information from partner CR users, and determining if a transmission opportunity exists based on the combined spectrum sensing information. The method also includes transmitting a message to a CR user if the transmission opportunity exists, and storing the message if the transmission opportunity does not exist.

Transceiver Design for Multi-User Multi-Antenna Two-Way Relay Channels

Abstract: In this paper, we design transceivers in a multi-user multi-antenna two-way relay system, where a single multi-antenna base station exchanges information with multiple users via a single multi-antenna relay station. We consider the half-duplex amplify-and-forward relay protocol. We aim to maximize the bidirectional sum rate under the constraint of no interference among different users. Suboptimal solutions to the problem that respectively maximizing the uplink and downlink rates are derived. We then introduce a threshold to balance the uplink and downlink rates so as to maximize the bidirectional sum rate. Simulation results show that the proposed scheme achieves considerably higher bidirectional sum rate than existing schemes.

Distributed coordinated multi-point downlink transmission with over-the-air communication

Abstract: Base station cooperation transmission, which is also known as coordinated multi-point (CoMP) transmission, is a promising technique to improve system spectrum efficiency in cellular networks, especially for multiple antenna systems. However, the performance gain is significant only when cooperative base stations can gather channel state information (CSI) from all their serving users and can share CSI without delay. In this paper, we propose a distributed downlink CoMP transmission scheme through over-the-air communication among all base stations, which can reduce not only the infrastructural overhead but also the delay of acquiring CSI. We first analyze the CSI acquisition methods in centralized and decentralized CoMP framework. Then we propose a distributed multi-user scheduling and precoding method. Simulation results show that the proposed framework and scheduling algorithm outperforms the centralized and decentralized CoMP when the outdated CSI led by backhaul latency is considered.

Robust Multiuser Precoder for Base Station Cooperative Transmission with Non-Ideal Channel Reciprocity

Abstract: In this paper we present a method to alleviate the performance degradation led by non-ideal channel reciprocity in TDD downlink base station (BS) cooperative transmission systems, which comes from imperfect antenna calibration among BSs. By exploiting the statistics of the ambiguity factors between uplink and downlink channels, a robust multiuser precoder is proposed aimed at maximizing the lower bound of the average signal-to-leakage-plus-noise ratio (SLNR). The precoder is able to adaptively control the cooperation level among the coordinated BSs according to the antenna calibration accuracy among BSs. Simulation results demonstrate an evident performance gain of the proposed robust precoder over the non-robust precoder.

Adaptive interference alignment for femtocell networks

Abstract: In this paper, we propose a spectral efficient transmission scheme for femtocell networks, which includes an adaptive subband partition method and an adaptive interference alignment transceiver. By introducing random frequency hopping to achieve the subband partition, we do not need a central coordinator to control the interference among the femtocells, and we can adjust the number of the interference in each subband. By employing adaptive interference alignment transceiver, the interference led by the subband collision can be suppressed. Simulation results show that the transmission scheme improves the system throughput significantly.

Secondary Transceiver Design in the Presence of Frequency Offset between Primary and Secondary Systems

Abstract: When both primary and secondary systems are orthogonal frequency division multiplexing modulated and are non-cooperative, carrier frequency offset between the systems is inevitable to cause harmful interference. In this paper, we jointly optimize secondary transceivers assuming that the frequency offset between the secondary transmitter (ST) and the primary receiver (PR) and different channel information from the ST to the PR are known at the ST. We first derive unified interference constraints and obtain the secondary transceivers minimizing the mean square error through convex optimization techniques. We then derive closed-form transceivers for several special cases to reveal the impact of the frequency offset on the secondary transceivers. We show that when there is no frequency offset between the ST and the PR, the optimal processing at the ST is power allocation. Otherwise, both power allocation and precoding are necessary. The impact of the frequency offset on the performance of both systems increases as the interference constraints become tighter and the bandwidth of the primary system becomes smaller. When the proposed transceivers are used, the performance of the secondary system is robust to the frequency offset and the performance of the primary system degrades little due to the remanent frequency offset.

Joint Transmitter–Receiver Frequency-Domain Equalization in Generalized Multicarrier Code-Division Multiplexing Systems

Abstract: Joint transmitter-receiver optimization in generalized multicarrier code-division multiplexing (GMC-CDM) systems is investigated in this paper. The optimization consists of a one-tap post-frequency-domain equalizer (post-FDE) and a one-tap pre-FDE. While the one-tap post-FDE is optimized under the criterion of minimum mean square error (MMSE), the one-tap pre-FDE is achieved through three stages of optimization, which are operated at different levels and motivated to achieve, possibly, different objectives, including maximum throughput and maximum reliability. Specifically, in our three-stage pre-FDE, the first-stage pre-FDE is operated at the symbol level, concerning only the symbols within a group. The second-stage pre-FDE is carried out at the group level for harmonization among the groups. Finally, the third-stage pre-FDE handles group partition. In this paper, the error and throughput performance of the GMC-CDM systems is investigated when assuming communications over frequency-selective Rayleigh fading channels. It can be shown that the reliability or throughput of the GMC-CDM systems can be significantly improved by employment of the proposed pre- and post-FDE schemes. Furthermore, the pre- and post-FDE algorithms obtained can be implemented with high flexibility, which facilitates a GMC-CDM system to achieve a good tradeoff between its throughput and reliability.

Channel allocation for cooperative relays in cognitive radio networks

Abstract: In this paper, we investigate channel allocation for cooperative relays in cognitive radio networks. Different from conventional cooperative relay channels, cognitive radio relay channels are actually a combination of three kinds of channels: direct, dual-hop, and relay channels, which belong to different spectrum bands and provide parallel end-to-end transmission. In order to maximize the achievable end-to-end throughput, we propose two channel allocation approaches with different complexities to assign all the channels cooperatively. Numerical results illustrate the performance improvement in different number of available channels. In particular, it has about 40% improvement in throughput when the average SNR is 15 dB and eight available channels are used.

Joint channel estimation for downlink base station cooperative transmission exploiting channel asymmetry

Abstract: In this paper, we study uplink joint channel estimation for downlink base station cooperative transmission orthogonal frequency division multiplexing systems. We present three linear estimators which jointly estimates the channel coefficients from users in different cells with minimum mean square error (MSE), robust design and least square criterion, then we analyze their performance. We find that due to the asymmetric feature of the multi-cell channels in cooperative transmission systems, the MSE for different links with three joint estimators differ. When the channel energy information is exploited for channel estimation, the cross channels that experience large attenuation have smaller estimation MSEs than the local channels, and using non-orthogonal training sequences among users in different cells leads to minor performance loss. This is in contrast to the traditional understanding. To reduce the complexity of the estimators when non-orthogonal training sequences are used, we also present an interference cancelation based estimator. Both the channel estimation errors and the throughput of the cooperative transmission system are simulated to verify our analysis.

Type-based multiple-access with bandwidth extension for the decentralized estimation in wireless sensor networks

Abstract: Type-based multiple-access (TBMA) is a bandwidth efficient transmission scheme for decentralized estimation in wireless sensor networks (WSNs) over non-orthogonal multiple-access channels, whereas its performance degrades severely in zero-mean fading channels. In this paper, we improve its performance in the fading channels by using more resources for transmitting each type, which is named as TBMA with bandwidth extension (TBMA-BE). This transmission scheme needs no feedback of channel state information as existing solutions, thus it is feasible for large scale WSNs. We then develop an approximate maximum likelihood (ML) estimator, and derive the Cramer-Rao lower bound to reveal how the performance and the bandwidth efficiency are related. It is shown from the simulations that the TBMA-BE transmission scheme with the approximate ML estimator performs fairly well in non-orthogonal multiple-access channels with Rayleigh fading, and is even superior to the orthogonal multiple-access protocol at low communication signal to noise ratio (SNR). It also shows that the TBMA-BE is bandwidth efficient for the typical scenarios of WSNs where the communication SNR is low, since the total bandwidth consumption is proportional to the SNR.

Cooperative spectrum sensing with realistic reporting channel

Abstract: In cooperative spectrum sensing, each cognitive radio (CR) user needs to report its local sensing results to a common CR user for a final decision. Most of existing contributions assume that such a reporting is perfect, i.e., there always exists an idle channel to accomplish an error free reporting. However, it is not true in practice since the reporting actually belongs to secondary transmission and may cause interference to primary users. In this paper, we propose a practical reporting approach in cooperative spectrum sensing, called "Listen-Before-Reporting", which considers both the availability and the reliability of the reporting channels. We design the detectors at the FC in both single and multiple realistic reporting bands. Simulation results demonstrate that the proposed scheme performs fairly well under imperfect reporting channels.

Spectral-Efficiency of TDD Multiuser Two-Hop MC-CDMA Systems Employing Egocentric-Altruistic Relay Optimization

Abstract: In this contribution we investigate the spectral-efficiency of a two-hop cooperative network using multicarrier code-division multiple-access (MC-CDMA) transmission scheme. The two-hop network constitutes K source users transmitting signals to K destinations with the aid of N relays. Our focus is on the relay optimization, when assuming that the N relays cooperate or do not cooperate with each other. Specifically, in this contribution the egocentric-altruistic (E-A) optimization is introduced, which constitutes an E-optimization motivating to suppress the multiuser interference (MUI) of the source-relay channels and an A-optimization aiming to pre-mitigate the potential MUI of the relay-destination channels. Both the minimum mean-square error (MMSE) and zero-forcing (ZF) optimization criteria are considered. Furthermore, the spectral-efficiency performance of the two-hop MC-CDMA systems using the proposed E-A relay optimization is investigated by simulations, when assuming communications over frequency-selective fading channels.

Decentralized Estimation over Orthogonal Multiple-access Fading Channels in Wireless Sensor Networks - Optimal and Suboptimal Estimators

Abstract: Optimal and suboptimal decentralized estimators in wireless sensor networks (WSNs) over orthogonal multiple-access fading channels are studied in this paper. Considering multiple-bit quantization before digital transmission, we develop maximum likelihood estimators (MLEs) with both known and unknown channel state information (CSI). When training symbols are available, we derive a MLE that is a special case of the MLE with unknown CSI. It implicitly uses the training symbols to estimate the channel coefficients and exploits the estimated CSI in an optimal way. To reduce the computational complexity, we propose suboptimal estimators. These estimators exploit both signal and data level redundant information to improve the estimation performance. The proposed MLEs reduce to traditional fusion based or diversity based estimators when communications or observations are perfect. By introducing a general message function, the proposed estimators can be applied when various analog or digital transmission schemes are used. The simulations show that the estimators using digital communications with multiple-bit quantization outperform the estimator using analog-and-forwarding transmission in fading channels. When considering the total bandwidth and energy constraints, the MLE using multiple-bit quantization is superior to that using binary quantization at medium and high observation signal-to-noise ratio levels.

Spatial capacity of UWB networks with space-time focusing transmission

Abstract: Space-time focusing transmission in impulse-radio ultra-wideband (IR-UWB) systems resorts to the large number of resolvable paths to reduce the interpulse interference as well as the multiuser interference and to simplify the receiver design. In this paper, we study the spatial capacity of IR-UWB systems with space-time focusing transmission where the users are randomly distributed. We will derive the power distribution of the aggregate interference and investigate the collision probability between the desired focusing peak signal and interference signals. The closed-form expressions of the upper and lower bound of the outage probability and the spatial capacity are obtained. Analysis results reveal the connections between the spatial capacity and various system parameters and channel conditions such as antenna number, frame length, path loss factor, and multipath delay spread, which provide design guidelines for IR-UWB networks.

A joint power control and link scheduling strategy for consecutive transmission in TDMA wireless networks

Abstract: This paper addresses joint power control and link scheduling problem in Spatial-reuse Time Division Multiple Access (STDMA) networks with consecutive transmission constraint, where multiple links should be scheduled in a number of consecutive time slots. By multi-link management with power control and scheduling, the network can provide lower transmission latency or support more users. In this paper, a new heuristic solution is proposed. Different from previous works, our method is based on the lagrangian dual theory and can be applied to more general scenarios. Moreover, it can be efficiently implemented in a polynomial time with the help of binary searching. Simulation results show that the method can reduce the transmission latency significantly.

User grouping based on large scale channel information in multi-cell downlink cooperative multi-antenna systems

Abstract: In this paper, we study low feedback user grouping in downlink cooperative transmission multi-antenna systems. Users in the same group are served on the same frequency and time resource by multiple cooperative BSs using the zero-forcing precoder. We first derive the user's average received signal to interference and noise ratio (SINR). It shows that each user's SINR is an increasing function of the large scale signal to interference ratio (SIR) of users in neighbor cells, which can be obtained using the large scale channel information. We then propose a novel fair user grouping algorithm based on the large scale SIR. Simulation results show that our algorithm achieves better cell edge throughput compared with existing methods with minor reduction of cell throughput.

Spatial User Capacity of UWB Networks with Space-Time Focusing Transmission

Abstract: Space-time focusing transmission in impulse-radio ultra-wideband (IR-UWB) systems resorts to the large number of resolvable paths to reduce the inter-pulse interference as well as the multiuser interference, and to simplify the receiver design. In this paper, we study the spatial user capacity of IR-UWB systems with space-time focusing transmission where the users are randomly distributed. We will derive the power distribution of the aggregate interference and investigate the collision probability between the desired focusing peak signal and interference signals. The closed-form expressions of the upper and lower bound of the outage probability and the spatial user capacity are obtained. Analysis results reveal the connections between the spatial user capacity and various system and channel parameters such as antenna gain, frame length, path loss factor, and multipath delay spread, which provide design guidelines for IR-UWB networks.

Secondary Transceiver Design in the Presence of Frequency Offset between OFDM-Based Primary and Secondary Systems

Abstract: In a cognitive network where both primary and secondary systems are Orthogonal Frequency Division Multiplexing modulated, carrier frequency offset between the two systems is inevitable and may cause harmful interference. In this paper, we jointly optimize the transceiver for the secondary system considering the frequency offset between secondary transmitter (ST) and primary receiver (PR). We first derive unified interference constraints with different information of inference channels known at ST, and formulate the transceiver design problem based on minimum mean square criterion as a convex optimization problem. To reveal the structure of secondary transceiver, we derive closed-form pre- processors in two special cases. It is shown from the simulations that with the increase of frequency offset, the performance of primary system degrades evidently when its bandwidth is smaller than that of secondary system, whereas the performance of secondary system using the proposed transceiver improves due to frequency diversity.

Estimation Feedback and Bandwidth Allocation with the Type-Based Multiple-Access in Wireless Sensor Networks

Abstract: In this paper, we consider the decentralized estimation over non-orthogonal multiple-access fading channels (MACs) with zero-mean channel coefficients in wireless sensor networks (WSNs). Using the type-based multiple-access (TBMA) with bandwidth extension transmission scheme, we introduce a low-complexity two-stage estimator and analyze its performance. It shows that the performance can be improved by allocating bandwidth among types, but the optimal bandwidth allocation scheme requires a priori information of the parameter being observed. We then introduce a broadcasting feedback method for the sensors to obtain the required information. This feedback scheme is more efficient and feasible than the channel state information (CSI) feedback over orthogonal MACs. Besides the estimation accuracy, we also evaluate the bandwidth efficiency of the proposed methods by introducing a metric as the bandwidth normalized performance gain in simulations. It shows that the proposed feedback and bandwidth allocation schemes outperform the TBMA with CSI feedback in non-orthogonal MACs and the optimal transmission scheme in orthogonal MACs at typical operating signal-to-noise ratio levels of the WSNs.

Impact of Channel Asymmetry on Base Station Cooperative Transmission with Limited Feedback

Abstract: Base station (BS) cooperative transmission, also known as coordinated multi-point transmission (CoMP), is an effective way to avoid inter-cell interference in universal frequency reuse cellular systems. To gain the promised benefit, however, huge feedback overhead is in demand to gather the channel information. In this paper, we analyze the impact of channel asymmetry, which is inherent in CoMP systems, on downlink BS cooperative transmission with limited feedback. We analyze the per-user rate loss of a multi-user CoMP system led by quantization. Per-cell quantization of multicell channels is considered, which quantizes the local channel and cross channel separately and is more feasible in practice. From both the analytical and simulation results, we provide a whole picture on various critical factors that lead to the performance loss. Specifically, we show that the per user rate loss led by limited feedback depends on the location of its paired users, except for relying on its own signal to noise ratio and the quantization errors as in single cell multi-user multiple antenna systems. This implies that the quantization accuracy required for local and cross channel of each user depends on the locations of its own as well as its paired users.

A precoder for reducing downlink interference from femtocells to macro users based on cognitive radio

Abstract: This paper proposes a preprocessor for femtocell base stations to avoid the downlink interference to the macro users, where both systems are multicarrier systems. By using the concept of cognitive radio, we design a low complexity transmitter including a precoder followed by a power allocation. With the judicious design of the interference tolerant constraints, the performance degradation of the macro user is controlled to be less than a predetermined value. Simulation results show that the spectrum efficiency of the femtocell system with the presented transmitter is improved significantly.

Impact of Channel Asymmetry on Performance of Channel Estimation and Precoding for Downlink Base Station Cooperative Transmission

Abstract: Base station (BS) cooperative transmission can improve the spectrum efficiency of cellular systems, whereas using which the channels will become asymmetry. In this paper, we study the impact of the asymmetry on the performance of channel estimation and precoding in downlink BS cooperative multiple-antenna multiple-carrier systems. We first present three linear estimators which jointly estimate the channel coefficients from users in different cells with minimum mean square error, robust design and least square criterion, and then study the impact of uplink channel asymmetry on their performance. It is shown that when the large scale channel information is exploited for channel estimation, using non-orthogonal training sequences among users in different cells leads to minor performance loss. Next, we analyze the impact of downlink channel asymmetry on the performance of precoding with channel estimation errors. Our analysis shows that although the estimation errors of weak cross links are large, the resulting rate loss is minor because their contributions are weighted by the receive signal to noise ratio. The simulation results verify our analysis and show that the rate loss per user is almost constant no matter where the user is located, when the channel estimators exploiting the large scale fading gains.

Communication-oriented cooperative spectrum sensing in cognitive radio

Abstract: Cooperative spectrum sensing improves detection performance over fading and shadowing channels by using multiple cognitive radio (CR) users. Compared with local spectrum sensing with individual CR user, it can either reduce sensitivity requirements for each CR user or shorten the overall sensing duration. These advantages are from spatial diversity when all cooperative users sense a common primary user. However, in practice, CR users at different locations may receive signals from different primary users, which may confuse the fusion user and result in the loss of communication opportunities. In this paper, we propose a cooperative sensing approach by taking the reliability of each partner user into account. Simulation results illustrate significant performance improvement of the proposed approach.

Channel Norm-Based User Scheduling Exploiting Channel Asymmetry in Base Station Cooperative Transmission Systems

Abstract: We study low-signalling overhead scheduling for downlink coordinated multi-point (CoMP) transmission with multi-antenna base stations (BSs) and single-antenna users. By exploiting the asymmetric channel feature, i.e., the pathloss differences towards different BSs, we derive a metric to judge orthogonality among users only using their average channel gains, based on which we propose a semi-orthogonal scheduler that can be applied in a two-stage transmission strategy. Simulation results demonstrate that the proposed scheduler performs close to the semi-orthogonal scheduler with full channel information, especially when each BS is with more antennas and the celledge region is large. Compared with other overhead reduction strategies, the proposed scheduler requires much less training overhead to achieve the same cell-average data rate.

User Scheduling for Cooperative Base Station Transmission Exploiting Channel Asymmetry

Abstract: We study low-signalling overhead scheduling for downlink coordinated multi-point (CoMP) transmission with multi-antenna base stations (BSs) and single-antenna users. By exploiting the asymmetric channel feature, i.e., the pathloss differences towards different BSs, we derive a metric to judge orthogonality among users only using their average channel gains, based on which we propose a semi-orthogonal scheduler that can be applied in a two-stage transmission strategy. Simulation results demonstrate that the proposed scheduler performs close to the semi-orthogonal scheduler with full channel information, especially when each BS is with more antennas and the celledge region is large. Compared with other overhead reduction strategies, the proposed scheduler requires much less training overhead to achieve the same cell-average data rate.