Dou Li

Resource allocation using a reverse iterative combinatorial auction for device-to-device underlay cellular networks

An innovative auction-based allocation scheme is proposed to improve the performance of device-to-device (D2D) communications as an underlay in the downlink (DL) cellular networks. To optimize the system sum rate over the resource sharing of both D2D and cellular modes, we introduce a reverse iterative combinatorial auction as the allocation mechanism. In the auction, all the spectrum resources are considered as a set of resource units, which compete to obtain business as bidders while packages of D2D pairs are auctioned off as goods in each auction round. We first formulate the valuation of each resource unit for packages of D2D links. And then a detailed non-monotonic descending price auction algorithm is explained. Further, we prove that the proposed scheme is cheat-proof, converges in a finite number of iteration rounds, and has lower complexity compared to a traditional combinatorial allocation. The simulation results demonstrate that the algorithm efficiently leads to a good performance on the system sum rate.

Analysis of CSMA/CA in IEEE 802.15.4

The release of IEEE 802.15.4 medium access control and physical layer specifications, employing carrier sense multiple access/collision avoidance (CSMA/CA) strategies, represents a significant milestone in promoting deployment of wireless sensor networks. In this study, the authors first analyse the performance of the slotted CSMA/CA strategy specified in the contention access period (CAP) of IEEE 802.15.4 by integrating the discrete-time Markov chain models of the node states and the channel states; and then, extend the Markov chain models by adopting a modification to the CAP. The extended models could be used to analyse the performance of the unslotted CSMA/CA strategy specified in IEEE 802.15.4 as well as that of the slotted CSMA/CA strategy. Extensive simulations demonstrate the accuracy and effectiveness of the proposed models and conclusions.

Analysis and Compare of Slotted and Unslotted CSMA in IEEE 802.15.4

This paper analyzes the non-persistent Carrier Sense Multiple Access (CSMA) mechanism, and simulates its application in IEEE 802. 15. 4. We find that throughputs are relevant to the normalized propagation delay and the Basic Time Period (BTP) of slotted CSMA. The throughputs of slotted and unslotted CSMA are almost equal when the normalized propagation delay is much less than 1 and the BTP is equal to the propagation delay. Moreover, for slotted CSMA, the BTP selection should be cautious because an unsuitable BTP will make the performance of the protocol worse. In IEEE 802. 15. 4, the normalized propagation delay is much less than 1, so it is inappropriate for the usage of a Slotted Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) mechanism whose BTP is 20 symbols. Finally, we compare the performance of slotted and unslotted CSMA/CA in IEEE 802. 15. 4 and verify the conclusion by simulation.

On Analysis of the Contention Access Period of IEEE 802.15.4 MAC and its Improvement

The release of IEEE 802.15.4 medium access control (MAC) and physical layer specifications represents a significant milestone in promoting deployment of wireless sensor networks. This paper first analyzes the performance of the contention access period (CAP) specified in the IEEE 802.15.4 standard by integrating the discrete-time Markov chain models of the node states and the channel states. Then a modified CAP is analyzed, which could significantly improve the performance of the system. Based on the theoretical analysis of the CAP and the effect of the modification, we have that in the saturated network of IEEE 802.15.4, the collision probability is large, and the throughput is small. The main reasons are the employment of slotted carrier sense multiple access-collision avoidance and the mechanism that if a frame transmission cannot be completed before the end of the CAP, it has to wait until the start of the next CAP. Thus an enhanced collision-avoidance MAC protocol, which achieves a better performance and is compatible with the IEEE 802.15.4 standard, is proposed to improve the performance of the system. Extensive simulations validate our conclusions.

Investigation on DL and UL power control in full-duplex systems

In this paper, we focus on the downlink (DL) and uplink (UL) power control issue in a wireless full-duplex system consisting of a full-duplex base station with single antenna and mobile stations working in time-division duplex mode. First, we investigate the signal-to-interference ratios (SIRs) of DL and UL and obtain a formula reflecting their relationship. Then, by employing the Lagrange multiplier method, we derive an optimal joint power control solution to maximize the sum rate of DL and UL channels. Consequently, we propose an efficient switching scheme between the full-duplex mode and the opportunistic half-duplex mode that can be considered as a compensation mode in sum rate maximization. The proposed switching scheme is verified by numerical simulations.

Prediction of Self-Similar Traffic and its Application in Network Bandwidth Allocation

In this paper, traffic prediction models based on chaos theory are studied and compared with FARIMA (fractional autoregressive integrated moving average) predictors by means of the adopted measurements of predictability. The traffic prediction results are applied in the bandwidth allocation of a mesh network, and the OPNET simulation platform is developed in order to compare their effects. The adopted predictability measurements are inadequate because although the chaotic predictor based on the Lyapunov exponent with worse values of the measurements can timely predict the burstiness of self- similar traffic, the FARIMA predictor forecasts the burstiness with a time-delay. The DAMA (dynamic assignment multiaccess) bandwidth allocation strategy combined with the chaotic predictor can provide better QoS performance.

Sampling Frequency Offset Estimation for Pilot-Aided OFDM Systems in Mobile Environment

In orthogonal frequency division multiplexing (OFDM) systems, most of the conventional sampling frequency offset (SFO) estimation methods work under the assumption of time-invariant or slow time-variant channels. In mobile environment, the time-variant channel significantly degrades the accuracy of SFO estimation. To solve the problem, we first analyze the properties of time-variant channels. If terminal moves within some tens of the wavelength of radio frequency (RF) signal, channel path delay almost remains unchanged. For most practical OFDM systems, our analysis indicates that channel path delay can be regarded as unchanged during the interval of some tens of OFDM symbols in time-variant channels. Based on the analysis, we propose a novel SFO estimation method for pilot-aided OFDM systems. Different from the conventional methods, the proposed method estimates SFO by detecting the variation of the symbol timing error caused by SFO. The detection is finished by implementing correlation between the channel impulse responses (CIRs) estimated by different OFDM symbols. Performance of the proposed method is simulated and compared with that of two conventional post-FFT methods. Numerical results show that, the proposed SFO estimation method performs better than the conventional methods not only in time-variant channels, but also at low SNRs and large residual carrier frequency offsets (CFOs).

Implementation of accelerated BCH decoders on GPU

With the development of communication systems, the requirement for efficient error correcting code becomes an important issue. In this paper, we address a parallel software implementation of BCH(Bose-Chaudhuri-Hocquenghen) decoding for HINOC (High Performance Network Over Coax) standard, which is an access network technology aiming at solving the bandwidth limit in user area. A highly efficient parallel decoding algorithm of BCH codes based on CU-DA (Compute Unified Device Architecture) is presented. An iterative decoding algorithm is adopted to implement BCH decoders by virtue of the massively parallel architecture advantage of GPU (Graphic Process Unit). The major concern of the BCH decoders devised on GPU is concentrated on the parallel process capability of the decoding algorithm. Through flexible threads assignment and efficient scheduling strategy, the GPU-based BCH decoders are implemented significantly. Specially, we design and carry out BCH(n=504, k=432, t=7) decoders using the proposed approach. Besides, we evaluate its performance with respect to its CPU-based single-threaded counterpart developed in the C++ language. The experimental results show that our proposed GPU-based BCH decoders achieve a significant speedup of more than 50 times improvement. Furthermore, the implementation of the proposed BCH decoder architecture is scalable to various block lengths and various correctable error numbers, thus providing an efficient and convenient approach to do parallel BCH decoding processes on GPU.

Double-space-cooperation method for increasing channel capacity

Shannon channel capacity theorem poses highest bit-rate of error free transmission over additive white Gaussian noise channel. In addition, he proved that there exists channel code that can theoretically achieve the channel capacity. Indeed fortunately, the latter researchers found some practical channel codes approaching the channel capacity with insignificant losses of spectral efficiency under ignorable bit error rate (BER). The authors note, in general, that bits of the channel codes are not independent of each other in code space. Further, we note that the modulated symbols are not independent among them, as well, in Euclidean Space. By exploiting a usage of the dependencies jointly to signal design, we can transmit two independent signal streams through an additive white Gaussian channel and separate them in Euclidean space at the receiver. The capacity of this approach is found larger than that of Shannon capacity in the same channel assumptions. The numerical results confirm the theoretical procedures.

Channel Estimation for Pilot-Aided OFDM Systems in Single Frequency Network

In single frequency network (SFN), there exist some special long delay spread channels as well as conventional multipath channels. For pilot-aided orthogonal frequency division multiplex (OFDM) systems, channel estimation is usually accomplished by interpolation with pilots. However, few pilot sub-carriers exist within the coherent bandwidth of the long delay spread channels in SFN. In this case, conventional frequency domain interpolation methods cannot work properly. In a narrowband channel, this paper indicates that both the real and the imaginary parts of channel frequency response can be accurately approximated as a sine-wave with DC offset. For many practical pilot-aided OFDM systems, the bandwidth of the narrowband channel mentioned before is comparable with the interval between several adjacent pilot sub-carriers. Then this paper proposes a sine-wave based frequency domain interpolation method for the channel estimation of pilot-aided OFDM systems in SFN. As simulation results show, the proposed method performs well in the long delay spread channel, whereas the mismatched Wiener interpolation filter (WIF) estimates channel response inaccurately. Moreover, the proposed method gives accurate channel estimation in conventional multipath channels, especially for the systems which adopt high order modulations.