Deepshikha Garg

Broadband CDMA techniques

A very high-speed wireless access of 100 Mb/s to 1 Gb/s is required for fourth-generation mobile communications systems. However, for such high-speed data transmissions, the channel is severely frequency-selective due to the presence of many interfering paths with different time delays. A promising wireless access technique that can overcome the channel frequency-selectivity and even take advantage of this selectivity to improve the transmission performance is CDMA. There may be two approaches in CDMA technique: direct sequence CDMA and multicarrier CDMA. A lot of attention is paid to MC-CDMA. However, recently it has been revealed that DS-CDMA can achieve good performance comparable to MC-CDMA if proper frequency domain equalization is adopted. This article discusses their similarities and performances. A major transmission mode in 4G systems is packet-based. Automatic repeat request combined with channel coding is a very important technique. Recent research activity on this technique is also introduced.

Throughput of turbo coded hybrid ARQ using single-carrier MIMO multiplexing

Broadband wireless packet access will be the core technology of the next generation mobile communication systems. Very high speed and high quality packet transmissions can be achieved by the joint use of multiple-input multiple-output (MIMO) multiplexing and hybrid ARQ (HARQ). However, if single-carrier (SC) transmission is used, the throughput performance significantly degrades due to large intersymbol interference (ISI) resulting from severe frequency-selective channels. Recently, we proposed an iterative parallel interference cancellation (PIC) for MIMO multiplexing in a frequency nonselective fading channel. We now propose a frequency-domain iterative PIC for SC-MIMO multiplexing to separate signals transmitted from different antennas while achieving frequency and antenna diversity gain. We evaluate, by computer simulation, the throughput performance of turbo coded HARQ using SC-MIMO multiplexing in a frequency-selective Rayleigh fading channel.

Packet access using DS-CDMA with frequency-domain equalization

The next-generation mobile communications system is anticipated to support very high-speed data rates exceeding several tens megabits per second. In this paper, we consider high-speed downlink packet access for direct-sequence code-division multiple access (DS-CDMA) as in third-generation wideband code-division multiple-access systems. Adaptive modulation and coding (AMC), multicode operation and hybrid automatic repeat request (HARQ) will be the enabling technologies. With such high-speed data transmissions, however, multicode operation severely suffers from the loss of orthogonality among the spreading codes since the wireless channel becomes severely frequency-selective. In this paper, we apply frequency-domain equalization (FDE) based on minimum mean-square error (MMSE) criterion instead of conventional rake combining for receiving the packet. A new MMSE-FDE weight is derived for packet combining. The throughput in a frequency-selective Rayleigh-fading channel is evaluated by computer simulation for Chase combining and incremental redundancy (IR) packet combining. It is shown that the use of MMSE-FDE for the reception of multicode DS-CDMA packet gives an improved throughput irrespective of the channels frequency-selectivity.

Throughput Comparison of Turbo-Coded HARQ in OFDM, MC-CDMA and DS-CDMA with Frequency-Domain Equalization

OFDM, MC-CDMA and DS-CDMA are being researched vigorously as the prospective signaling technique for the next generation mobile communications systems, which will be characterized by the broadband packet technology. With packet transmissions, hybrid ARQ (HARQ) will be inevitable for error control. HARQ with rate compatible punctured turbo (RCPT) codes is one of the promising techniques. Data rate equivalent to the OFDM system can be attained with MC-CDMA and DS-CDMA by assigning all the available codes to the same user resulting in what is commonly referred to as multicode MC-CDMA and multicode DS-CDMA. A rake receiver is used for receiving the DS-CDMA signals. However, recently minimum mean square error frequency-domain equalization (MMSE-FDE) has been proposed for the reception of DS-CDMA signals. In this paper, we introduce RCPT HARQ to DS-CDMA with MMSE-FDE and compare its throughput performance with OFDM, multicode MC-CDMA and multicode DS-CDMA with rake combining. MMSE weight for packet combining is introduced and the soft value generation for turbo coding in MC-CDMA and DS-CDMA with MMSE-FDE is presented. The throughput is theoretically evaluated for the uncoded case. For RCPT-HARQ, the comparison is done by computer simulations. It is found that the throughput of HARQ using DS-CDMA with MMSE-FDE is the same as or better than using MC-CDMA. However, with higher level modulation, type I HARQ using OFDM is better than using either MC-CDMA or DS-CDMA; for type II HARQ without redundancy in the first transmission, however, MC-CDMA and DS-CDMA gives a higher throughput.

Rate compatible punctured turbo-coded hybrid ARQ for OFDM in a frequency selective fading channel

Recently orthogonal frequency division multiplexing (OFDM) has gained a lot of attention in mobile radio communications because of its ability to allow high data rate transmission in a severe frequency selective fading channel. Rate compatible punctured turbo coded hybrid ARQ (RCPT HARQ) has been found to give improved throughput performance in a DS-CDMA system. However the extent to which the RCPT HARQ improves the throughput performance of the OFDM system has not been fully understood. In this paper we evaluate by computer simulations the performance of the RCPT HARQ for the OFDM system. It is found that the type II RCPT HARQ has the highest throughput when minimum amount of redundancy bits are transmitted with each retransmission, typical case is when the puncturing period for the parity sequences is 8. It was found that the OFDM system with RCPT HARQ outperforms the DS-CDMA system with RCPT HARQ in a frequency selective channel.

Diversity-Coding-Orthogonality Trade-off for Coded MC-CDMA with High Level Modulation

In MC-CDMA, the data rate can be increased by reducing the spreading factor SF or by allowing multicode transmission. In this paper, we evaluate by computer simulations which gives a better bit error rate (BER) performance lower SF or multicode operation - when high level modulation is used in addition to error control coding. For a coded system in a frequency selective channel, there is a tradeoff between frequency diversity gain due to the spreading of symbols, coding gain due to better frequency interleaving effect and orthogonality destruction. It is shown that for QPSK, the performance of OFDM (MC-CDMA with SF=1) is almost the same as that of a fully spread MC-CDMA system. However, for 16QAM and 64QAM, the BER performance is better for lower SF unlike the uncoded system, wherein higher SF gives a better BER

Performance comparison of turbo-coded DS-CDMA, MC-CDMA and OFDM with frequency-domain equalization and higher-level modulation

For high-speed data communications, channel coding and high-level modulation will be inevitable. In this paper, we compare the turbo coded performance of DS-CDMA and MC-CDMA which are the major contenders for the next generation wireless signaling technique. Minimum mean square frequency-domain equalization (MMSE-FDE) is assumed for both DS-CDMA and MC-CDMA. The log-likelihood ratio (LLR) computation is used to generate the soft decision value needed for turbo decoding. It is found that the DS-CDMA performance is the same for all spreading factors (SF) and equivalent to a fully spread MC-CDMA for all modulation levels and coding rates. For an uncoded system, DS-CDMA is better than MC-CDMA for smaller SF due to a larger frequency diversity gain. However, with turbo coding, MC-CDMA with smaller SF provides better performance due to larger coding gain owing to better frequency interleaving and severe orthogonality destruction for larger SF and DS-CDMA wherein a symbol is spread over the entire bandwidth.

Effect of limited number of retransmissions of RCPT hybrid ARQ for DS-CDMA mobile radio

In this paper, we evaluate by computer simulations the signal energy per information bit (E/sub b//N/sub 0/) for rate compatible punctured turbo (RCPT) coded hybrid ARQ for direct sequence code division multiple access (DS-CDMA) with antenna diversity reception and Rake combining in a frequency selective Rayleigh fading channel when the number of allowable retransmissions is limited. When the number of allowable retransmissions is unlimited, transmitting the minimum amount of redundancy bits with each retransmission would result in the highest throughput as unnecessary redundancy is avoided. However in a practical system, the number of retransmissions allowed is limited to avoid unacceptable time delay before the successful transmission of a packet. When the number of retransmissions is limited, residual bit error is produced. In this paper, we consider different RCPT hybrid ARQ schemes with limited number of retransmissions. It is found that the type II hybrid ARQ scheme is the most favorable in terms of required E/sub b//N/sub 0/ for a given bit error rate (BER), while the type I hybrid ARQ scheme is the most favorable for delay sensitive systems. For the type II hybrid ARQ schemes, the minimum required E/sub b//N/sub 0/ is attained after all the parity bits are transmitted and hence depends on the puncturing period.

DS-CDMA with frequency-domain equalization for high speed downlink packet access

The next generation mobile communications system is anticipated to support data rates up to and exceeding 100 Mbps. In this paper, we consider a DS-CDMA high-speed downlink packet access (HSDPA) as in 3G W-CDMA systems. Adaptive modulation and coding (AMC), multicode operation and hybrid automatic repeat request (HARQ) will be the technologies. With such high-speed data transmissions, however, multicode operation severely suffers from destruction of orthogonality among the spreading codes since the wireless channel becomes severely frequency-selective. In this paper, we apply frequency-domain equalization (FDE) instead of conventional RAKE combining for receiving the packet. It is shown that the use of FDE for the reception of the multicode signal gives an improved throughput irrespective of the channels frequency selectivity.

Turbo coded MIMO multiplexing with iterative adaptive soft parallel interference cancellation

An iterative adaptive soft parallel interference canceller (ASPIC) is proposed for turbo coded multiple-input multiple-output (MIMO) multiplexing. ASPIC is applied to transform a MIMO channel into single-input multiple-output (SIMO) channels for maximum ratio diversity combining (MRC). In the iterative ASPIC, replicas of the interference from different transmit antennas are generated and subtracted from the received signals. The log-likelihood ratio (LLR) sequence, obtained as the turbo decoder output, is fed back for iterative interference cancellation. At the transmitter, the information bit sequences and parity bit sequences are transmitted from different antennas. The achievable bit error rate (BER) performance of turbo coded MIMO multiplexing with the proposed iterative ASPIC in a Rayleigh fading channel is evaluated by computer simulation.