Showing papers by "Devendra Jalihal published in 2005"
TL;DR: The concept of space sampling at the receiver where antennas are placed relatively close to each other is introduced and it is shown that even with a separation of only 0.44/spl lambda/, the required spatial correlation in the channel frequency response becomes sufficiently low.
Abstract: Multiple antennas are useful in orthogonal frequency division multiplexing (OFDM) systems for providing transmit and receive diversity to overcome fading. Typically, these designs require considerable separation between the antennas. Spatial correlation is introduced when antennas are not well separated, and it often leads to performance degradation in a flat fading environment. However, in frequency selective fading channels with rich multipath diversity, OFDM receivers can overcome this performance degradation due to antenna correlation. This is due to transformation of a highly spatially correlated channel impulse response to a less spatially correlated channel frequency response inherently by an OFDM system in the presence of rich multipath diversity. We illustrate this for a simple receive diversity OFDM system and hence introduce the concept of space sampling at the receiver where antennas are placed relatively close to each other. The minimum separation required between the antennas under such circumstances is derived analytically, and it is shown that even with a separation of only 0.44/spl lambda/, the required spatial correlation in the channel frequency response becomes sufficiently low. Simulated performance results with such spacing for various multiple antenna OFDM systems corroborate the analytical results.
21 citations
TL;DR: A closed-loop STBC for five and six transmit antennas with rate 3/4, where the one parameter feedback angle can be evaluated in closed form and the novel contribution of this work is in the approach taken to derive the feedback parameter as a single phase term, which is purely a function of the channel gains.
Abstract: Closed-loop, rate-one, channel orthogonalized space-time block codes (CO-STBCs) for three and four transmit antennas using a single (real) phase feedback term have been proposed . These codes achieve full diversity and result in maximum likelihood (ML) decoding with only linear processing at the receiver similar to OSTBCs. In this paper, we propose a closed-loop STBC for five and six transmit antennas with rate 3/4, where the one parameter feedback angle can be evaluated in closed form. These codes achieve full diversity and are delay optimal. Simulation results comparing the error-rate performance of the CO-STBC with open-loop OSTBC as well as some quasiorthogonal space-time block codes are also provided. While the 1- to 5-dB gain accrued by the closed-loop scheme (over the open-loop methods) is not surprising, the novel contribution of this work is in the approach taken to derive the feedback parameter as a single phase term, which is purely a function of the channel gains.
19 citations
TL;DR: Open-loop (no feedback) preprocessing methods for improving the error-rate performance of space-time trellis codes when used in channels with a high K factor are proposed.
Abstract: Space-time trellis codes (STTCs) have been designed for quasi-static Rayleigh channels. When these codes are directly used for additive white Gaussian noise (AWGN) channels or Ricean channels with a high K factor, their frame error-rate (FER) performance is no longer optimal. We propose open-loop (no feedback) preprocessing methods for improving the error-rate performance of these codes when used in channels with a high K. For K=0 (Rayleigh channels), these open-loop schemes generally do not alter the FER performance of the STTCs. The preprocessing operation at the transmitter is either to rotate the constellation points in one of the transmit antennas relative to the other by some angle, or suitably divide (unequally) the total power between the two transmit antennas. The angle of rotation and power division is optimized by maximizing the d/sub free/ of the STTCs. Simulation results show that for AWGN or Ricean fading channels with a large K, the FER performance can be significantly improved by doing either rotation or power division at the transmitter. While the FER performance is nearly unchanged for quasi-static Rayleigh channels, the improvement in performance is between 1 to 3 dB for Ricean or AWGN channels.
11 citations
TL;DR: This letter proposes a one-bit feedback method, that improves the performance of the power division method in Rayleigh channels, and results show that this improvement is significant for the higher state trellis codes.
Abstract: Open-loop (no feedback) pre-processing methods for improving the error-rate performance of space-time trellis codes in additive white Gaussian noise (AWGN) and Ricean channels which nearly preserve the performance in Rayleigh channels are proposed. The rotation based pre-processing preserves the Rayleigh channel performance, whereas there is a small degradation in performance in the power division based pre-processing. This degradation depends on the trellis complexity, and is more pronounced for the lower state trellis codes. In this letter, we propose a one-bit feedback method, that improves the performance of the power division method in Rayleigh channels. Simulation results for a 2/spl times/1 system show that this improvement is significant for the higher state trellis codes. The one-bit feedback is used to determine which of the two channels has a higher gain, and the higher power is assigned to the corresponding transmitter.
7 citations
16 May 2005
TL;DR: The design criteria to obtain space-frequency LDC codes for MIMO-OFDM systems along with the error-rate performance of such codes in frequency selective fading channels are provided.
Abstract: Linear dispersion codes (LDC) designed for multiple input multiple output (MIMO) wireless systems are examples of space-time codes which try to maximize the mutual information between the transmitter and receiver. However, such codes are typically designed for flat fading channels. It is possible to extend LDC designs for frequency selective fading channels using orthogonal frequency division multiplexing (OFDM). Additionally, the multipath diversity available as frequency diversity could be potentially exploited by properly designed space-frequency LDC codes for MIMO-OFDM systems. In this paper, the design criteria to obtain such space-frequency LDC along with the error-rate performance of such codes in frequency selective fading channels are provided.
7 citations
TL;DR: An overview of multiple-input multiple-output (MIMO) wireless communication systems followed by some of the new techniques proposed by us to enhance performance and the symbol error rate (SER) performance of the various schemes are compared through computer simulations.
Abstract: We present an overview of multiple-input multiple-output (MIMO) wireless communication systems followed by some of the new techniques proposed by us to enhance performance. Firstly, some of the popular transmit and receive diversity schemes available in the literature are discussed and issues like diversity, rate, power distribution, decoding complexity, decoding delay, amount of feedback and feedback delay and their tradeoffs to achieve reliable and cost effective communications are described. We then analyze the merits and demerits of some of the open-loop (no feedback) and closed-loop (with feedback) schemes, and discuss various methods proposed by us recently to improve the performance of such schemes. The symbol error rate (SER) performance of the various schemes are compared through computer simulations.
2 citations