Sigit P. W. Jarot

Each Carrier Transmission Power Control with Antenna Carrier Diversity for OFDMDS-CDMA System

In OFDM-DS/CDMA system, frequency distortion caused by multipath propagation occurs at different frequencies in the bandwidth and generally affects selected subcarriers within the larger bandwidth. It has been investigated that each carrier transmission power control is effective to compensate for subchannel variation due to multipath, by assigning different transmission power level to different subcarriers based on the attenuation level suffered by them. The effectiveness of power control is mainly affected by the accuracy of the transmission power adjustment at the transmitter. As a matter of fact, however, the dynamic range of transmission power is very limited, due to the restriction on power consumption of a mobile station (transmitter side of reverse link). When a subcarrier is subjected to strong fading, power control is not able to compensate for channel attenuation by transmitting signals with large power, and this condition is very potential to degrade system performance. In this paper, we propose to combine each carrier transmission power control with antenna-carrier diversity reception to lighten the burden imposed on power control. This system allows the base station (receiver side) to select the best combination among the carriers and antennas after receiving signals from mobile station (transmitter side), thus power control only compensates for smaller attenuation level. System performance is evaluated in terms of Bit Error Rate (BER) performance as a function of Doppler frequency, number of users, number of branches, updating period and dynamic range. The result of computer simulations clearly show that the proposed system reduces significantly the burden imposed on power control.

MIMO System with Relative Phase Difference Time-Shift Modulation in Rician Fading Environments

In line-of sight (LOS) environments, the performance of Multiple-Input Multiple-Output (MIMO) systems depends largely on the difference of the phase difference of direct paths from transmit antennas to each receive antenna. When the phase difference of direct paths are close to each other, the spatial division multiplexing (SDM) channel is not orthogonal to each other so that the signal detection becomes difficult. In this paper, we propose a MIMO system with relative phase difference time-shift modulation (RPDTM) in Rician fading environments. The proposed scheme transmits independent signals from each antenna at each time slot where relative phase difference between signal constellations used by transmit antennas varies with a pre-determined pattern. This transmission virtually changes the phase difference of direct paths from transmit antennas to each receive antenna without lowering data rate and without knowledge of the channels. In addition, forward error correction coding (ECC) is applied to exploit the time slots where the receiver can detect the signals easily to improve the detection performance. From the results of computer simulation, we show that MIMO system with RPDTM can achieve the better bit error rate (BER) than the conventional MIMO system. We also show that the MEMO system with RPDTM is effective by about Rician factor K = 10 dB.

Size Compatible (SC)-Array LDPC Codes

Array low density parity check (LDPC) codes are high-rate codes that can achieve good error rate performance in additive white Gaussian noise (AWGN) channels. However, array LDPC codes do not support arbitrary code lengths, because the code length of an array LDPC code with good error rate performance is limited to a multiple of a prime number. This paper proposes Size Compatible (SC)-array LDPC codes; they achieve good error rate performance while supporting arbitrary code lengths. We conduct computer simulations to evaluate the block error rate (BLER) performance of SC-array LDPC codes in AWGN channels. We also evaluate the corresponding performance of SC-array LDPC coded multiband OFDM systems for data transmission rates over 1 Gbps in the UWB multipath channel model CM 3. We show that if the submatrix size is a non- prime number, SC-array LDPC codes achieve better error rate performance than the conventional array LDPC codes in AWGN channels. We also show that the SC-array LDPC codes achieve better error rate performance than the punctured array LDPC codes in AWGN channels. We also show that if the submatrix size is a prime number, SC-array LDPC codes achieve the same error rate performance as conventional array LDPC codes in AWGN channels. Moreover, we show that for multiband OFDM systems with data transmission rates of over 1 Gbps, SC-array LDPC codes achieve better error rate performance than conventional array LDPC codes.

Transmission power control techniques for the reverse link of OFDM-DS-CDMA system

A method of transmission power control (TPC) for orthogonal frequency division multiplexing direct sequence code division multiple access (OFDM-DS-CDMA), in order to compensate for power attenuation at each subcarrier, is proposed. Instead of assigning the same power levels for all-subcarriers, different transmission power levels are assigned to different subcarriers, according to the attenuation of the subcarriers. System performance, in terms of bit error rate (BER), has been evaluated by Monte Carlo simulation. The simulation results presented significant improvement; the proposed system performed much better than the system without TPC. It is shown that the each-carrier TPC performed better than all-carrier TPC, hence each-carrier TPC is more suitable for OFDM-DS-CDMA systems.

MIMO System with Relative Phase Difference Time-Shift Modulation for Rician Fading Environment

Multiple-Input Multiple-Output (MIMO) systems that realize high-speed data transmission with multiple antennas at both transmitter and receiver are drawing much attention. In line-of sight (LOS) environments, the performance of MIMO systems depends largely on the difference of the phase difference of direct paths from transmit antennas to each receive antenna. When the phase difference of direct paths are close to each other, the spatial division multiplexing (SDM) channels are not orthogonal to each other so signal detection becomes difficult. In this paper, we propose a MIMO system with relative phase difference time-shift modulation (RPDTM) in Rician fading environments. The proposed scheme transmits independent signals from each antenna at each time slot where the relative phase difference between signal constellations used by transmit antennas varies in a pre-determined pattern. This transmission virtually changes the phase difference of direct paths from transmit antennas to each receive antenna without lowering data rate and without knowledge of the channels. In addition, forward error correction coding (ECC) is applied to exploit the time slots where the receiver can detect the signals easily to improve the detection performance. If there are time slots where the receiver can separate the received signal, the receiver can decode the data by using the time slots and the correlation between data. From the results of computer simulation, we show that MIMO system with RPDTM can achieve the better bit error rate (BER) than the conventional MIMO system. We also show that the MIMO system with RPDTM is effective by about Rician factor K=10dB.

Analysis of Performance Degradation due to Channel Estimation Error in Pre-Rake TDD/CDMA

In recent years, Pre-Rake combining technique has become a hot topic of research as it decreases the complexity of the portable mobile unit, while achieving the same multipath diversity effect of the Rake receiver. The technique is based on precoding of the transmitted signal relying on knowledge of the channel estimation response before transmission. This a priori channel state information is available in Time Division Duplexing (TDD) systems, since the same channel is used both in uplink and downlink. In practice, the error in channel estimation in Pre-Rake system occurs due to time variability in mobile radio channel. Most previous works on Pre-Rake in TDD CDMA have not taken into consideration the effect of imperfect channel estimation. In this paper, we present Pre-Rake performance under imperfect channel estimation due to time variability in TDD system, depending on Doppler Frequency and compare it with the ideal Pre-Rake system. Numerical analysis and computer simulations were carried out to obtain the system error probability.

Analysis of performance degradation due to channel estimation error in pre-Rake TDD/CDMA

In recent years, pre-Rake combining technique has become a hot topic of research as it decreases the complexity of the portable mobile unit, while achieving the same multipath diversity effect of the Rake receiver. In practice, the error in channel estimation in pre-Rake system occurs due to time variability in mobile radio channel. In this paper, we present pre-Rake performance under imperfect channel estimation due to time variability in TDD system, depending on Doppler frequency and compare it with the ideal pre-Rake system.

LDPC Codes for High Data Rate Multiband OFDM Systems over 1Gbps

In this paper, we evaluate packet error rate (PER) performances of high data rate multiband OFDM systems over 1 Gbps in CM 3 using irregular structured LDPC code, simplified LDPC code, and array LDPC code. We also evaluate the decoding complexities of simplified LDPC code and array LDPC code with code rates of R = 3/4,7/8, when belief propagation (BP) algorithm is used. From simulation results, we show that at data transmission rates of 640, 960 Mbps, simplified LDPC codes have the best error rate performances, and at data transmission rate of 1120 Mbps, array LDPC code has better error rate performances than simplified LDPC code. We also show that the order of decoding complexity for array LDPC code and simplified LDPC codes are same.

MultiBand-OFDM System with Concatenated Coding Scheme

Among new technologies, ultra wideband (UWB) is gathering interests as a short-reach transmission system. At present, as one of the prevailing UWB systems, the multiband-orthogonal frequency division multiplexing (MB-OFDM) system has attracted much attention where convolutional code (CC) is used. The main contribution of this paper is to evaluate the bit error rate (BER) and decoding complexity performances of MB-OFDM with concatenated coding scheme and dual carrier modulation (DCM) on IEEE 802.15.3a UWB multipath channel model. We employ the CC for the inner code and the Reed Solomon (RS) code for the outer code in the concatenated coding scheme. From the results of our computer simulation, we show good combination of coding rates in the concatenated coding scheme at the data rates of about 320, 400 480 Mbps. We also show that MB-OFDM with concatenated coding scheme can achieve the better BER than MB-OFDM with only CC. In addition, we show that the BER improvement of MB-OFDM with concatenated coding scheme over that with only CC becomes larger as the data rate becomes higher.

Subcarrier selection with variable spreading factor for reverse link of TDD multi-carrier/DS-CDMA system

This paper proposes subcarrier selection in conjunction with variable spreading factor to be applied in the reverse link of TDD multi-carrier/DS-CDMA system, to improve the system performance, against multiple access interference (MAI), in frequency selective fading channel, without reducing the throughput of the system. In frequency selective fading channel, MC/DS-CDMA subcarrier suffers from different attenuation, and when some subcarriers are severely faded, it may cause significant effect of MAI, and thus degrade the system performance. To avoid this, we apply subcarrier selection method that selects the subcarriers that are in good channel condition for transmission, thus the transmission will use a part of available subcarriers. In order to avoid throughput degradation due to this method, we employ variable spreading factor that adjust the spreading factor of each user depends on the number of subcarriers used for the transmission. Moreover, the proposed method is very easy to be realized in TDD system using the reciprocity of forward and reverse link. The performance of proposed method is evaluated using computer simulation.