Khoirul Anwar - Academia.edu (original) (raw)

Papers by Khoirul Anwar

In this paper, we propose a joint design of serially concatenated source channel coding for binar... more In this paper, we propose a joint design of serially concatenated source channel coding for binary Markov sources over AWGN channels. To exploit the memory structure inherent within the sequence output from the source, modifications are made on the BCJR algorithm. To decode the outer code, the modified version of the BCJR algorithm is used, while the inner code by the standard version of the algorithm. Since optimal design of serially concatenated convolutional code falls into the problem of curve matching between the extrinsic information transfer (EXIT) curves of the inner and outer codes, we first evaluate the EXIT curve of the outer code decoded by the modified BCJR algorithm. It is then shown that the EXIT curve obtained by the modified BCJR algorithm is better matched with short memory inner convolutional code, which significantly reduces coding/decoding complexity. Numerical results demonstrate significant gains over the systems in which source statistics are not exploited (i.e., the standard BCJR algorithm is used for the both codes), and thereby narrowing the performance gap to the Shannon limit. We also compare in this paper the performance of the proposed design with the algorithm presented in [1], designed also for transmission of binary Markov source using parallel concatenated convolutional code (the authors of Ref. [1] refer the technique as Joint Source Channel Turbo Code (JSCTC)). It is shown that our proposed system is superior in both system complexity and BER performance to the JSCTC technique presented in [1].

Chained turbo equalization (CHATUE) is a simple detection scheme for block transmission without c... more Chained turbo equalization (CHATUE) is a simple detection scheme for block transmission without cyclic prefix (CP), where turbo equalizers for the several consecutive frames exchange information about the interference components in the form of a posteriori Log-Likelihood Ratio (LLR). Although elimination of CP in the block transmission can significantly increase bandwidth efficiency, the received signal suffers from inter-block interference (IBI), which can not be effectively compensated by block-by-block equalization. In this paper, we apply the CHATUE algorithm to effectively suppress inter-symbol interference (ISI), inter-carrier interference (ICI), and multiple-access interference (MAI) in a Multiuser Single Input Multiple Output Orthogonal Frequency Division Multiplexing (MU-SIMO-OFDM) system without CP. Results of computer simulations show the superiority of our proposed MU-SIMO-OFDM to conventional CP-aided systems, without requiring high computational complexity. The convergence property of the proposed technique is analyzed using the extrinsic information transfer (EXIT) chart. Doped accumulator is used to achieve a mutual information point close enough to the (1,1) point, and also to obtain better matching between the equalizer and decoder EXIT curves.

This paper proposes a novel Single Carrier Frequency Division Multiple Access (SC-FDMA) System wi... more This paper proposes a novel Single Carrier Frequency Division Multiple Access (SC-FDMA) System without Cyclic Prefix based on the Chained Turbo Equalization (CHATUE) algorithm, which connects turbo equalizers neighboring in time to eliminate the Inter-Block Interference (IBI). Results of Extrinsic Information Transfer (EXIT) chart demonstrate that the performance can be improved by performing iterations between neighboring blocks. In addition, this paper also proposes a complexity reduction technique. The numerical results verify the performance improvement and superiority of the proposed technique over the conventional SC-FDMA transmission, without requiring high computational complexity.

The major objective of this paper is to reveal the performance of the turbo equalization using do... more The major objective of this paper is to reveal the performance of the turbo equalization using doped accumulator (TEQ-ACC) when it is combined with channel estimation, in broadband single carrier signalling. It is shown that the accuracy of the channel estimate in TEQ-ACC is inferior to that in conventional turbo equalization (TEQ-CVT), if TEQ-ACC generates the soft replica only from the soft information fed back from the decoder. To cope with this problem, this paper proposes a new soft replica generation method for TEQ-ACC which achieves asymptotically the equivalent accuracy of channel estimates to that with TEQ-CVT. Simulation results show that the gain in frame-error-rate (FER) with TEQ-ACC utilizing the proposed soft replica generation method is significant, 1 dB better than TEQ-CVT at FER = 10-3 under pedestrian mobility channel assumptions. Furthermore, this paper verifies that TEQ-ACC with channel estimation implemented by an adaptive subspace tracking technique, Low rank adaptive filtering (LO-RAF), achieves exactly the same FER performance as that with the sliding window singular value decomposition (SVD)-based technique.

A frequency division multiplexing technique, nonorthogonal frequency division multiplexing (n-OFD... more A frequency division multiplexing technique, nonorthogonal frequency division multiplexing (n-OFDM), is proposed in [1]-[2] to enhance the efficiency of bandwidth utilization. This paper reveals that the smaller the frequency separation, the larger sum capacity can be achieved compared with the conventional OFDM technique. However, n-OFDM system introduces inter-carrier interference (ICI) at the transmitter because the orthogonality between the subcarriers no longer holds. Moreover, since the channel covariance matrix of n-OFDM has high condition number when the overlapping factor, 1 − α, is large, conventional linear detectors suffers from severe noise enhancement. To solve this problem, this paper proposes the use of soft cancellation-minimum mean-squared error (SC-MMSE) turbo equalization. Binary constellation constrained mutual information (CCMI) is calculated by utilizing the area property for the EXtrinsic Information Transfer (EXIT) chart of the SC-MMSE equalizer. Results of the EXIT chart analysis and bit-error-rate (BER) simulations in additive white Gaussian noise (AWGN) channel are presented.

This paper proposes chained turbo equalization (CHATUE) algorithm, a simple detection scheme for ... more This paper proposes chained turbo equalization (CHATUE) algorithm, a simple detection scheme for block transmission without guard interval (GI), where turbo equalizers for the several consecutive frames exchange information about the interference components. Although the block transmission without GI can significantly increase bandwidth efficiency, the received signal suffers from inter-block interference (IBI) and intersymbol interference (ISI), which can't be effectively compensated by block-by-block equalization.With the CHATUE algorithm, the IBI components are cancelled by utilizing a posteriori feedback from the decoders of the neighboring equalizers (future and past). To best utilize the latest results of the sub-optimal reduced complexity equalization, frequency domain soft cancellation and minimum mean squared error filtering (FD/SC-MMSE), we exploit J matrix to convert the Toeplitz channel matrix structure to a circulant matrix. Results of the computer simulation for single carrier block transmission (SCBT) show that the proposed scheme can achieve good performance.

This paper proposes a structure that combines iterative equalization and turbo decoding, denoted ... more This paper proposes a structure that combines iterative equalization and turbo decoding, denoted as spatial turbo coding (STC), for single carrier signaling to achieve near capacity performance in multipath-rich fading channels. Instead of multiplexing the encoded bits in the time domain as in the standard turbo codes, the proposed STC transmits coded bits in the space domain by employing multiple-input multiple-output (MIMO) transceiver to exploit space diversity, path diversity and coding gains through the decoding branches at the receiver. The considered MIMO detector is a MIMO frequency domain softcancelation and minimum mean square error equalizer (FD/SC-MMSE). Extrinsic Information Transfer (EXIT) chart analysis confirms that bit-error-rate (BER) pinch-off is achieved in 64path Rayleigh fading channels with equal average path gains power which is just by about 1dB away from the static channel capacity/dimension of 2×2 MIMO systems.

In this paper, we propose three low-complexity strategies for Multiple Access Relay Channel (MARC... more In this paper, we propose three low-complexity strategies for Multiple Access Relay Channel (MARC) based on the concept of bit-flipping model. By estimating the error rates of the links between the sources and the relay, and by utilizing the estimate as side information at the destination, it is shown that the proposed relay strategies are superior to the Decode-and-Forward (DF) relay strategy when all links are suffering from Additive White Gaussian Noise (AWGN). Furthermore, extrinsic information transfer (EXIT) analysis is used to track the convergence property with the aim of being able to search for suitable codes, the proposed relay strategies can achieve the performance close to those strategies utilizing relatively strong codes, such as turbo code or LDPC code, with iterative decoding at the relay in order to make intra links error free.

Electronics and Communications in Japan Part Ii-electronics, 2007

The new digital terrestrial television transmission signals use the OFDM system. This system has ... more The new digital terrestrial television transmission signals use the OFDM system. This system has the fundamental problem that the peak power to average power ratio (PAPR) of the signal is significant. In this paper, a satellite communication system is proposed as a supplemental network for digital terrestrial broadcasting. It is shown that effective use of the satellite power becomes possible at a large PAPR by using the FM system with a constant signal envelope as the secondary modulation, so that the problem can be resolved. It is shown analytically that a larger FM gain can be obtained by limiting the peak voltage of the OFDM signal by clipping. The degradation of the BER caused by clipping is derived by simulation and the system is optimized with regard to the degree of clipping. Taking the size and operational cost of the receiving station into account, channel design is performed assuming a real satellite channel. The overall transmission characteristics such as the transmission capacity of the satellite repeater and the channel quality are discussed. © 2007 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 90(11): 74–84, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.20385

Abstract—This paper proposes a design framework based on efficient peak-to-average power (PAPR) f... more Abstract—This paper proposes a design framework based on efficient peak-to-average power (PAPR) for orthogonal frequency division multiplexing (OFDM) and carrier interferometry OFDM (CI/OFDM) systems with iterative clipping. In this paper, we utilize the efficient PAPR ...

In this paper, we propose a new design of efficient carrier interferometry orthogonal frequency d... more In this paper, we propose a new design of efficient carrier interferometry orthogonal frequency division multiplexing (CI/OFDM) by using fast Fourier transform (FFT) as spreading codes. First, we propose the use of both positive and negative frequency of the IFFT rather than only using the positive part as in conventional method. It results new CI/OFDM waveforms which only requires a half of FFT points. Second, we replace CI spreading with FFT for achieving lower complexity because complexity of CI is similar to that of discrete Fourier transform (DFT). Our results confirm that the proposed design is capable of achieving high efficiency and low complexity while providing high performance.

This paper proposes an effective iterative clipping (IC) on a coded carrier interferometry OFDM (... more This paper proposes an effective iterative clipping (IC) on a coded carrier interferometry OFDM (CI/OFDM) system for improving the system performances over the nonlinearity of a solid state power amplifier (SSPA). An additional filtering is not required to remove the out-of-band (OOB) spectrum radiation as required in the conventional clipping method. The IC is designed with a minimized computational complexity though the iteration is performed. The results confirm that the CI/OFDM with the proposed IC presents many advantages in terms of BER performance and OOB spectrum radiation. Based on the optimization on BER and OOB spectrum, it is found that 2 and 3 iterations is an effective number for the IC in OFDM and CI/OFDM systems, respectively. The total degradation (TD) improvement by the effective IC in CI/OFDM is about 0.32dB for QPSK and 2.1dB for 64QAM symbol on an SSPA with an output-back-off (OBO) 4dB and 8dB, respectively.

The new digital terrestrial television transmission signals use the OFDM system. This system has ... more The new digital terrestrial television transmission signals use the OFDM system. This system has the fundamental problem that the peak power to average power ratio (PAPR) of the signal is significant. In this paper, a satellite communication system is proposed as a supplemental network for digital terrestrial broadcasting. It is shown that effective use of the satellite power becomes possible at a large PAPR by using the FM system with a constant signal envelope as the secondary modulation, so that the problem can be resolved. It is shown analytically that a larger FM gain can be obtained by limiting the peak voltage of the OFDM signal by clipping. The degradation of the BER caused by clipping is derived by simulation and the system is optimized with regard to the degree of clipping. Taking the size and operational cost of the receiving station into account, channel design is performed assuming a real satellite channel. The overall transmission characteristics such as the transmission capacity of the satellite repeater and the channel quality are discussed. © 2007 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 90(11): 74-84, 2007; Published online in Wiley InterScience (www.interscience. wiley.com).

Generally peak-to-average power ratio (PAPR) reduction imposes transmission performance degradati... more Generally peak-to-average power ratio (PAPR) reduction imposes transmission performance degradation or system capacity reduction. This paper proposes a method for reducing the PAPR of OFDM signals by combining carrier interferometry (CI) spreading code and iterative processing (IP). The proposed method is capable of reducing the PAPR significantly while minimizing the bit error rate (BER) performance degradation. Computer simulation results prove the effectiveness of the proposed method. Further, the proposed method does not introduce out-of-band interference, which requires filtering whose complexity depends on the filtering requirement.

Recently, there is an interest to increase capacity of multicarrier code division multiplexing (M... more Recently, there is an interest to increase capacity of multicarrier code division multiplexing (MC-CDMA) and orthogonal frequency division multiplexing (OFDM) by using spreading codes. Some spreading codes only support up to K = N+M, where M < N. In this paper, we propose a new spreading code with low cross correlation and high autocorrelation while capable of supporting K = 2N+1 users. It means that 2N+1 users or data symbols are able to be transmitted over only N subcarriers. The numerical results show that our proposed code outperforms current Pseudo-Orthogonal Carrier Interferometry (POCI) code with gain of 3dB in additive white Gaussian noise (AWGN) channel and more than 4dB in multipath fading channel at bit-error-rate (BER) level of 10 -5 . By the proposed code, peak-to-average power ratio (PAPR) problem as a typical problem in multicarrier system is also able to be reduced up to the PAPR level of single carrier system (with roll-of-factor=0.10).

Abstract: This paper proposes new simplified design of Pseudo-orthogonal Carrier Interferometry O... more Abstract: This paper proposes new simplified design of Pseudo-orthogonal Carrier Interferometry Orthogonal Frequency Division Multiplexing (PO-CI/OFDM) system by utilizing IFFT/FFT algorithm as a substitute of PO-CI spreading and despreading. We also propose a separator design for guaranteeing that the resulted codes are still (pseudo) orthogonal to each other. This solution is capable of reducing the complexities of PO-CI/OFDM implementation significantly. Difficulties of generating PO-CI codes as complex spreading ...

In this paper, we propose a novel fast and low complexity algorithm of computation for minimum-me... more In this paper, we propose a novel fast and low complexity algorithm of computation for minimum-mean-square-error (MMSE) equalizer or combiner without divisions. Multiplicative effect of fading channel should be compensated by divisions at the receiver. Therefore, equalizer or combiner at the receiver is derived by inverting the channel impulse responses. Here, the number of divisions equals to the number of subcarriers. For the next generation with high bit rate applications, these divisions are necessary to be computed in a very short time and may impact to the increasing of hardware complexities. The main contribution of this paper is a proposed fast algorithm by replacing the large number of divisions with multiplications and subtraction due to its lower complexity. We improve the performance of Newton-Raphson Method by a range extension so that the Newton-Raphson Method is applicable for MMSE computation with small number of iterations. Our results in Carrier Interferometry Orthogonal Division Multiplexing (CI/OFDM) confirm that with only two iterations, performance of the proposed algorithm can achieve the similar performance as the normal computation with divisions.

This paper proposes a simplified method for computation of Fourier Transform by employing double ... more This paper proposes a simplified method for computation of Fourier Transform by employing double square-waves, called double square-waves Fourier transform (DS-FT). Compared to the conventional single square-wave, double square-waves can increase the resolution of signals. Thus, the orthogonality is kept so the bit-error-rate (BER) performance is better than that of Fourier transform using a conventional single square-wave. Our results confirm that the BER performance is significantly improved. The BER performance by the conventional waveform has residual bit error at BER level of 1.5 x 10-3, while BER of the proposed method has no residual bit error up to the BER level of 1 x 1CT7. In addition, the proposed DS-FT has no restriction in its size (e.g. power of two, etc.) as in fast Fourier transform (FFT). Therefore DS-FT is applicable to any size of Fourier transform.

Ieice Transactions, 2008

In this paper, a new large spreading code set with a uniform low cross-correlation is proposed. T... more In this paper, a new large spreading code set with a uniform low cross-correlation is proposed. The proposed code set is capable of (1) increasing the number of assigned user (capacity) in a multicarrier code division multiple access (MC-CDMA) system and (2) reducing the peak-to-average power ratio (PAPR) of an orthogonal frequency division multiplexing (OFDM) system. In this paper, we derive a new code set and present an example to demonstrate performance improvements of OFDM and MC-CDMA systems. Our proposed code set with code length of N has K = 2N + 1 number of codes for supporting up to (2N + 1) users and exhibits lower cross correlation properties compared to the existing spreading code sets. Our results with subcarrier N = 16 confirm that the proposed code set outperforms the current pseudo-orthogonal carrier interferometry (POCI) code set with gain of 5 dB at bit-error-rate (BER) level of 10 −4 in the additive white Gaussian noise (AWGN) channel and gain of more than 3.6 dB in a multipath fading channel. key words: spreading code, OFDM, MC-CDMA, peak-to-average power ratio (PAPR), pseudo-orthogonal code, Carrier Interferometry (CI)

This paper proposes a new technique for peak-to-average-power ratio (PAPR) reduction without spec... more This paper proposes a new technique for peak-to-average-power ratio (PAPR) reduction without spectrum broadening for Carrier Interferometry orthogonal frequency division multiplexing (CI/OFDM) system. Recently, Carrier Interferometry (CI) code is becoming an interesting code for reducing the PAPR of OFDM system. However, for guaranteeing the low PAPR as the PAPR of single carrier (i.e. by about 4 -6dB with roll-of-factor of 0.2 -0.5), we propose low complexity clipping technique for CI/OFDM system. However, it can be easily adapted for other OFDM systems. The main contribution of this paper is the introduction of a new clipping technique with low complexity frequency domain filtering (FDF) which is employed prior to the IFFT. The benefits of the proposed technique are low complexity of FDF and no spectrum broadening. Our results confirm that the PAPR of clipped CI/OFDM can be reduced by about 1dB better than that of the conventional clipped OFDM signals. We clarified that the PAPR is now similar to the PAPR of single carrier system. In the frequency selective fading channel, the bit-error-rate (BER) performance is also improved by about 7dB better than that of the traditional OFDM system due to the frequency diversity benefits in CI/OFDM system.

In this paper, we propose a joint design of serially concatenated source channel coding for binar... more In this paper, we propose a joint design of serially concatenated source channel coding for binary Markov sources over AWGN channels. To exploit the memory structure inherent within the sequence output from the source, modifications are made on the BCJR algorithm. To decode the outer code, the modified version of the BCJR algorithm is used, while the inner code by the standard version of the algorithm. Since optimal design of serially concatenated convolutional code falls into the problem of curve matching between the extrinsic information transfer (EXIT) curves of the inner and outer codes, we first evaluate the EXIT curve of the outer code decoded by the modified BCJR algorithm. It is then shown that the EXIT curve obtained by the modified BCJR algorithm is better matched with short memory inner convolutional code, which significantly reduces coding/decoding complexity. Numerical results demonstrate significant gains over the systems in which source statistics are not exploited (i.e., the standard BCJR algorithm is used for the both codes), and thereby narrowing the performance gap to the Shannon limit. We also compare in this paper the performance of the proposed design with the algorithm presented in [1], designed also for transmission of binary Markov source using parallel concatenated convolutional code (the authors of Ref. [1] refer the technique as Joint Source Channel Turbo Code (JSCTC)). It is shown that our proposed system is superior in both system complexity and BER performance to the JSCTC technique presented in [1].

Chained turbo equalization (CHATUE) is a simple detection scheme for block transmission without c... more Chained turbo equalization (CHATUE) is a simple detection scheme for block transmission without cyclic prefix (CP), where turbo equalizers for the several consecutive frames exchange information about the interference components in the form of a posteriori Log-Likelihood Ratio (LLR). Although elimination of CP in the block transmission can significantly increase bandwidth efficiency, the received signal suffers from inter-block interference (IBI), which can not be effectively compensated by block-by-block equalization. In this paper, we apply the CHATUE algorithm to effectively suppress inter-symbol interference (ISI), inter-carrier interference (ICI), and multiple-access interference (MAI) in a Multiuser Single Input Multiple Output Orthogonal Frequency Division Multiplexing (MU-SIMO-OFDM) system without CP. Results of computer simulations show the superiority of our proposed MU-SIMO-OFDM to conventional CP-aided systems, without requiring high computational complexity. The convergence property of the proposed technique is analyzed using the extrinsic information transfer (EXIT) chart. Doped accumulator is used to achieve a mutual information point close enough to the (1,1) point, and also to obtain better matching between the equalizer and decoder EXIT curves.

This paper proposes a novel Single Carrier Frequency Division Multiple Access (SC-FDMA) System wi... more This paper proposes a novel Single Carrier Frequency Division Multiple Access (SC-FDMA) System without Cyclic Prefix based on the Chained Turbo Equalization (CHATUE) algorithm, which connects turbo equalizers neighboring in time to eliminate the Inter-Block Interference (IBI). Results of Extrinsic Information Transfer (EXIT) chart demonstrate that the performance can be improved by performing iterations between neighboring blocks. In addition, this paper also proposes a complexity reduction technique. The numerical results verify the performance improvement and superiority of the proposed technique over the conventional SC-FDMA transmission, without requiring high computational complexity.

The major objective of this paper is to reveal the performance of the turbo equalization using do... more The major objective of this paper is to reveal the performance of the turbo equalization using doped accumulator (TEQ-ACC) when it is combined with channel estimation, in broadband single carrier signalling. It is shown that the accuracy of the channel estimate in TEQ-ACC is inferior to that in conventional turbo equalization (TEQ-CVT), if TEQ-ACC generates the soft replica only from the soft information fed back from the decoder. To cope with this problem, this paper proposes a new soft replica generation method for TEQ-ACC which achieves asymptotically the equivalent accuracy of channel estimates to that with TEQ-CVT. Simulation results show that the gain in frame-error-rate (FER) with TEQ-ACC utilizing the proposed soft replica generation method is significant, 1 dB better than TEQ-CVT at FER = 10-3 under pedestrian mobility channel assumptions. Furthermore, this paper verifies that TEQ-ACC with channel estimation implemented by an adaptive subspace tracking technique, Low rank adaptive filtering (LO-RAF), achieves exactly the same FER performance as that with the sliding window singular value decomposition (SVD)-based technique.

A frequency division multiplexing technique, nonorthogonal frequency division multiplexing (n-OFD... more A frequency division multiplexing technique, nonorthogonal frequency division multiplexing (n-OFDM), is proposed in [1]-[2] to enhance the efficiency of bandwidth utilization. This paper reveals that the smaller the frequency separation, the larger sum capacity can be achieved compared with the conventional OFDM technique. However, n-OFDM system introduces inter-carrier interference (ICI) at the transmitter because the orthogonality between the subcarriers no longer holds. Moreover, since the channel covariance matrix of n-OFDM has high condition number when the overlapping factor, 1 − α, is large, conventional linear detectors suffers from severe noise enhancement. To solve this problem, this paper proposes the use of soft cancellation-minimum mean-squared error (SC-MMSE) turbo equalization. Binary constellation constrained mutual information (CCMI) is calculated by utilizing the area property for the EXtrinsic Information Transfer (EXIT) chart of the SC-MMSE equalizer. Results of the EXIT chart analysis and bit-error-rate (BER) simulations in additive white Gaussian noise (AWGN) channel are presented.

This paper proposes chained turbo equalization (CHATUE) algorithm, a simple detection scheme for ... more This paper proposes chained turbo equalization (CHATUE) algorithm, a simple detection scheme for block transmission without guard interval (GI), where turbo equalizers for the several consecutive frames exchange information about the interference components. Although the block transmission without GI can significantly increase bandwidth efficiency, the received signal suffers from inter-block interference (IBI) and intersymbol interference (ISI), which can't be effectively compensated by block-by-block equalization.With the CHATUE algorithm, the IBI components are cancelled by utilizing a posteriori feedback from the decoders of the neighboring equalizers (future and past). To best utilize the latest results of the sub-optimal reduced complexity equalization, frequency domain soft cancellation and minimum mean squared error filtering (FD/SC-MMSE), we exploit J matrix to convert the Toeplitz channel matrix structure to a circulant matrix. Results of the computer simulation for single carrier block transmission (SCBT) show that the proposed scheme can achieve good performance.

This paper proposes a structure that combines iterative equalization and turbo decoding, denoted ... more This paper proposes a structure that combines iterative equalization and turbo decoding, denoted as spatial turbo coding (STC), for single carrier signaling to achieve near capacity performance in multipath-rich fading channels. Instead of multiplexing the encoded bits in the time domain as in the standard turbo codes, the proposed STC transmits coded bits in the space domain by employing multiple-input multiple-output (MIMO) transceiver to exploit space diversity, path diversity and coding gains through the decoding branches at the receiver. The considered MIMO detector is a MIMO frequency domain softcancelation and minimum mean square error equalizer (FD/SC-MMSE). Extrinsic Information Transfer (EXIT) chart analysis confirms that bit-error-rate (BER) pinch-off is achieved in 64path Rayleigh fading channels with equal average path gains power which is just by about 1dB away from the static channel capacity/dimension of 2×2 MIMO systems.

In this paper, we propose three low-complexity strategies for Multiple Access Relay Channel (MARC... more In this paper, we propose three low-complexity strategies for Multiple Access Relay Channel (MARC) based on the concept of bit-flipping model. By estimating the error rates of the links between the sources and the relay, and by utilizing the estimate as side information at the destination, it is shown that the proposed relay strategies are superior to the Decode-and-Forward (DF) relay strategy when all links are suffering from Additive White Gaussian Noise (AWGN). Furthermore, extrinsic information transfer (EXIT) analysis is used to track the convergence property with the aim of being able to search for suitable codes, the proposed relay strategies can achieve the performance close to those strategies utilizing relatively strong codes, such as turbo code or LDPC code, with iterative decoding at the relay in order to make intra links error free.

Electronics and Communications in Japan Part Ii-electronics, 2007

The new digital terrestrial television transmission signals use the OFDM system. This system has ... more The new digital terrestrial television transmission signals use the OFDM system. This system has the fundamental problem that the peak power to average power ratio (PAPR) of the signal is significant. In this paper, a satellite communication system is proposed as a supplemental network for digital terrestrial broadcasting. It is shown that effective use of the satellite power becomes possible at a large PAPR by using the FM system with a constant signal envelope as the secondary modulation, so that the problem can be resolved. It is shown analytically that a larger FM gain can be obtained by limiting the peak voltage of the OFDM signal by clipping. The degradation of the BER caused by clipping is derived by simulation and the system is optimized with regard to the degree of clipping. Taking the size and operational cost of the receiving station into account, channel design is performed assuming a real satellite channel. The overall transmission characteristics such as the transmission capacity of the satellite repeater and the channel quality are discussed. © 2007 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 90(11): 74–84, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.20385

Abstract—This paper proposes a design framework based on efficient peak-to-average power (PAPR) f... more Abstract—This paper proposes a design framework based on efficient peak-to-average power (PAPR) for orthogonal frequency division multiplexing (OFDM) and carrier interferometry OFDM (CI/OFDM) systems with iterative clipping. In this paper, we utilize the efficient PAPR ...

In this paper, we propose a new design of efficient carrier interferometry orthogonal frequency d... more In this paper, we propose a new design of efficient carrier interferometry orthogonal frequency division multiplexing (CI/OFDM) by using fast Fourier transform (FFT) as spreading codes. First, we propose the use of both positive and negative frequency of the IFFT rather than only using the positive part as in conventional method. It results new CI/OFDM waveforms which only requires a half of FFT points. Second, we replace CI spreading with FFT for achieving lower complexity because complexity of CI is similar to that of discrete Fourier transform (DFT). Our results confirm that the proposed design is capable of achieving high efficiency and low complexity while providing high performance.

This paper proposes an effective iterative clipping (IC) on a coded carrier interferometry OFDM (... more This paper proposes an effective iterative clipping (IC) on a coded carrier interferometry OFDM (CI/OFDM) system for improving the system performances over the nonlinearity of a solid state power amplifier (SSPA). An additional filtering is not required to remove the out-of-band (OOB) spectrum radiation as required in the conventional clipping method. The IC is designed with a minimized computational complexity though the iteration is performed. The results confirm that the CI/OFDM with the proposed IC presents many advantages in terms of BER performance and OOB spectrum radiation. Based on the optimization on BER and OOB spectrum, it is found that 2 and 3 iterations is an effective number for the IC in OFDM and CI/OFDM systems, respectively. The total degradation (TD) improvement by the effective IC in CI/OFDM is about 0.32dB for QPSK and 2.1dB for 64QAM symbol on an SSPA with an output-back-off (OBO) 4dB and 8dB, respectively.

The new digital terrestrial television transmission signals use the OFDM system. This system has ... more The new digital terrestrial television transmission signals use the OFDM system. This system has the fundamental problem that the peak power to average power ratio (PAPR) of the signal is significant. In this paper, a satellite communication system is proposed as a supplemental network for digital terrestrial broadcasting. It is shown that effective use of the satellite power becomes possible at a large PAPR by using the FM system with a constant signal envelope as the secondary modulation, so that the problem can be resolved. It is shown analytically that a larger FM gain can be obtained by limiting the peak voltage of the OFDM signal by clipping. The degradation of the BER caused by clipping is derived by simulation and the system is optimized with regard to the degree of clipping. Taking the size and operational cost of the receiving station into account, channel design is performed assuming a real satellite channel. The overall transmission characteristics such as the transmission capacity of the satellite repeater and the channel quality are discussed. © 2007 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 90(11): 74-84, 2007; Published online in Wiley InterScience (www.interscience. wiley.com).

Generally peak-to-average power ratio (PAPR) reduction imposes transmission performance degradati... more Generally peak-to-average power ratio (PAPR) reduction imposes transmission performance degradation or system capacity reduction. This paper proposes a method for reducing the PAPR of OFDM signals by combining carrier interferometry (CI) spreading code and iterative processing (IP). The proposed method is capable of reducing the PAPR significantly while minimizing the bit error rate (BER) performance degradation. Computer simulation results prove the effectiveness of the proposed method. Further, the proposed method does not introduce out-of-band interference, which requires filtering whose complexity depends on the filtering requirement.

Recently, there is an interest to increase capacity of multicarrier code division multiplexing (M... more Recently, there is an interest to increase capacity of multicarrier code division multiplexing (MC-CDMA) and orthogonal frequency division multiplexing (OFDM) by using spreading codes. Some spreading codes only support up to K = N+M, where M < N. In this paper, we propose a new spreading code with low cross correlation and high autocorrelation while capable of supporting K = 2N+1 users. It means that 2N+1 users or data symbols are able to be transmitted over only N subcarriers. The numerical results show that our proposed code outperforms current Pseudo-Orthogonal Carrier Interferometry (POCI) code with gain of 3dB in additive white Gaussian noise (AWGN) channel and more than 4dB in multipath fading channel at bit-error-rate (BER) level of 10 -5 . By the proposed code, peak-to-average power ratio (PAPR) problem as a typical problem in multicarrier system is also able to be reduced up to the PAPR level of single carrier system (with roll-of-factor=0.10).

Abstract: This paper proposes new simplified design of Pseudo-orthogonal Carrier Interferometry O... more Abstract: This paper proposes new simplified design of Pseudo-orthogonal Carrier Interferometry Orthogonal Frequency Division Multiplexing (PO-CI/OFDM) system by utilizing IFFT/FFT algorithm as a substitute of PO-CI spreading and despreading. We also propose a separator design for guaranteeing that the resulted codes are still (pseudo) orthogonal to each other. This solution is capable of reducing the complexities of PO-CI/OFDM implementation significantly. Difficulties of generating PO-CI codes as complex spreading ...

In this paper, we propose a novel fast and low complexity algorithm of computation for minimum-me... more In this paper, we propose a novel fast and low complexity algorithm of computation for minimum-mean-square-error (MMSE) equalizer or combiner without divisions. Multiplicative effect of fading channel should be compensated by divisions at the receiver. Therefore, equalizer or combiner at the receiver is derived by inverting the channel impulse responses. Here, the number of divisions equals to the number of subcarriers. For the next generation with high bit rate applications, these divisions are necessary to be computed in a very short time and may impact to the increasing of hardware complexities. The main contribution of this paper is a proposed fast algorithm by replacing the large number of divisions with multiplications and subtraction due to its lower complexity. We improve the performance of Newton-Raphson Method by a range extension so that the Newton-Raphson Method is applicable for MMSE computation with small number of iterations. Our results in Carrier Interferometry Orthogonal Division Multiplexing (CI/OFDM) confirm that with only two iterations, performance of the proposed algorithm can achieve the similar performance as the normal computation with divisions.

This paper proposes a simplified method for computation of Fourier Transform by employing double ... more This paper proposes a simplified method for computation of Fourier Transform by employing double square-waves, called double square-waves Fourier transform (DS-FT). Compared to the conventional single square-wave, double square-waves can increase the resolution of signals. Thus, the orthogonality is kept so the bit-error-rate (BER) performance is better than that of Fourier transform using a conventional single square-wave. Our results confirm that the BER performance is significantly improved. The BER performance by the conventional waveform has residual bit error at BER level of 1.5 x 10-3, while BER of the proposed method has no residual bit error up to the BER level of 1 x 1CT7. In addition, the proposed DS-FT has no restriction in its size (e.g. power of two, etc.) as in fast Fourier transform (FFT). Therefore DS-FT is applicable to any size of Fourier transform.

Ieice Transactions, 2008

In this paper, a new large spreading code set with a uniform low cross-correlation is proposed. T... more In this paper, a new large spreading code set with a uniform low cross-correlation is proposed. The proposed code set is capable of (1) increasing the number of assigned user (capacity) in a multicarrier code division multiple access (MC-CDMA) system and (2) reducing the peak-to-average power ratio (PAPR) of an orthogonal frequency division multiplexing (OFDM) system. In this paper, we derive a new code set and present an example to demonstrate performance improvements of OFDM and MC-CDMA systems. Our proposed code set with code length of N has K = 2N + 1 number of codes for supporting up to (2N + 1) users and exhibits lower cross correlation properties compared to the existing spreading code sets. Our results with subcarrier N = 16 confirm that the proposed code set outperforms the current pseudo-orthogonal carrier interferometry (POCI) code set with gain of 5 dB at bit-error-rate (BER) level of 10 −4 in the additive white Gaussian noise (AWGN) channel and gain of more than 3.6 dB in a multipath fading channel. key words: spreading code, OFDM, MC-CDMA, peak-to-average power ratio (PAPR), pseudo-orthogonal code, Carrier Interferometry (CI)

This paper proposes a new technique for peak-to-average-power ratio (PAPR) reduction without spec... more This paper proposes a new technique for peak-to-average-power ratio (PAPR) reduction without spectrum broadening for Carrier Interferometry orthogonal frequency division multiplexing (CI/OFDM) system. Recently, Carrier Interferometry (CI) code is becoming an interesting code for reducing the PAPR of OFDM system. However, for guaranteeing the low PAPR as the PAPR of single carrier (i.e. by about 4 -6dB with roll-of-factor of 0.2 -0.5), we propose low complexity clipping technique for CI/OFDM system. However, it can be easily adapted for other OFDM systems. The main contribution of this paper is the introduction of a new clipping technique with low complexity frequency domain filtering (FDF) which is employed prior to the IFFT. The benefits of the proposed technique are low complexity of FDF and no spectrum broadening. Our results confirm that the PAPR of clipped CI/OFDM can be reduced by about 1dB better than that of the conventional clipped OFDM signals. We clarified that the PAPR is now similar to the PAPR of single carrier system. In the frequency selective fading channel, the bit-error-rate (BER) performance is also improved by about 7dB better than that of the traditional OFDM system due to the frequency diversity benefits in CI/OFDM system.