Adrian Boariu

Minimal non-orthogonality rate 1 space-time block code for 3+ Tx antennas

We propose a full rate space-time block code for 3+ Tx antennas. The code is chosen to minimize the non-orthonormality that arises from increasing the rate above the maximum allowed by orthogonality. A linear decoding based on iterative interference cancellation between parts of the code approaches the maximal likelihood decoding performance.

A class of nonorthogonal rate-one space-time block codes with controlled interference

Complex, rate one, orthogonal block space-time codes (STCs) exist only for two transmit antennas and were proposed by Alamouti (1998). Designing rate-one block STCs for more than two transmit antennas implies giving up orthogonality. For more than two transmit antennas a class of rate one block STCs with a controlled number of interference terms per detected symbol is presented. Conditions that link the matrix formulation of the receive equation to the matrix formulation of a multiuser detection scheme are discussed. The block STCs design problem is closely related to the class of linear real Hadamard codes.

Predictability of uplink channels from downlink channels in frequency division duplex systems: a power control application

A novel method for performing predictive closed-loop power control is used to illustrate the predictability of uplink channels from downlink channels in a frequency division duplex (FDD) wireless link. In an uplink power control scenario, the proposed scheme performs uplink prediction using downlink-derived (linear) predictor coefficients. Essentially, the lack of correlation between the fading processes on uplink and downlink (due to FDD and frequency selective fading) is circumvented by using the uplink fading history, together with a set of link-independent, autoregressive (AR) coefficients. An AR model is used to formally describe the frequency selective fading process on either link, regardless of the number of Rayleigh paths. It is also proven that the relevant AR coefficients are common to both links, up to a proper normalization, irrespective of whether the number of paths on the uplink differs from that on the downlink. Both an uncoded and a coded system are simulated, and their performance results are compared, on equal grounds, with those of the corresponding nonpredictive systems.

Multiuser detection in multipath environments for variable spreading-factor CDMA systems

A way to allow variable data rates in CDMA is by varying the spreading factor in accordance with the data rate requested by the user. A detection scheme suitable for multirate transmission is required in addition to one that combats multiple access interference of other users and intersymbol interference from multipath. In this letter, an energy-add multiuser detection method is combined with a scheme called Cholesky-iterative detection to cope with these challenges. It allows the users to be estimated on a symbol-by-symbol basis, making receiver complexity independent of data package length. Simulation results correspond closely to a single-user lower bound. The results also show that the bit-error probability performances of the various users for the Cholesky-iterative detector are closely clustered, while those of a previously published scheme, decorrelation feedback detection, are more widely spread.

Effect of delayed commands on error probability and throughput capacity in a multiple access system

The channel correlation coefficient is a function of maximum Doppler frequency shift and delay. The effect of the correlation coefficient on a multiple access system, that schedules for transmission to the user with the strongest channel, is characterized in terms of the probability density function (pdf). It is shown that the system performance-in terms of error probability and throughput capacity-degrades significantly with the decrease of correlation coefficient. Thus, in order to achieve the desired target performance in terms of error probability, additional transmission power is required to compensate for a low correlation coefficient. Error probability curves for fading channels can he derived quasi-analytically based on the corresponding curves obtained in additive white Gaussian noise (AWGN) channels. Also, for a constant transmission power, an analytical expression of throughput-capacity, a function of the correlation coefficient, is provided.

A resource allocation algorithm for downlink CDMA system

A resource allocation algorithm, that assigns the users to be served together with their number of Walsh codes and power, is proposed for a downlink CDMA system. The optimal solution of maximizing the reward function is achieved by serving at most two users simultaneously. In the proposed algorithm the maximum number of users served simultaneously, m, is variable. This allows to trade the reward and the throughput-capacity in a flexible manner. Simulations performed in the 1xEV-DV framework show that the proposed algorithm has a graceful degradation of the reward function as m increases. For m=2 the algorithm performs close to the optimal solution.

Energy-add multistage detector: a novel multiuser detection method for synchronous CDMA systems

In synchronous code division multiple access (CDMA) systems, both multiple access interference (MAI) and intersymbol interference (ISI) arise. MAI is present due to the CDMA format while ISI is present due to channel multipath. One approach to overcome these problems is to detect the entire package of data for all users at once. This implies working with matrices with dimensions proportional to the product of the data package length and the number of active users. In this paper, we present an alternative approach. This alternative consists of a type of multistage detector that overcomes the MAI and ISI problems; in addition, it has complexity proportional to the number of CDMA users, is computationally efficient, and is suitable for pipeline implementation which allows fast data processing. Simulations are compared to a derived single user bound for the average bit error probability and the results prove the validity of the proposed detection method.