Hsuan-Jung Su

Joint Subcarrier Pairing and Power Allocation for OFDM Transmission With Decode-and-Forward Relaying

In this paper, a point-to-point orthogonal-frequency- division multiplexing (OFDM) system with a decode-and- forward (DF) relay is considered. The transmission consists of two hops. The source transmits in the first hop, and the relay transmits in the second hop. Each hop occupies one time slot. The relay is half-duplex, and capable of decoding the message on a particular subcarrier in one time slot, and re-encoding and forwarding it on a different subcarrier in the next time slot. Thus, each message is transmitted on a pair of subcarriers in two hops. It is assumed that the destination is capable of combining the signals from the source and the relay pertaining to the same message. The goal is to maximize the weighted sum rate of the system by jointly optimizing subcarrier pairing and power allocation on each subcarrier in each hop. The weighting of the rates is to take into account the fact that different subcarriers may carry signals for different services. Both total and individual power constraints for the source and the relay are investigated. For the situations where the relay does not transmit on some subcarriers because doing so does not improve the weighted sum rate, we further allow the source to transmit new messages on these idle subcarriers. To the best of our knowledge, such a joint optimization inclusive of the destination combining has not been discussed in the literature. The problem is first formulated as a mixed integer programming problem. It is then transformed to a convex optimization problem by continuous relaxation, and solved in the dual domain. Based on the optimization results, algorithms to achieve feasible solutions are also proposed. Simulation results show that the proposed algorithms almost achieve the optimal weighted sum rate and outperform the existing methods in various channel conditions.

Space-time turbo codes with full antenna diversity

In attempting to find a spectrally and power efficient channel code which is able to exploit maximum diversity from a wireless channel whenever available, we investigate the possibility of constructing a full antenna diversity space-time turbo code. As a result, both three-antenna and two-antenna (punctured) constructions are shown to be possible and very easy to find. To check the decodability and performance of the proposed codes, we derive non-binary soft-decoding algorithms. The performance of these codes are then simulated and compared with two existing space-time convolutional codes (one has minimum worst-case symbol-error probability; the other has maximal minimum free distance) having similar decoding complexity. As the simulation results show, the proposed space-time turbo codes give similar or slightly better performance than the convolutional codes under extremely slow fading. When fading is fast, the better distance spectra of the turbo codes help seize the temporal diversity. Thus, the performance advantage of the turbo codes becomes evident. In particular, 10/sup -5/ bit-error rate and 10/sup -3/ frame-error rate can be achieved at less than 6-dB E/sub b//N/sub 0/ with 1 b/s/Hz and binary phase-shift keying modulation. The practical issue of obtaining the critical channel state information (CSI) is also considered by applying an iteratively filtered pilot symbol-assisted modulation technique. The penalty when the CSI is not given a priori is about 2-3 dB.

On Secrecy Rate of the Generalized Artificial-Noise Assisted Secure Beamforming for Wiretap Channels

In this paper we consider the secure transmission with multiple-input, single-output, single-antenna eavesdropper (MISOSE) in fast fading channels where the transmitter knows perfect legitimate channel state information but only the statistics of the eavesdroppers channel. For the MISOSE channels, the artificial noise assisted beamforming proposed by Goel and Negi is a promising technique, where the artificial noise is imposed on the null space of the legitimate channel to disrupt the eavesdroppers reception. Here we propose a generalized artificial noise scheme which allows the injection of the artificial noise to the legitimate channel. Although the generalized artificial noise may cause the leakage of artificial noise at the legitimate receiver, the secrecy rate can still be improved since the covariance matrix of it is more flexible than the heuristic one selected by Goel and Negi. To fully characterize the proposed scheme, we investigate the optimization of its secrecy rate. We first derive the conditions under which the beamformers of the message bearing signal and the generalized artificial noise being the same is optimal. Based on this choice, the complicated secrecy rate optimization problem over the covariance matrices of the message-bearing signal and the generalized artificial noise can be reduced to a much simpler power allocation problem. We also develop an efficient algorithm to solve this non-convex power allocation problem. Numerical results show that our generalized artificial noise scheme outperforms Goel and Negis heuristic selection, especially in the near eavesdropper settings. In particular, with the aid of the proposed scheme, the regime with non-zero secrecy rate is enlarged, which can significantly improve the connectivity of the network.

Adaptive closed-loop power control with quantized feedback and loop filtering

Power control is important in maintaining the communication link quality under fading and interference situations. In recent years, research on power control has been conducted toward reducing power consumption while maintaining reliable link quality. These previous works, however, did not try to optimize power control algorithms under practical constraints. In this paper, we first observe and provide some insights into the previous works on different aspects of power control schemes. These observations motivate us to propose a new power control scheme which has several novel features: it uses an adaptive optimal quantizer at the receiver for transmitting discrete feedback information and an adaptive quantization scaler/restorer followed by a loop filter at the transmitter. Optimal quantization minimizes the feedback information loss and the additional power control error caused by it, while the loop filter can be designed to achieve the lowest power control error. Optimization of the loop filter requires some computational power. Fortunately, filter self-design capability can be implemented. In that case, closed-loop power control can be seen as an instance of a general channel identification problem. Intuitive explanation, together with analysis and simulation results of the proposed scheme will be given.

Low-complexity joint channel estimation and decoding for pilot symbol-assisted modulation and multiple differential detection systems with correlated Rayleigh fading

Joint channel estimation and decoding in a time-varying Rayleigh fading channel is considered. Knowing that the optimal solution or even the truncated near-optimal solution using iterative processing has an exponential complexity which hinders the practicability, a reduced complexity approach is proposed. This approach keeps the existing channel estimation and decoding schemes almost intact, while applying iterative processing to effectively exchange information between them. Thus, the complexity is rendered linear, and estimator adaptability can be easily established. We apply this approach to pilot symbol-assisted modulation (PSAM) and differentially modulated systems. It turns out that the performance is improved and the robustness to fading parameters is enhanced. Through simulations, we also show that the proposed method performs almost as well as the near-optimal design.

Performance of hybrid ARQ for high speed downlink packet access in UMTS

An expanded effort is underway to support the evolution of the UMTS standard to meet the rapidly developing needs associated with wireless Internet applications. The support of packet-switched high-speed data users is provided by means of a new, shared channel called HS-DSCH (high speed downlink shared channel) that preferably serves one packet data user at a time in a time-multiplexed manner. A number of performance enhancing technologies are included in the high speed downlink packet access (HSDPA) proposal to ensure high peak and average packet data rates while supporting circuit switched voice and packet data on the same spectrum. These techniques include adaptive modulation and coding (AMC), hybrid ARQ (HARQ), fat-pipe scheduling, fast cell site selection (FCSS) and multiple input multiple output (MIMO) antenna techniques. In this paper, we provide the performance comparison of different HARQ schemes being considered for HSDPA.

A Novel Low-Complexity Precoded OFDM System With Reduced PAPR

The computational complexity and peak-to-average power ratio (PAPR) of conventional precoded orthogonal frequency division multiplexing (OFDM) systems can be reduced using a T-OFDM precoded system based on the Walsh-Hadamard matrix. The present paper proposes a novel precoding scheme for further reducing the computational complexity and PAPR of T-OFDM. In the proposed scheme, the precoding matrix is combined with an inverse discrete Fourier transform to construct a new transform matrix at the transmitter. Notably, the transform matrix is both unitary and circulant, with each column being a perfect Gaussian integer sequence containing just four non-zero elements of {±1,±j}. A low-complexity receiver is additionally constructed for the proposed precoding scheme. A closed-form expression is derived for the bit error rate (BER) in T-OFDM and the proposed precoding scheme under frequency-selective fading channels. The simulation results for the BER are shown to be in good agreement with the mathematical derivations. In addition, it is demonstrated that T-OFDM and the proposed scheme have an equivalent BER performance when their precoding matrices are designed in such a way as to obtain full frequency diversity. However, the proposed scheme has a better PAPR performance and a lower computational complexity than T-OFDM.

Performance of UMTS radio link control

The radio link control (RLC) protocol in Universal Mobile Telecommunication System (UMTS) contains a suite of features and options which make the protocol very flexible and adaptive to operation in different radio environments and for various applications. We analyze the various features and functions associated with the RLC protocol and evaluate the performance over UMTS specific physical link through simulations. Specifically, we examine the detail behavior of the protocol under various conditions and options, including polling, status transmission mechanism, and packet discard functions. We compare different polling mechanisms and examine their impacts on the delay and throughput as well as acknowledgement overhead. We also compare different discard parameters and service rates and evaluate their impacts on the overall performance. Finally we provide quantitative results for web-browsing traffic which is considered as one of the future applications in 3G wireless networks.

A distributed power allocation algorithm with adaptive modulation for multi-cell OFDM systems

A heuristic distributed algorithm for allocating power among OFDM subchannels is proposed. Its performance and convergence in a multi-cell system with frequency reuse factor 4 are evaluated through simulation. With a realistic simulation which considers uniformly distributed mobiles, propagation loss and fading, it is shown that this simple distributed algorithm enhances the throughput of OFDM systems as compared to equal power allocation.

Random access design for clustered wireless machine to machine networks

In this paper, we consider the infrastructure mode wireless machine-to-machine (M2M) network, such as the machine-type communication (MTC) in the LTE-Advanced, where there could be tens of thousands of machines within a macro cell and intending to access the same macro base station. Since most of the M2M communications are of low duty cycle and the machine traffic is usually bursty, the bottleneck for M2M communications is usually at the random access stage. In the LTE-Advanced, the access class barring (ACB) and extended access barring (EAB) have been proposed for the random access of MTC. For the one-shot random access attempts which correspond to the extremely low-duty-cycle scenarios where the network has enough time before the next traffic burst to resolve random access collisions, ACB and EAB have been shown to be sufficient if the machines can tolerate long access delay. However, when the machine traffic is recurrent with higher duty cycle, the ACB and EAB will fail to resolve collisions before the next wave of traffic comes in. To accommodate the M2M traffic and reduce the random access delay under limited random access resource, a clustered network structure is considered for which the machines in a macro cell are divided into clusters, and the machines belonging to a cluster communicate to the cluster head which then aggregates the traffic and relays to the macro base station. This clustered M2M network spatially reuses the random access resource. Thus it can increase the number of machines supported by the network and/or reserve more random access resource for the conventional (human) devices such that their qualities of service will not be affected too much by the M2M communications. Our analysis shows that full reuse of the random access resource among the clusters is feasible. In addition, simulations are conducted to study the random access performance of the clustered network, and to determine the number of clusters that should be formed and the number random access preambles that should be allocated to the machines when the total number of machines is given.