Pre-Rake Diversity Research Articles

Spatial Multiplexing Receivers in UWB MIMO Systems based on Prerake Combining

In this paper, various ultra-wideband (UWB) spatial multiplxing (SM) multiple input multiple output (MIMO) receivers based on a prerake diversity combining scheme are discussed and their performance is analyzed. Several UWB MIMO detection approaches such as zero forcing (ZF), minimum mean square error (MMSE), ordered successive interference cancellation (OSIC), sorted QR decomposition (SQRD), and maximum likelihood (ML) are considered in order to cope with inter-channel interference. The UWB SM systems based on transmitter-side multipath preprocessing and receiver-side MIMO detection can either boost the transmission data rate or offer significant diversity gain and improved BER performance. The error performance and complexity of linear and nonlinear detection algorithms are comparatively studied on a lognormal multipath fading channel.

English

The transmitter-based pre-rake diversity combining technique reduces the complexity, size and cost of the mobile unit (MU), while achieving the same inter symbol interference (ISI) mitigation effects of rake receiver for direct sequence-code division multiple access (DS-CDMA) systems. The technique is based on preprocessing of transmitted signal relying on knowledge of the channel state information (CSI) before transmission. In most of the previous works, this a priori information is either assumed or estimated for the uplink and the same is applied to the downlink in time division duplex (TDD) systems due to channel reciprocity. In this paper, a method for channel prediction to evaluate the pre-rake system using binary phaseshift keying (BPSK) modulation in frequency-division duplex (FDD) through analytical and computer simulations for DS-CDMA downlink has been proposed. The performance of the system was also evaluated under ideal and predicted channel conditions using different spreading codes. The findings will have widespread applications in defence communication equipment. Defence Science Journal, 2010, 60(3), pp.282-289 , DOI:http://dx.doi.org/10.14429/dsj.60.355

Error Performance of Prerake Diversity Combining-Based UWB Spatial Multiplexing MIMO Systems over Indoor Wireless Channels

In this letter, we consider a novel ultra-wideband (UWB) spatial multiplexing (SM) multiple input multiple output (MIMO) structure, which consists of prerake diversity combiners in the transmitter and a zero forcing (ZF) detector in the receiver. For a UWB SM MIMO system with N transmit antennas, M receive antennas, and L resolvable multipath components, it is shown that the proposed prerake combining-based MIMO detection scheme has the diversity order of (LN - M + 1) and its BER performance is analytically presented in a log-normal fading channel and also compared with that of a rake combining-based ZF scheme.

Adaptive Interference Avoidance with Pre-RAKE Diversity Combining for High-Data-Rate UWB Systems

Multipath energy capture and inter-symbol interference (ISI) are two intractable problems in high-data-rate Ultra-wideband (UWB) systems. To tackle the problems and simplify the receiver, we propose an adaptive interference avoidance scheme based on Pre-RAKE combining technique. The symbol repetition period (SRP) is regarded a changeable parameter in an ordered set to avoid severe interference paths and guarantee high data-rate. The set is known to both the transmitter and receiver. The index of the selected SRP is then sent to the receiver to coordinate the transmitter and receiver. The SRP can be updated adaptively according to the variations of the channels. Both theoretical analysis and simulations show that the ISI is mitigated and the transmission rate is improved simultaneously compared to the constant SRP transmission scheme.

Transmitter-Side Multipath Preprocessing for Pulsed UWB Systems Considering Pulse Overlapping and Narrow-Band Interference

We analyze a prerake diversity combining scheme for pulsed ultrawideband systems to shift signal processing needs from the receiver to the transmitter. We consider the realistic case that received pulses carrying the same transmitted symbol could overlap with one another in optimizing the prerake scheme based on zero-forcing and eigenanalysis techniques. We show that in the presence of interpulse interference caused by pulse overlapping, the optimum prerake combining scheme in the sense of maximizing the received signal-to-noise ratio derived by using the eigenanalysis technique performs the same as a conventional rake with maximal ratio combining. We also analyze the different behaviors of prerake and rake schemes in the presence of tone interferers and provide numerical results to validate the conclusions in the presence of in-band-modulated interferers. We find that when the number of narrow-band interference (NBI) signals is small, the prerake scheme outperforms the rake scheme; as the number of NBI sources increases, however, this performance gap reduces.

Performance Analysis of Pre-Rake Diversity With Generalized Orthogonal Codes for DS-CDMA

The pre-rake technique is mainly adopted in the time-division duplexing (TDD) direct-sequence code-division multiple access (DS-CDMA) system to provide multipath diversity. In this paper, generalized orthogonal (GO) codes are applied to the TDD/DS-CDMA system employing the pre-rake technique to reduce the self and multiaccess interference and improve system capacity. Based on the theoretical analysis and simulation results, it is shown that the pre-rake diversity with GO codes outperforms the one with other spreading codes.

Interference suppression and diversity exploitation for multiantenna CDMA with ultra-low complexity receivers

We consider transmitter precoding for interference suppression and diversity exploitation for multiantenna, multipath code division multiple access (CDMA). The receivers are constrained to matched filter detection without channel state information (CSI) or receiver-based multiuser detection. We first develop precoders for flat-fading scenarios where perfect downlink channel information is available at the transmitter. We show that full transmit antenna diversity is achievable, even for nonorthogonal spreading codes, although we suffer an SNR loss that is a simple function of the spreading code crosscorrelations. We also develop precoders for cases in which the transmitter has partial channel information, which is modeled as knowledge of conditional channel correlation matrices, and we apply them to Jakes' fading model. Finally, we show that the precoding approach for flat fading can be modified for multipath fading to fully exploit multipath diversity when used in conjunction with prerake diversity combining. We show, in summary, that transmitter precoding offers a reasonable alternative to receiver-based multiuser signal processing when minimizing computational complexity at the mobile unit is a priority.

Pre-Rake Diversity Combining for UWB Systems in IEEE 802.15 UWB Multipath Channel

Since Ultra Wideband Impulse Radio (UWB-IR) system can resolve many paths and is thus rich in multipath diversity, the use of Rake diversity combining is very effective. In the Rake diversity combining, the bit error rate (BER) is improved with the increase of the number of fingers. The Pre-Rake diversity combining is known as another technique to achieve the performance equivalent to the Rake diversity combining without increasing the receiver complexity. In the Pre-Rake diversity combining, the transmitted signals are scaled and delayed according to the delay and strength of the multipath. In this paper, we propose Pre-Rake diversity combining techniques for UWB systems, All-Pre-Rake (A-Pre-Rake) diversity combining using perfect channel information, Selective-Pre-Rake (S-Pre-Rake) diversity combining using the information on the L strongest paths, and Partial-Pre-Rake (P-Pre-Rake) diversity combining using the information on the first L paths. From the results of our computer simulation for UWB-IR systems in IEEE 802.15 UWB multipath channel model, we show that the proposed Pre-Rake diversity combining techniques are effective for the UWB-IR systems to achieve good error rate performance, while keeping the complexity of the receiver low. We also show that the S-Pre-Rake diversity combining is effective to achieve good error rate performance with less channel information.

MISO CDMA transmission with simplified receiver for wireless communication handsets

The next-generation wireless personal and mobile communication systems are expected to accommodate not only high-quality voice services, but also a broad range of other multirate services. Of the various multiaccess techniques, wide-band code-division multiple access (CDMA) has been regarded as an important part of the third-generation wireless communication systems because of its high frequency utilization efficiency and suitability for handling multimedia and multirate services. In this paper, we consider a system with a simplified receiver structure for direct-sequence CDMA (DS/CDMA) wireless communication handsets, in which improved performance is demonstrated when compared to a conventional DS/CDMA system with a RAKE receiver at the mobile station. We arrive at this system by finding the optimal solution to a general multiple-input single-output (MISO) DS/CDMA smart antenna system. We find that this solution reduces to a pre-RAKE with space transmit diversity system under the assumption that a simple one-finger matched filter is used at the receiver. This system combines the advantages of pre-RAKE diversity and transmit antenna diversity. It is shown that significant system performance and capacity improvements are possible. The numerical results also reveal that this system is not too sensitive to channel estimation errors.