Interleaved Subcarrier Allocation Research Articles

Sensitivity Analysis of OFDMA and SC-FDMA Uplink Systems to Carrier Frequency Offset

Orthogonal frequency division multiple access (OFDMA) and single carrier frequency division multiple access (SC-FDMA) are two technologies for the uplink transmission of present and next generation of broadband wireless systems. This paper studies and compares OFDMA and SC-FDMA in terms of sensitivity to carrier frequency offset (CFO) in the uplink. In order to calculate signal-to-interference ratio (SIR), we use a simple superposition principle approach where the contributions of different users are studied separately. We derive closed-form mathematical expressions for the desired signal, interference terms, and consequently SIR for both OFDMA and SC-FDMA systems in the uplink. It is pointed out that there is a strong relationship between sensitivity analysis to CFO and subcarrier allocation schemes. Also, we prove that the derived expressions for both systems are reduced to very simple forms in interleaved subcarrier allocation. Finally, the theoretical analysis are verified using Monte Carlo simulations in block, interleaved, and block-interleaved subcarrier allocations, and the two systems are compared upon these set of results.

Improving the OFDMA Performance with Common Carrier Frequency Offset Correction

In an OFDMA system, different carrier frequency offsets (CFOs) are possibly raised due to the mismatch of local oscillators and the Doppler effect because of multiple-antenna channels. In spite of an initial CFO synchronization scheme able to be applied at transmitters, the compensation of residual CFOs is required at the receiver in order to eliminate the inter-carrier interference (ICI) for individual subscribers, especially with an interleaved subcarrier allocation scheme. Unfortunately, conventional ICI mitigation methods cannot simultaneously remove the CFOs caused by multiple subscribers, and therefore, the multiuser interference (MUI) remains. In this case, a common CFO (CCFO) existing among the multiple CFOs is found in relation to the overall OFDMA system performance. In this paper, a CCFO estimation method is proposed at the OFDMA receiver, and the CCFO is then corrected to reach the minimum weighted mean square error (MSE) performance. Numerical results show that new OFDMA receivers after correcting the estimated CCFO significantly improve the overall bit error rate (BER) performance over the conventional receivers.

Spectral Precoding for Cyclic-Prefixed OFDMA With Interleaved Subcarrier Allocation

Spectrally precoded orthogonal frequency-division multiple access (OFDMA) using cyclic prefix and interleaved subcarrier allocation (IOFDMA) is a useful rectangularly pulsed OFDMA signaling format for it exploits high spectral compactness and frequency diversity characteristics. In this paper, two new spectral precoders of the largest precoding rate are developed for IOFDMA in order to provide improved spectral compactness and reduce implementation complexity, respectively. Specifically, a new correlative precoder is developed and shown to minimize the dominant spectral coefficient among all correlative precoders of the largest precoding rate and thus can construct IOFDMA signals providing extremely high spectral compactness. Besides, a multiple-stage spectral precoding technique is proposed to construct a low-complexity orthogonally precoded IOFDMA with power spectral sidelobes decaying as f^{-2L-2} for L>1, where L is the number of stages. The corresponding orthogonally precoded IOFDMA signals conveying independent zero-mean data are shown to provide the same power spectrum as existing orthogonally precoded IOFDMA signals of the largest precoding rate while requiring much less complexity.

Semi‐analytic selection of sub‐carrier allocation schemes in uplink orthogonal frequency division multiple access

The authors propose a generalised framework that analytically compares different sub-carrier allocation (SA) or sub-channelisation schemes (such as interleaved SA (ISA), localised SA (LSA) or hybrid schemes) in uplink (UL) orthogonal frequency division multiple access (OFDMA). The ultimate goal of the proposed framework is to systematically determine the best SA scheme among the considered candidates for a given condition of inter-user frequency offsets (IUFOs) and the target bit error rate (BER). As an illustration, two typical SA schemes are considered, that is, ISA and LSA for comparison. First, based on the well-known fact that multiple access interference by IUFO and frequency diversity gain are both dictated by the employed SA scheme, the authors propose a semi-analytic approach to derive coded BER curve formula as a function of frequency offset bound (FOB) for each scheme. Then, the signal-to-noise ratio gain of ISA over LSA for various target BERs and FOBs is derived. Finally, the cutoff FOB over which ISA obtains worse than LSA is obtained and its functional relationship with the target BER is investigated. By following the overall procedure in the proposed framework, the best SA scheme among the various SA candidates for the general UL OFDMA systems with arbitrary system parameters can be chosen.

Diversity-Multiplexing Tradeoff in OFDMA Systems: An H-Matching Approach

OFDMA is a promising technique because it is capable of improving the transmission reliability and efficiency of multi-user wireless communications. However, previous works on the performance of OFDMA did not properly consider the fundamental relationship between multiplexing and diversity in OFDMA systems. As a comprehensive performance metric, the diversity-multiplexing tradeoff will be applied in this paper to evaluate the subcarrier allocation scheme. The OFDMA system will be formulated into a correlated random bipartite graph model, in which, whether the edges occur or not depends on the distribution of the channel fading. The H-matching method, which is used to determine the maximum collection of vertex-disjoint copies of a fixed sub-graph H contained in a given graph, will then be developed to address the optimal subcarrier allocation problem. Theoretical analysis will show that the proposed H-matching method achieves the optimal outage performance at a given target multiplexing gain, which implies that the optimal diversity-multiplexing tradeoff can be achieved by only allocating subcarriers. Although the H-matching problem is NP-complete, the proposed Random Rotation and Expansion based Hopcroft-Karp (R2EHK) algorithm can still achieve the asymptotically optimal outage performance (i.e., optimal diversity-multiplexing tradeoff) with a sub-linear complexity. Furthermore, the channel state information needed is only one bit per subcarrier. Simulation results will verify the theoretical analysis and will show that the performance loss of the R2EHK algorithm is negligible compared to the exhaustive search method. In addition, it is also shown that the R2EHK algorithm has at least a 2 dB SNR gain compared to the interleaved subcarrier allocation with water-filling power allocation in IEEE 802.16 standards.

Semiblind Channel Estimation for IFDMA in Case of Channels with Large Delay Spreads

Discrete Fourier Transform- (DFT-) precoded Orthogonal Frequency Division Multiple Access (OFDMA) with interleaved subcarrier allocation per user is considered which is denoted as Interleaved Frequency Division Multiple Access (IFDMA). For IFDMA, the received signal exhibits a cyclostationarity with the parameter which defines the number of allocated subcarriers per user. In previous works, we have shown that this cyclostationarity can be exploited for the application of subspace-based semiblind channel estimation to IFDMA. Nevertheless, the formerly proposed algorithm is restricted to channels with small delay spreads, meaning that the number of channel delay taps needs to be smaller than . In this paper, we introduce a new semiblind subspace-based channel estimation algorithm which identifies a subspace of the received signal that is spanned by the elements of the so-called cyclic channel impulse response. By doing so, the number of elements to be estimated is reduced and the proposed algorithm is able to cope with channels with large delay spreads.

Modular system-level simulator concept for OFDMA systems

Modeling and simulation is essential for performance evaluation of wireless systems and specific algorithms used, for instance, during resource allocation. For current and future wireless systems, several properties have to be considered such as data traffic, frequency selective fading channels, and radio resource management. Throughout this article, a new system-level simulator concept is presented for packet switched systems using OFDMA, which is called snapshot-based SLS and represents a compromise between state-of-the-art static and dynamic SLSs. The proposed methodology only considers short time intervals within the busy hour by their statistics so that RRM and data traffic can be considered, as in dynamic SLS, but with lower computational complexity. In this article a description is given of how the cellular setup and traffic generation are performed for the proposed snapshot concept. Furthermore, a new methodology is proposed for a quality measure of resource units that is applicable to future wireless systems using an interleaved subcarrier allocation. Exemplary simulation results show that the developed concept is able to evaluate the performance of OFDMA systems considering the impact of, for example, data traffic and resource allocation strategies.

Low-Complexity Interleaved Subcarrier Allocation in Multicarrier Multiple-Access Systems

We propose a new method to efficiently carry out an interleaved subcarrier assignment in multicarrier (MC) multiple-access (MA) systems. According to the conventional approach, the assignment of the subcarriers to the different users is accomplished prior to the MC modulation that is performed over the whole set of subcarriers within the transmission bandwidth. The proposed method enables the MC modulation/demodulation of only the subcarriers assigned to a specific user, thus being much simpler to implement than the conventional approach