• Title/Summary/Keyword: access timing offset

Search Result 16, Processing Time 0.027 seconds

Access timing offsets-resilient SC-FDMA (접속동기 오차에 강한 SC-FDMA 기법)

  • Kim, Bong-Seok;Choi, Kwonhue
    • Journal of Satellite, Information and Communications
    • /
    • v.7 no.3
    • /
    • pp.26-29
    • /
    • 2012
  • In this paper, we propose a Single Carrier Frequency Division Multiple Access(SC-FDMA) scheme with greatly enhanced tolerance of timing offset among the users. The type of the proposed scheme is similar to code spread Multiple Carrier Direct Spread Code Division Multiple Access(MC DS CDMA). The proposed scheme performs partial Discrete Fourier Transform(DFT) in order to solve high Peak to Average Power Ratio(PAPR) of the MC DS CDMA before Inverse Fast Fourier Transform(IFFT). Exploiting the property Properly Scrambled Walsh-Hadamard(PSW) code has zero correlation despite ${\pm}1$ chip timing offset, the proposed scheme achieves Multiple Access Interference free performance with the timing offset up to ${\pm}1$ OFDM symbol duration with low PAPR. In contrast, the other existing schemes in comparison undergo severe performance degradation even with small timing offset in multipath fading channel.

Access timing offsets-resilient uplink OFDMA for satellite systems (액세스 타이밍 오차에 강한 위성 시스템 상향링크 OFDMA 기법)

  • Kim, Bong-Seok;Choi, Kwon-Hue
    • Journal of Satellite, Information and Communications
    • /
    • v.5 no.2
    • /
    • pp.92-96
    • /
    • 2010
  • We propose a new satellite OFDMA(Orthogonal Frequency Division Multiple Access) scheme with greatly enhanced tolerance of timing offset among the users. In uplink OFDMA systems, timing misalignment among users destroys subcarrier orthogonality and thus, it degrades the performance. In order to avoid this performance degradation, the accurate processing, so called 'ranging', is required to synchronize among users. However, ranging scheme is not available in the satellite systems due to the very long round trip delay. Exploiting the property that PSW(Propoerly Scrambled Walsh-code) code has zero correlation despite ${\pm}1$ chip timing offset, the proposed OFDMA achieves MAI free performance with the timing offset up to ${\pm}1$ OFDM symbol duration for the satellite systems.

Allowing a Large Access Timing Offset in OFDM-CDMA Using ZCZ Code and Block Spreading (ZCZ 부호와 블록 확산을 이용한 사용자 동기화 경감 OFDM-CDMA)

  • Na, Donj-jun;Choi, Kwonhue
    • The Journal of Korean Institute of Communications and Information Sciences
    • /
    • v.41 no.1
    • /
    • pp.23-36
    • /
    • 2016
  • We propose a new type of OFDM-CDMA scheme which allows large inter-user timing offset using zero correlation zone(ZCZ) code in conjunction with block spreading technique. Moreover to maximize spectral efficiency, the proposed OFDMA does not have guard time(GT). This is opposite to the trends in the conventional schemes where GT are supposed to be larger to allow larger inter-user timing offset. It is remarkable that the proposed GT-free OFDM-CDMA scheme completely cancels inter-user interference in the multipath fading simply by despreading process. This inter-user interference-free feature still remains even there exist inter-user timing offsets as large as multiple OFDM symbols. Although the self-user interference exists due to no GT, it can be effectively suppressed by simple successive interference cancellation(SIC) from the first symbol in spread block as it is free from inter symbol interference(ISI).

A Random Access Scheme Robust to Timing Offsets for Uplink OFDMA Systems (타이밍오차에 강건한 상향링크 OFDMA 시스템의 랜덤 액세스 기법)

  • Song, Hyun-Joo;Rim, Dae-Woon;Jeong, Byung-Jang;Noh, Tae-Gyun;Rim, Min-Joong
    • The Journal of Korean Institute of Communications and Information Sciences
    • /
    • v.33 no.6A
    • /
    • pp.659-665
    • /
    • 2008
  • If ranging processes are not frequent in an uplink OFDMA system, the timing synchronization between base and mobile stations may not be maintained. Especially in the case of infrequent random accesses in a large cell, timing offsets can be considerable and the destroyed orthogonality of OFDMA signals may result in the degraded performance. This paper proposes a novel random access scheme for uplink OFDMA systems, which support the orthogonality of OFDMA signals with timing offsets by allowing a random access OFDMA symbol shorter than a normal one. In order to make uplink OFDMA systems robust to timing offsets, the proposed scheme inserts a zero-padding region in an OFDMA symbol.

The Scheme for Improving the Performance of Ranging Code Detection over OFDMA Systems in Uplink (OFDMA 시스템 상향링크의 레인징 부호 검출 성능 향상 기법)

  • Kim Ki-Nam;Kim Jin-Ho;Cho Sung-Joon
    • The Journal of Korean Institute of Communications and Information Sciences
    • /
    • v.31 no.6A
    • /
    • pp.575-585
    • /
    • 2006
  • In Orthogonal Frequency Division Multiple Access (OFDMA) systems, timing synchronization in uplink is accomplished by an initial uplink synchronization called an initial ranging process. The Base Station's receiver synchronizes the symbol timing to specific user's symbol and the other user's symbols have some Symbol Timing Offset (STO). Linear phase shift is occurred by each user's STO in an OFDMA symbol. The Multiple Access Interference (MAI) caused by the summation of each user's linear phase shift degrades the performance of ranging code detection. In this paper, we propose an initial ranging symbol structure with common ranging code for phase shift estimation and compensation. We car estimate the average of phase shift that is generated by each user's STO and compensate this phase shift by using common ranging code. This scheme will suppress the MAI and provide better detection performance than conventional process.

Analysis of Initial Synchronization Performance in OFDMA/TDD Systems (OFDMA/TDD 시스템의 초기 동기 성능 분석)

  • Seung Young-Min;Kim Ki-Nam;Cho Sung-Joon
    • Proceedings of the Korean Institute of Information and Commucation Sciences Conference
    • /
    • 2006.05a
    • /
    • pp.410-414
    • /
    • 2006
  • In the present, Orthogonal Frequency Division Multiple Access (OFDMA) that wireless access scheme for high speed data transmission is noticed in mobile communication market and OFDMA/TDD scheme will be used combining Time Division Duplex (TDD) scheme based on OFDMA. The Base Station's receiver synchronizes the symbol timing to anyone user's symbol and the other user's symbols have some Symbol Timing Offset (STO). Linear phase shift is occurred by each user's STO in an OFDMA symbol and the Multiple Access Interference (MAI) caused by the summation of each user's linear phase shift degrades the performance of ranging code detection. In this paper, we analyze the ranging code detection performance for each users STO in OFDMA/TDD system. Simulation results show that the more users access and mobile speed increase, the more ranging code detection performance degrades.

  • PDF

Effect of Synchronization Errors on the Performance of Multicarrier CDMA Systems

  • Li Ying;Gui Xiang
    • Journal of Communications and Networks
    • /
    • v.8 no.1
    • /
    • pp.38-48
    • /
    • 2006
  • A synchronous multicarrier (MC) code-division multiple access (CDMA) system using inverse fast Fourier transform (IFFT) and fast Fourier transform (FFT) for the downlink mobile communication system operating in a frequency selective Rayleigh fading channel is analyzed. Both carrier frequency offset and timing offset are considered in the analysis. Bit error rate performance of the system with both equal gain combining and maximum ratio combining are obtained. The performance is compared to that of the conventional system using correlation receiver. It is shown that when subcarrier number is large, the system using IFFT/FFT has nearly the same performance as the conventional one, while when the sub carrier number is small, the system using IFFT/FFT will suffer slightly worse performance in the presence of carrier frequency offset.

Frequency Offset Estimation in OFDMA Uplink System Using Low Cross-correlation Property of PN Sequences (OFDMA 상향 링크 시스템에서 PN 시퀀스의 낮은 Cross correlation 특성을 이용한 주파수 오차 추정)

  • Min, Hyun-Kee;Kim, Seok-Joong;Bang, Keuk-Joon;Hong, Dae-Sik
    • Proceedings of the IEEK Conference
    • /
    • 2008.06a
    • /
    • pp.239-240
    • /
    • 2008
  • A new frequency offset estimator using low cross-property of PN sequences is proposed in the orthogonal frequency division multiple access (OFDMA) uplink systems. Simulation results show that the proposed estimator improves the mean square error (MSE) performance compared to conventional estimator [2] where each user have both different timing offset and different frequency offset. In addition, the MSE performance is improved as the length of PN sequence increases.

  • PDF

Effect Analysis of Timing Offsets for Asynchronous MC-CDMA Uplink Systems (비동기 MC-CDMA 상향 링크 시스템에서의 시간 옵셋 영향 분석)

  • Ko, Kyun-Byoung;Woo, Choong-Chae
    • Journal of the Institute of Electronics Engineers of Korea TC
    • /
    • v.47 no.8
    • /
    • pp.1-8
    • /
    • 2010
  • This paper models a symbol timing offset (STO) with respect to the guard period and the maximum access delay time for asynchronous multicarrier code division multiple access (MC-CDMA) uplink systems over frequency-selective multipath fading channels. Analytical derivation shows that STO causes desired signal power degradation and generates self-interferences. This effect of the STO on the average bit error rate (BER) and the effective signal-to-noise ratio (SNR) is evaluated. The approximated BER and the SNR loss caused by STO are then obtained as closed-form expressions. The tightness between the analytical result and the simulated one is verified for the different STOs and SNRs. Furthermore, the derived analytical results are verified via Monte Carlo simulations.

An Inter-Cell Interference Estimation Algorithm for Cellular OFDMA Systems (셀률러 OFDMA 시스템을 위한 셀간 간섭추정 알고리즘)

  • Rim, Min-Joong
    • Journal of the Institute of Electronics Engineers of Korea TC
    • /
    • v.43 no.10 s.352
    • /
    • pp.55-59
    • /
    • 2006
  • In cellular OFDMA(Orthogonal Frequency Division Multiple Access) systems, each subcarrier may suffer from different amount of interferences from neighbor cells. Suppose that it is possible to accurately estimate inter-cell interferences for each subcarrier, the performance can be considerably improved by applying SINR(Signal to Interference and Noise Ratio) weighting. This paper proposes an inter-cell interference estimation method for cellular OFDMA systems. The proposed method extracts amounts of noise and interferencesby eliminating the channel variation effects of pilot symbols caused by frequency offset, timing offset mobile velocity, and delay spread.