7 research outputs found
Interference Localization for Uplink OFDMA Systems in Presence of CFOs
Multiple carrier frequency offsets (CFOs) present in the uplink of orthogonal
frequency division multiple access (OFDMA) systems adversely affect subcarrier
orthogonality and impose a serious performance loss. In this paper, we propose
the application of time domain receiver windowing to concentrate the leakage
caused by CFOs to a few adjacent subcarriers with almost no additional
computational complexity. This allows us to approximate the interference matrix
with a quasi-banded matrix by neglecting small elements outside a certain band
which enables robust and computationally efficient signal detection. The
proposed CFO compensation technique is applicable to all types of subcarrier
assignment techniques. Simulation results show that the quasi-banded
approximation of the interference matrix is accurate enough to provide almost
the same bit error rate performance as that of the optimal solution. The
excellent performance of our proposed method is also proven through running an
experiment using our FPGA-based system setup.Comment: Accepted in IEEE WCNC 201
Enhanced MIMO-DCT-OFDM System Using Cosine Domain Equalizer
Discrete Cosine Transform (DCT) can be used instead of conventional Discrete
Fourier Transform (DFT) for the Orthogonal Frequency Division Multiplexing
(OFDM) construction, which offers many advantages. In this paper, the
Multiple-Input-Multiple-Output (MIMO) DCT-OFDM is enhanced using a proposed
Cosine Domain Equalizer (CDE) instead of a Frequency Domain Equalizer (FDE).
The results are evaluated through the Rayleigh fading channel with Co-Carrier
Frequency Offset (Co-CFO) of different MIMO configurations. The average bit
error probability and the simulated time of the proposed scheme and the
conventional one is compared, which indicates the importance of the proposed
scheme. Also, a closed formula for the number of arithmetic operations of the
proposed equalizer is developed. The proposed equalizer gives a simulation time
reduction of about 81.21%, 83.74% compared to that of the conventional LZF-FDE,
and LMMSE-FDE, respectively for the case of 4x4 configuration
Low-Complexity Orthogonal Spectral Signal Construction for Generalized OFDMA Uplink With Frequency Synchronization Errors
In orthogonal frequency-division multiplexing, the total spectral resource is partitioned into multiple orthogonal subcarriers. These subcarriers are assigned to different users for simultaneous transmission in orthogonal frequency-division multiple access (OFDMA). In an unsynchronized OFDMA uplink, each user has a different carrier frequency offset (CFO) relative to the common uplink receiver, which results in the loss of orthogonality, among subcarriers and thereby multiple access interference. Hence, OFDMA is very sensitive to frequency synchronization errors. In this paper, we construct the received signals in frequency domain that would have been received if all users were frequency synchronized. A generalized OFDMA framework for arbitrary subcarrier assignments is proposed. The interference in the generalized OFDMA uplink due to frequency synchronization errors is characterized in a multiuser signal model. Least squares and minimum mean square error criteria are proposed to construct the orthogonal spectral signals from one OFDMA block contaminate with interference that was caused by the CFOs of multiple users. For OFDMA with a large number of subcarriers, a low-complexity implementation of the proposed algorithms is developed based on a banded matrix approximation. Numerical results illustrate that the proposed algorithms improve the system performance significantly and are computationally affordable using the banded system implementation
Inter-carrier interference suppression in orthogonal frequency division multiple access (OFDMA) systems uplink
Ph.DDOCTOR OF PHILOSOPH
Timing and Frequency Synchronization in Practical OFDM Systems
Orthogonal frequency-division multiplexing (OFDM) has been adopted by many broadband wireless communication systems for the simplicity of the receiver technique to support high data rates and user mobility. However, studies also show that the advantage of OFDM over the single-carrier modulation schemes could be substantially compromised by timing or frequency estimation errors at the receiver. In this thesis we investigate the synchronization problem for practical OFDM systems using a system model generalized from the IEEE 802.11 and IEEE 802.16 standards. For preamble based synchronization schemes, which are most common in the downlink of wireless communication systems, we propose a novel timing acquisition algorithm which minimizes false alarm probability and indirectly improves correct detection probability. We then introduce a universal fractional carrier frequency offset (CFO) estimator that outperforms conventional methods at low signal to noise ratio with lower complexity. More accurate timing and frequency estimates can be obtained by our proposed frequency-domain algorithms incorporating channel knowledge. We derive four joint frequency, timing, and channel estimators with different approximations, and then propose a hybrid integer CFO estimation scheme to provide flexible performance and complexity tradeoffs. When the exact channel delay profile is unknown at the receiver, we present a successive timing estimation algorithm to solve the timing ambiguity. Both analytical and simulation results are presented to confirm the performance of the proposed methods in various realistic channel conditions. ..