Panayiotis D. Papadimitriou

Zero-padded OFDM with improved performance over multipath channels

It has been proposed recently to replace the cyclic prefix (CP) in OFDM transmission by zero-padding (ZP); this guarantees symbol recovery even when channel nulls are located on a subcarrier. This, though, has the disadvantage that the simple DFT-based receiver does not perform well, but results have shown that if much higher complexity turbo demodulation is used, ZP-OFDM outperforms CP-OFDM. However, by viewing N-carrier OFDM as multicode CDMA with complex spreading codes from the N-PSK alphabet, one can apply to ZP-OFDM the numerous suboptimum multiuser receivers developed for CDMA. The performance of these receivers is dictated by the correlation properties of the IDFT matrix. Therefore, we propose to modify the IDFT matrix used to modulate the data in ZP-OFDM so that the resulting matrix possesses better correlation properties, and hence improved performance over multipath channels can be achieved. Simulation results verify the large gains in the uncoded performance of ZP-OFDM with modified IDFT matrix compared to CP-OFDM and ZP-OFDM with the IDFT matrix.

A physical layer framework for interference analysis of LTE and Wi-Fi operating in the same band

The recent digital transition of TV broadcasting has created new opportunities for wireless usage for voice, video and data services in these newly released bands (known as TV white spaces). Some possible wireless technologies that can be deployed in this new spectrum are LTE and Wi-Fi. In this paper, we conduct an in-depth analysis of the physical layer performance of LTE and Wi-Fi in the presence of cross-technology interference. We propose a physical layer simulation framework in order to co-simulate the link level performance of LTE and Wi-Fi and show the impact of different interference patterns on the error rate. Our results show that the Wi-Fi performance is slightly exacerbated in the presence of LTE interference (compared to similar Wi-Fi interference). We also show that LTE pilot boosting has negligible impact on Wi-Fi performance. Finally, we also show the impact of Wi-Fi interference on the LTE performance.

Code-search for optimal TSC binary sequences with low cross-correlation spectrum

It has been shown that signature (multi) sets that meet Welchs lower bound on the total squared correlation (TSC), maximize the sum capacity of the synchronous code division multiple access (CDMA) channel with equal average input energy constraints. However, a signature set that is TSC-optimum (i.e. achieves the lower bound on TSC), doesnt necessarily have a low cross-correlation spectrum. Hence, for various system loads there may be users that experience high probability of error due to high cross-correlation of their assigned signature to others in the set. In this paper we present a search method to obtain new optimum TSC binary signature sets with low cross-correlation spectrum. We also show, based on our search method, that if we relax the TSC optimality constraint, codes with better cross-correlation spectrum may be found. In addition, a catastrophic binary code (i.e. a code having complementary codewords) can be TSC-optimal. Therefore, practically speaking, it is important that the TSC-optimality be sought along with the minimization of the cross-correlation spectrum. Finally our codes are tested on fully-loaded and overloaded synchronous CDMA systems, and simulation results show that they are a promising alternative to the Hadamard-Walsh codes, since they offer higher capacity.

Channel estimation and signal detection for multi-carrier CDMA systems with pulse-shaping filter

We consider the problem of digital communication in a fading environment using the multi-carrier CDMA technology. By incorporating the effect of the pulse-shaping filter in the channel estimation and signal detection modules, we develop new estimator and detector structures which significantly outperform the commonly used time-domain equalizers and matched filter detector. Our simulation results demonstrate the elimination of some of the error floors which one would experience if usual time-domain techniques were used to cancel the effect of non-ideal pulse-shaping filter.

On asymptotically optimum rate 1/n convolutional codes for a given constraint length

The criterion used to find good convolutional codes has traditionally been the maximization of the free distance for a given constraint length and code rate. In this letter we approach convolutional code design from a different perspective. We assume no constraints in the rate of the code, and the only restriction is in the constraint length. That is, we assume we can utilize as much bandwidth as necessary but we are complexity-limited by the constraint length we can afford. The results are optimum codes for a given constraint length, which can find application, for example, in CDMA systems.

Linear MMSE chip equalization and parallel interference cancellation as applied to 1xEV-DV

1xEV-DV is a 3G CDMA standard, capable of achieving data rates up to 3 Mbps. This standard employs high-order modulation (up to 16-QAM) which makes the detection at the receiver difficult, due to the multipath distortion. In this paper, we investigated the linear minimum mean square error (LMMSE) chip equalizer, as well as the parallel interference canceller (PIC), and their hybrid approaches, as applied to the 1xEV-DV downlink receiver. The results showed that the hybrid receivers perform excellently in all scenarios tested, and outperform previous art LMMSE and PIC receivers.

On binary code design for the noncoherent block fading channel

In this paper, we study the binary code design problem for the fading channel with noncoherent maximum-likelihood detection. The /spl rho/-spectrum (i.e. all possible pairwise absolute cross-correlations) provides a way to define the optimal codes, based on the union bound on the error probability, as the codes achieving the optimal /spl rho/-spectrum for some code rate. We introduce some limited search methods based on the /spl rho/-spectrum, including a nested search, which for some code rates yields optimal codes. With our nested search, good codes were obtained easily up to code rate 12/n, n/spl les/4096. The codes developed offer good performance in the fading channel and can be used in place of the Hadamard-Walsh codes in synchronous CDMA systems to increase system capacity.

Link-level performance of an LTE UE receiver in synchronous and asynchronous networks

The downlink link-level performance assessment, through simulations, is given in this paper for an LTE (and LTE-A) UE receiver, in interference-limited scenarios, and in the presence of RF impairments for both synchronous and asynchronous networks. The data channel (PDSCH) receiver is based on the linear MMSE filter adapted accordingly to the interference environment through the covariance estimation and its inclusion into the filter calculation. A suboptimum MMSE-based receiver is also derived for the transmit-diversity scheme in presence of interference, showing excellent performance. In the baseband simulator, we have modelled typical RF impairments, AGC/ADC, as well as signal-related estimations, such as UE speed, SINR, channel, covariance, time/frequency offsets. The simulation results show excellent performance for the discussed receiver in the presence of strong other-cell interference and impairments.

New linear binary block codes for the AWGN channel

We study the design of linear binary block codes exhibiting large minimum distance, i.e., suitable for the AWGN channel. It is first shown that the n columns of length 2/sup k/ of any (n, k) linear, binary block code, are also columns of a linear 2/sup k/ Hadamard matrix. Based on this observation, we develop nested search methods to obtain linear binary block codes of almost any practical rate having large minimum distance. The codes compare favorably with upper and lower bounds on the minimum distance, as well as to existing codes.

A TDMA-based physical layer solution for high-rate synchronous CDMA systems

The wireless industrys demand for higher data rates from cellular CDMA systems, makes the use of higher order modulation (e.g. 16-QAM) a necessity. Unfortunately, the high data rate schemes of the current 3G cellular standards dont work well in wireless channels due to the poor equalization of the highly dense constellations. In this work we look at the performance of a fully-loaded CDMA system when we remove the spreading, if not the scrambling.