Roger Shu Kwan Cheng

Multiuser OFDM with adaptive subcarrier, bit, and power allocation

Multiuser orthogonal frequency division multiplexing (OFDM) with adaptive multiuser subcarrier allocation and adaptive modulation is considered. Assuming knowledge of the instantaneous channel gains for all users, we propose a multiuser OFDM subcarrier, bit, and power allocation algorithm to minimize the total transmit power. This is done by assigning each user a set of subcarriers and by determining the number of bits and the transmit power level for each subcarrier. We obtain the performance of our proposed algorithm in a multiuser frequency selective fading environment for various time delay spread values and various numbers of users. The results show that our proposed algorithm outperforms multiuser OFDM systems with static time-division multiple access (TDMA) or frequency-division multiple access (FDMA) techniques which employ fixed and predetermined time-slot or subcarrier allocation schemes. We have also quantified the improvement in terms of the overall required transmit power, the bit-error rate (BER), or the area of coverage for a given outage probability.

Channel estimation for OFDM transmission in multipath fading channels based on parametric channel modeling

We present an improved channel estimation algorithm for orthogonal frequency-division multiplexing mobile communication systems using pilot subcarriers. This algorithm is based on a parametric channel model where the channel frequency response is estimated using an L-path channel model. In the algorithm, we employ the ESPRIT (estimation of signal parameters by rotational invariance techniques) method to do the initial multipath time delays acquisition and propose an interpath interference cancellation delay locked loop to track the channel multipath time delays. With the multipath time delays information, a minimum mean square error estimator is derived to estimate the channel frequency response. It is demonstrated that the use of the parametric channel model can effectively reduce the signal subspace dimension of the channel correlation matrix for the sparse multipath fading channels and, consequently, improve the channel estimation performance.

A VLSI architecture of a K-best lattice decoding algorithm for MIMO channels

Lattice decoding algorithms have been proposed for implementing the maximum likelihood detector (MLD), which is the optimal receiver for multiple-input multiple-output (MIMO) channels. However the computational complexity of direct implementation of the lattice decoding algorithm is high and the throughput is variable. In this work, a K-best algorithm is proposed to implement the lattice decoding. It is computational inexpensive and has fixed throughput. It can be easily implemented in a pipelined fashion and has similar performance as the optimal lattice decoding algorithm if high value of K is used. In this paper, we describe a pipelined VLSI architecture for the implementation of the K-best algorithm. The architecture was designed and synthesized using a 0.35 /spl mu/m library. For a 4-transmit and 4-receive antennas system using 16-QAM, a decoding throughput of 10 Mbit/s can be achieved.

Gaussian multiaccess channels with ISI: capacity region and multiuser water-filling

The capacity region of a two-user Gaussian multiaccess channel with intersymbol interference (ISI) in which the inputs pass through respective linear systems and are superimposed before being corrupted by an additive Gaussian noise process is discussed. A geometrical method for obtaining the optimal input power spectral densities and the capacity region is presented. This method can be viewed as a nontrivial generalization of the single-user water-filling argument. It is shown that, as in the traditional memoryless multiaccess channel, frequency-division multiaccess (FDMA) with optimally selected frequency bands for each user achieves the total capacity of the multiuser Gaussian multiaccess channel with ISI. However, the capacity region of the two-user channel with memory is, in general, not a pentagon unless the channel transfer functions for both users are identical. >

Timing recovery for OFDM transmission

Orthogonal frequency division multiplexing (OFDM) is an effective modulation technique for high-rate and high-speed transmission over frequency selective fading channels. However, OFDM systems can be extremely sensitive and vulnerable to synchronization errors. In this paper, we present a scheme for performing timing recovery that includes symbol synchronization and sampling clock synchronization in OFDM systems. The scheme is based on pilot subcarriers. In the scheme, we use a path time delay estimation method to improve the accuracy of the correlation-based symbol synchronization methods, and use a delay-locked loop (DLL) to do the sampling clock synchronization. It is shown that by using this scheme, the mean square values of the symbol timing estimation error can be decreased by several orders of magnitude compared to the common correlation methods in both the AWGN and multipath fading channels. In addition, the scheme can track the symbol timing drift caused by the sampling clock frequency offsets.

Adaptive antennas at the mobile and base stations in an OFDM/TDMA system

Several smart antenna systems have been proposed and demonstrated at the base station (BS) of wireless communications systems, and these have shown that significant system performance improvement is possible. We consider the use of adaptive antennas at the BS and mobile stations (MS), operating jointly, in combination with orthogonal frequency-division multiplexing. The advantages of the proposed system includes reductions in average error probability and increases in capacity compared to conventional systems. Multiuser access, in space, time, and through subcarriers, is also possible and expressions for the exact joint optimal antenna weights at the BS and MS under cochannel interference conditions for fading channels are derived. To demonstrate the potential of our proposed system, analytical along with Monte Carlo simulation results are provided.

On the Performance of the MIMO Zero-Forcing Receiver in the Presence of Channel Estimation Error

By employing spatial multiplexing, multiple-input multiple-output (MIMO) wireless antenna systems provide increases in capacity without the need for additional spectrum or power. Zero-forcing (ZF) detection is a simple and effective technique for retrieving multiple transmitted data streams at the receiver. However the detection requires knowledge of the channel state information (CSI) and in practice accurate CSI may not be available. In this letter, we investigate the effect of channel estimation error on the performance of MIMO ZF receivers in uncorrelated Rayleigh flat fading channels. By modeling the estimation error as independent complex Gaussian random variables, tight approximations for both the post-processing SNR distribution and bit error rate (BER) for MIMO ZF receivers with M-QAM and M-PSK modulated signals are derived in closed-form. Numerical results demonstrate the tightness of our analysis

A real-time sub-carrier allocation scheme for multiple access downlink OFDM transmission

In this paper, we propose sub-carrier allocation algorithms for a multiple access scheme in downlink OFDM transmission. Knowing the channel characteristics of all the users at the base station, the sub-carrier allocation algorithm assigns sub-carriers to the users in a way that the total transmit power is minimized. An optimal algorithm is presented to provide the best sub-carrier assignment. The complexity of the optimal solution renders it impractical for real-time application. To deal with the frequently changing channel characteristic of a fast time-varying multipath fading channel, a heuristic algorithm based on constructive assignment and iterative improvement is proposed which can give out a valid solution in real time. Experimental results show that the performance of this real-time algorithm is close to that of the optimal allocation.

The evolution path of 4G networks: FDD or TDD?

Frequency-division duplexing and time-division duplexing are two common duplexing methods used in various wireless systems. However, there are advantages and technical issues associated with them. In this article we discuss in detail the features, and the design and implementation challenges of FDD and TDD systems for 4G wireless systems. In particular, we present a number of advantages and flexibilities an TDD system can bring to 4G systems that an FDD system cannot offer, and identify the major challenges, including cross-slot interference, in applying TDD in practice. Due to the fact that cross-slot interference is one of the critical challenges to employing TDD in cellular networks, we also provide a quantitative analysis on its impact on co-channel and adjacent channel interfering cells

Adaptive trellis coded MQAM and power optimization for OFDM transmission

We propose an adaptive trellis coded modulation orthogonal frequency division multiplexing (ATCM-OFDM) scheme to provide high rate transmission with high spectral efficiency for an indoor environment. Our objective is to minimize the overall transmit power in each OFDM transmission by optimizing the power distribution, the code rate and modulation scheme in each OFDM subchannel so as to maintain a constant data transmission rate and a fixed bit error rate (BER). In the simulation, the ATCM-OFDM scheme can achieve more than 7 dB power reduction compared with the OFDM scheme using fixed TCM.