State Key Laboratory of Mobile Communications

Biomedical sensors wirelessly located on/in the human body have drawn lots of attention for healthcare purposes. Due to stringent constraints of power, memory and computation capability, security mechanism in wireless body area biosensor networks should be quite different from generic sensor networks. This paper proposes a specific symmetric cryptosystem for those networks wherein physiological signals derived from the human body are utilized to ensure the security of key distribution without key exchange. Experimental data analysis shows that heart rate variability could be one of the proper physiological parameters that can be used to generate a good 'witness' to ensure the secure key distribution.

The distributed wireless quantum communication network (DWQCN) has a distributed network topology and transmits information by quantum states. In this paper, we present the concept of the DWQCN and propose a system scheme to transfer quantum states in the DWQCN. The system scheme for transmitting information between any two nodes in the DWQCN includes a routing protocol and a scheme for transferring quantum states. The routing protocol is on-demand and the routing metric is selected based on the number of entangled particle pairs. After setting up a route, quantum teleportation and entanglement swapping are used for transferring quantum states. Entanglement swapping is achieved along with the process of routing set up and the acknowledgment packet transmission. The measurement results of each entanglement swapping are piggybacked with route reply packets or acknowledgment packets. After entanglement swapping, a direct quantum link between source and destination is set up and quantum states are transferred by quantum teleportation. Adopting this scheme, the measurement results of entanglement swapping do not need to be transmitted specially, which decreases the wireless transmission cost and transmission delay.

Cooperative diversity has recently been proposed as a promising technology to achieve spatial diversity in wireless networks. In this paper, we analyze the performance of SNR-based hybrid decode-amplify-forward (HDAF) relaying cooperative diversity networks over independent non-identical flat Rayleigh fading channels with maximum ratio combining (MRC) technique. Closed-form expressions for the outage and bit error probability of the HDAF relaying scheme are derived. Computer simulations are carried out to illustrate and validate the correctness of analytical results. Besides, the impacts of the SNR threshold and relay location on the performances of outage and bit error probability are investigated.

Lifetime extension is a key design issue for wireless sensor networks (WSN) with battery-operated nodes. Compared with direct transmission, transmission power can be significantly reduced by cooperative communication, because it can effectively mitigate multi-path fading by introducing space diversity. In this paper, BER analysis for M-PSK and M-QAM modulation in noncooperative and cooperative situations is firstly presented. Then power is optimally allocated to source and relay nodes with the objective of minimizing the total transmission power under the average BER constraint. Based on these works, a new distributed cooperative MAC protocol is proposed to improve the lifetime of WSN. In this protocol, both channel state information (CSI) and residual energy information (REI) of sensor nodes are considered to choose the cooperative nodes. Simulation results show that when the access point (AP) is above a certain height, the proposed cooperative protocol can significantly prolong network lifetime compared with non-cooperative MAC protocols.

A new scheme, called pre-decorrelating strategy, is proposed for single user detection in the forward direction of centralized DS-CDMA systems. The basic idea is that, instead of the data themselves, the linear combinations of active users' data are transmitted, which can then decorrelate the cross-correlation between users at the receiver. In the flat Rayleigh fading environment, analysis and simulations show that the receiver is much simplified and can well cancel the multiple access interference regardless of the interference powers and without any knowledge of other users' signature waveforms.

In this letter, an improved selection cooperation (ISC) scheme for decode-and-forward relaying is proposed based on selection cooperation (SC) strategy. Assuming that channels suffer from independent non-identical Rayleigh fading, we derive closed-form expression for the exact symbol error rate (SER) of ISC scheme. The result holds for arbitrary number of relays and refers to both M-PSK and M-QAM modulations. Simulations are carried out to validate the theoretical analysis. Results show that the proposed ISC scheme outperforms SC for all the cases we investigate.

In this paper, we propose a modified DSR protocol in special scenario and combined this protocol with location information. The location information gives each node local information rather than a global one in the routing of wireless ad hoc networks. The simulation results show that the modified protocol is more efficient than the traditional DSR in the two given scenarios.

Sensing-throughput tradeoff is involved in opportunistic spectrum access (OSA). At the physical layer it's expressed as the tradeoff between false alarm and misdetection, while at the MAC layer it's between maximizing the throughput of secondary users and reducing collisions with primary users. To balance both tradeoffs together drives the optimization of MAC frame structure for OSA. Since channel handoff results in a time overhead for MAC frame, it's first researched by modeling OSA dynamics in the paper. Three handoff cases and their probabilities are deduced, which lead to three application scenarios of MAC frame. Thus the throughput model of secondary network involving channel handoff is proposed. By balancing the misdetection and false alarm probabilities and maximizing the throughput of secondary network subject to the sensing quality and collision avoidance constraints, the optimal sensing time and frame duration are deduced as closed forms. The characteristics of the optimal MAC frame structure are researched as well. Theoretical and simulated results disclose the impacts of channel handoff and spectrum sensing on MAC frame structure and the achievable throughput of secondary network, which provide an insight for OSA design and improvement.

Maximum-likelihood detection in MIMO communications amounts to solving a least-squares problem with a constellation (alphabet) constraint. One popular method that can be used to solve this problem is sphere decoding. We show in this letter that by employing a simple stopping criterion, it is possible to significantly reduce the complexity of sphere decoding over a wide range of SNRs, without a noticeable performance degradation. Specifically, simulation results demonstrate that a 10%-90% reduction of the average complexity could be achieved.

It is well known that time domain channel estimation can achieve better performance than frequency domain channel estimation with a time-multiplexed preamble in common OFDM systems. When it is applied to UWB/OFDM systems, however, its high complexity becomes the main obstacle because of the large number of resolvable paths. To solve this problem, a low-complexity channel estimation scheme is presented by exploiting a special construction of complementary sequence pairs.

In this paper, the puncturing pattern design criteria for rate-compatible punctured turbo (RCPT) codes are reviewed at first. Then the optimal puncturing pattern design problem for RCPT codes are highlighted for turbo codes with specific interleaver instead of the uniform interleaver assumption. By employing the constrained subcode algorithm to calculate the low weight spectrum sequence, the optimized weight spectrum sequence (OWSS) criterion is utilized to determine the optimal puncturing pattern. Two algorithms are utilized to realize the OWSS design criterion for both the periodic puncturing pattern design and the non-periodic puncturing pattern design. And simulation results are presented to validate the performance for the long term evolution (LTE) standard turbo codes. The analysis in this paper unveils that, non-periodic puncturing pattern design will always outperform the periodic puncturing pattern design. However, the performance difference between the periodic puncturing and the non-periodic puncturing will tend to be negligible with the increase in the puncturing period. As a conclusion, when the simplified implementation in practical application is concerned, it seems that the periodic puncturing pattern with long enough puncturing period is recommended.

The DFT based OFDM (DFT-OFDM) has currently drawn most of attention in the area of wireless communication. But, there is a long way to go to pursue higher bandwidth efficiency. Discrete wavelet transform have been proposed to substitute DFT, due to its higher spectral containment between subchannels, which can eliminate cyclic prefix (CP) that a must in DFT-OFDM system. In this paper, we propose turbo coded wavelet-based OFDM (TCWOFDM) and LDPC coded wavelet-based OFDM (LDPC-CWOFDM). And their BER performances are compared in different code parameter on AWGN channel. And we show that TCWOFDM performs better than LDPC-CWOFDM.

A new hybrid method based on artificial neural networks for predicting field strength in indoor environment for wireless LAN is introduced in this paper. In contrast to other prediction models, the new method shows a better prediction result and better generalization. This method is particularly suited for application in wireless LAN planning.

In this paper, a receive antenna selection algorithm based on the theory of convex optimization is proposed to improve the system performance over Rayleigh fading multiple-input multiple-output (MIMO) channels. The algorithm is based on the aim of minimizing error rate, and by relaxing the antenna selection variables from discrete to continuous, we arrive at a standard semidefinite convex problem that can be solved very efficiently. The Monte-Carlo simulations show that the algorithm proposed can provide the performance very close to that of the optimal selection based on exhaustive search.

In this study, the authors investigate the downlink performance of distributed antenna systems (DAS) in multicell environment with blanket transmission. In most existing works, the interference plus noise is treated as Gaussian random variable with fixed variance by the central limit theorem. However, doing this will ignore the effect of the short-term fading on interference that is called as restrictive condition. To avoid the restrictive condition, the authors consider the variance of interference plus noise as a random variable with changeable variance that is influenced by the short-term fading when propagation pathloss and transmit power are given. From the perspective of information theoretic, the closed-form and approximate analytical expressions of downlink achievable throughput, outage probability and average bit error rate in the cellular system are derived for no-shadowing and shadowing scenarios, respectively. Extensive simulation results validate the theoretical analysis and demonstrate that the system performances can be significantly improved for cell-edge users. Moreover, the proposed analytical method can obtain more accurate system performances.

In this paper, a novel selecting method based on Hopfield neural networks (HNN) is proposed, which is used to select the sensor nodes as the antenna-array in the sensor networks. In our method, we select the sensor nodes, which have the lowest cost and also satisfy the distance requirement of the multi-input-multi-output (MIMO) system, to act as multiple transmitting and receiving antennas. Using the HNN for selecting makes it feasible and easy to realize. Simulation results also indicate the effectiveness of the proposed method

Ultra wideband (UWB) wireless communication is a new type of radio communication. One of the main applications of UWB in the future focuses on wireless personal area networks (WPAN). Channel estimation is very important for OFDM based UWB system because of its coherent demodulation. In this paper, a new improved channel estimation method is proposed based on the characteristics of the practical indoor UWB multipath channels. Compared with the common estimator using discrete Fourier-transform, the proposed algorithm can properly choose suitable length of time-domain filter by distinguishing different channel conditions. It is shown by simulation that better performance can be achieved for UWB indoor channels

We propose a scheme of utilizing unitary space-time symbol based nonbinary turbo code (USTSBTC) for wireless communication systems with multiple antennas over extremely fast Rayleigh block fading channels, where no channel state information (CSI) is available. We give an effective way of applying USTSBTC as an alternative to the original unitary space-time modulation and present an effective unitary space-time (UST) symbol based BCJR-like maximum a posteriori (MAP) decoding algorithm, which can greatly improve the system performance. Since a constellation with a small size of T can also works well for a long coherence interval, our scheme fits for more conditions. With iterative decoding and/or multi-antennas, it can further improve the system performance, which helps to give an acceptable bit error rate. Specially, we demonstrate for the first time by simulations that when combined with a powerful error correcting code, transmit diversity would be more effective when a very low bit error rate is needed.

In this study, the authors investigate the outage probability and bit‐error rate (BER) of distributed antenna system in downlink multicell environment with blanket transmission. Different from the most existing works, the variance of interference plus noise is treated as a random variable other than constant, and it is influenced by the short term fading when propagation pathloss and transmit power are given. From the perspective of information theory, the closed‐form and approximate analytical expressions of downlink outage probability and average BER in the cellular system are derived for no shadowing and shadowing scenarios, respectively. Extensive simulation results validate the theoretical analysis and demonstrate that the system performances can be significantly improved for cell‐edge users. Moreover, the proposed analytical method can obtain more accurate system performances.

This paper investigates the performance of image transmission with unequal error protection (UEP) through typical satellite communication channel, and the UEP is achieved by mapping different bits of the image bytes to different positions of a progressive edge growth (PEG) constructed irregular Low-Density Parity-Check (LDPC) codes. The satellite communication channel consists of two main fading: one is the nonlinearity caused by on-board high power amplifier (HPA), and another is the time-varying fading caused by the propagation channel, a neural network (NN) based predistortion technique is adopted to deal with the nonlinearity.