OFDM-based Wireless Local Area Networks Research Articles

Range impairments of multi-band OFDM UWB communication systems in the presence of OFDM-based WLAN interference

In this paper, a closed form expression for the bit error rate performance of the multi-band orthogonal frequency division multiplexing (MB-OFDM) ultra wideband (UWB) communication system in the presence of interference is analytically analysed. The considered channel is the Saleh-Valenzuela multipath fading channel model. The interference is modelled as the OFDM-based IEEE802.11n wireless local area networks (WLANs). The dependence of the bit error rate performance of the MB-OFDM UWB communication system on the transmission range along with the impact of interference is also analytically investigated. Numerical results are validated with the aid of simulations. It is shown that the range performance of the MB-OFDM UWB communication system is severely deteriorated due to the presence of such interference.

HJam: Attachment Transmission in WLANs

Effective coordination can dramatically reduce radio interference and avoid packet collisions for multistation wireless local area networks (WLANs). Coordination itself needs consume communication resource and thus competes with data transmission for the limited wireless radio resources. In traditional approaches, control frames and data packets are transmitted in an alternate manner, which brings a great deal of coordination overhead. In this paper, we propose a new communication model where the control frames can be "attachedâ to the data transmission. Thus, control messages and data traffic can be transmitted simultaneously and consequently the channel utilization can be improved significantly. We implement the idea in OFDM-based WLANs called hJam, which fully explores the physical layer features of the OFDM modulation method and allows one data packet and a number of control messages to be transmitted together. hJam is implemented on the GNU Radio testbed consisting of eight USRP2 nodes. We also conduct comprehensive simulations and the experimental results show that hJam can improve the WLANs efficiency by up to 200 percent compared with the existing 802.11 family protocols.

A Wireless Local Area Network Channel Estimation Scheme

In this paper, a robust channel estimation scheme is proposed for the OFDM based wireless local area network systems. The proposed channel estimation scheme has been done targeting the IEEE 802.11a standard [, but it can be extended to other packet-based OFDM systems. A robust channel estimation scheme is proposed for the OFDM-based wireless local area network systems. Performance evaluation shows that the WLAN system with the proposed channel estimation scheme is with practical system performance under harsh fading channel.

Two symbol timing estimation methods using Barker and Kasami sequence as preamble for OFDM-based WLAN systems

The estimation of the exact point for the start of the symbol is significant as the orthogonal frequency division multiplexing (OFDM) systems are very sensitive to timing errors. In the IEEE 802.11a wireless local area network (WLAN), each data packet starts with a preamble consisting of ten short training symbols followed by two long training symbols. The timing metric is computed through autocorrelation of the received samples and their delayed copies. When the preambles are known to the receiver, the timing metric is obtained by crosscorrelation of the received samples with the locally generated samples. The correlation peak of the timing metric indicates the correct symbol time. The symbol timing synchronization schemes in IEEE 802.11a WLAN systems use short training symbols to estimate a coarse symbol time via autocorrelation and then use long symbols to find a fine symbol time via crosscorrelation. In this paper, Barker and Kasami codes are proposed to be used as preambles for timing estimation in OFDM WLAN systems. Timing estimation makes use of correlation of preamble. So preamble with good autocorrelation property has to be chosen. The proposed scheme develops a simple preamble structure and gives more accurate estimate of symbol timing.

Efficient implementation of complementary Golay sequences for PAR reduction and forward error correction in OFDM-based WLAN systems

In this paper the use of complementary Golay sequences (CGS) for peak-to-average power ratio (PAR) reduction and forward error correction (FEC) in an orthogonal frequency division multiplexing (OFDM)-based wireless local area network (WLAN) system is explored; performance is examined and complexity issues are analyzed. We study their PAR reduction performance depending on sequence lengths and we have found that, for the case that the number of sub-carriers differs from the sequence length, some interesting relationships can still be stated. Regarding their error correction capabilities, these sequences are investigated considering M-PSK constellations applied to the OFDM signal specified in IEEE 802.11a standard. Computational load for both Golay encoding and decoding processes is addressed and we provide an exhaustive analysis of their complexity. In order to overcome memory restrictions and speed up algorithmic operations, a novel algorithm for real-time generation of the Golay Base Sequences is proposed and evaluated giving as a conclusion that these sequences can be real-time generated with actual Digital Signal Processors (DSP). Our proposal lies on an efficient permutation algorithm that obtains the current permutation without the need for generating previous ones. Its complexity is calculated and turns out to be significantly low; the advantages are specially appreciated at the decoding stage. We also introduce a hybrid solution to get a trade-off between complexity and memory requirements. Moreover, the whole system is also implemented in a DSP to validate the proposal in a prototype, where its feasibility has been confirmed.

Robust channel estimation for the OFDM-based WLAN systems with imperfect synchronization

As an effective technique for combating multipath fading and for high data rate transmission over wireless channels, orthogonal frequ- ency division multiplexing (OFDM) is extensively used in wireless local area network (WLAN) systems to support high-performance bandwidth- efficient multimedia services. In this paper, a robust channel estimation scheme is proposed for the OFDM-based WLAN systems with imperfect synchronization. The frame structure information, the preamble information, the pilot information are efficiently utilized in the proposed channel estimation scheme. Simulation results are used to illustrate the performance of the proposed scheme.

Virtual Subcarrier Assignment (VISA) for Spatial Filtering of OFDM Signals

In this paper, we propose a novel spatial filtering technique for orthogonal frequency division multiplexing (OFDM) signals called Vlrtual Subcarrier Assignment (VISA). Here, virtual subcarrier is a subcarrier which is not used for data transmission. When a wireless terminal is equipped with multiple antennas, VISA can easily achieve a space division multiple access (SDMA) by assigning a different spectral position of virtual subcarrier to a different user. To realize VISA in an already-existing OFDM-based wireless local area network (WLAN), we discuss an antenna weight control method in the preamble of a signal burst format designed for the IEEE802.11a standard and evaluate the bit error rate (BER) performance in typical indoor wireless environments.

Channel Estimation for OFDM-Based WLANs in the Presence of Wiener Phase Noise and Residual Frequency Offset

SUMMARY In orthogonal frequency-division multiplexing (OFDM)based wireless local area networks (WLANs), phase noise (PHN) and residual frequency offset (RFO) can cause the common phase error (CPE) and the inter-carrier interferences (ICI), which seriously degrade the performance of systems. In this letter, we propose a combined pilot symbol assisted and decision-directed channel estimation scheme based on the leastsquares (LS) and the maximum-likelihood (ML) algorithms. Simulation results present that the proposed scheme significantly improves the performance of OFDM-based WLANs.

Efficient Time-Domain Residual ISI Cancellation for OFDM-Based WLAN Systems

In orthogonal frequency-division multiplexing (OFDM) systems, the multipath components whose delays exceed cyclic prefix (CP) cause intersymbol interference (ISI) and intercarrier interference (ICI), which may degrade system performance severely. In this paper, we propose two novel iterative cyclicity restoration (CR) schemes to efficiently cancel these interferences for OFDM-based wireless local area network (WLAN) systems. The time-domain parallel CR (PCR) scheme is introduced first. Using time-domain iterative solution, PCR provides much lower computational complexity than conventional CR. Furthermore, in order to improve the performance of PCR, we present successive CR (SCR) method based on Gauss-Seidel iteration. SCR offers faster convergence speed than PCR without increasing the computational cost. It is shown that both PCR and SCR need only a few iterations to approach the system performance with sufficient CP. SCR has approximately the same performance as conventional CR with much lower computational complexity for WLAN applications.

A phase noise mitigation scheme for MIMO WLANs with spatially correlated and imperfectly estimated channels

Applying multiple input multiple output (MIMO) technique to OFDM-based wireless local area networks (WLANs) promises impressive high capacity and spectral efficiency compared with conventional systems. However, similar to SISO-OFDM, MIMO-OFDM suffers significant performance degradation due to the presence of phase noise. Many methods have been developed to mitigate phase noise for a single antenna system with perfect channel estimation, whereas none has been proposed for correlated MIMO-OFDM scenarios. Therefore, in this letter, by using the phase noise correlation function, a new phase noise mitigation scheme is proposed for the general M/sub T//spl times/M/sub R/ MIMO WLANs system with channel estimation errors. Numerical results show that, compared with conventional approaches, the proposed scheme achieves significant performance gain with high spectral efficiency, requiring few pilots, and is robust to spatial correlation and channel estimation errors, which makes it very attractive for practical applications.

Performance of a novel carrier frequency offset estimation algorithm for OFDM-based WLANs

This paper presents a novel carrier frequency offset estimation (CFO) algorithm for orthogonal frequency division multiplexing (OFDM)-based Wireless Local Area Networks (WLANs). Compared with previous approaches, this paper extends the whole frequency offset acquisition range by embedding a synthetic algorithm according to the preamble structure of WLANs symbols. The numerical results presented support the effectiveness of this algorithm by which the estimation error of the whole carrier frequency offset in the WLANs is effectively decreased.

An OFDM-TDMA/SA MAC Protocol with QoS Constraints for Broadband Wireless LANs

Orthogonal frequency division multiplexing (OFDM) is an important technique to support high speed transmission of broadband traffic in wireless networks, especially broadband wireless local area networks (LANs). Based on OFDM, a new multiple access scheme, called OFDM-TDMA with subcarrier allocation (OFDM-TDMA/SA), is proposed in this paper. It provides more flexibility in resource allocation than other multiple access schemes such as OFDM-TDMA, OFDM-frequency division multiple access (OFDM-FDMA), and orthogonal frequency division multiple access (OFDMA). With OFDM-TDMA/SA, a medium access control (MAC) is designed for broad-band wireless LANs. It optimizes bit allocation in subcarriers so that maximum bits are transmitted in each OFDM symbol under a frequency selective fading environment. The OFDM-TDMA/SA MAC protocol also supports three classes of traffic such as guaranteed, controlled-load, and best effort services. Based on the optimum subcarrier bit-allocation algorithm and considering heterogeneous QoS constraints of multimedia traffic, a hierarchical scheduling scheme is proposed to determine the subcarriers and time slots in which a mobile terminal can transmit packets. In such a way, the OFDM-TDMA/SA MAC protocol significantly increases system throughput in a frequency selective fading environment and guarantees QoS of multimedia traffic. Computer simulation is carried out to evaluate the performance of the OFDM-TDMA/SA MAC protocol. Results show that the new MAC protocol outperforms other MAC protocols for OFDM-based wireless LANs.

Phase Error Correction for OFDM-Based WLANs

A novel phase error correction scheme is proposed for the high rate OFDM-based wireless local area networks (WLANs). The proposed scheme makes the system capable of efficiently compensating the whole phase error due to the residual sampling clock offset and frequency offset estimation error after timing and frequency offset compensation, as well as the phase noise.

Robust Time and Frequency Synchronization for OFDM-Based WLANs

A robust time and frequency synchronization scheme is proposed for the high rate OFDM-based wireless local area networks (WLANs). The IEEE 802.11a standardized preamble is efficiently utilized and makes the proposed scheme practical. Simulation results under different channel environments are presented to illustrate the effectiveness of the proposed scheme.

Efficient adaptive array receiver for OFDM based wireless local area networks (WLAN)

The paper considers low complexity and suboptimal frequency-domain process technology for orthogonal frequency division multiplexing (OFDM) adaptive array system. To reduce complexity, a novel receiver scheme is proposed which is a single beamformer in combination with channel equalization. The scheme computes a set of beamforming weight vector from the samples of a particular subcarrier, and applies the weight vector to all the subcarriers. In the frequency selective fading environment, in order to mitigate performance degradation arising from different channel fading across subcarriers, frequency domain channel estimation and equalization for the post-beamforming channel are employed at individual subcarrier. As an application case, the OFDM-based wireless local area networks (WLAN) standard IEEE 802.11a is considered. Simulation results show that the proposed method can achieve comparable performances with very low complexity.

Robust synchronization and channel estimation for the OFDM-based WLAN systems

As an effective technique for combating multipath fading and for high data rate transmission over wireless channels, orthogonal frequency division multiplexing (OFDM) is extensively used in wireless local area network (WLAN) systems to support high performance bandwidth-efficient multimedia services. In this paper, novel synchronization and channel estimation schemes are proposed for the OFDM-based WLAN systems. Analyses and simulation results show that the proposed schemes exhibit good performances and are robust in harsh fading channel environments and low signal-to-noise ratio (SNR) conditions.

A MIMO System with Backward Compatibility for OFDM-Based WLANs

Orthogonal frequency division multiplexing (OFDM) has been selected as the basis for the new IEEE 802.11a standard for high-speed wireless local area networks (WLANs). We consider doubling the transmission data rate of the IEEE 802.11a system by using two transmit and two receive antennas. We propose a preamble design for this multi-input multi-output (MIMO) system that is backward compatible with its single-input single-output (SISO) counterpart as specified by the IEEE 802.11a standard. Based on this preamble design, we devise a sequential method for the estimation of the carrier frequency offset (CFO), symbol timing, and MIMO channel response. We also provide a simple soft detector based on the unstructured least square approach to obtain the soft information for the Viterbi decoder. This soft detector is very simple since it decouples the multidimensional QAM symbol detection into multiple one-dimensional QAM symbol-and further PAM symbol-detections. Both the sequential parameter estimation method and the soft detector can provide excellent overall system performance and are ideally suited for real-time implementations. The effectiveness of our methods is demonstrated via numerical examples.

Parameter estimation and error reduction for OFDM-based WLANs

We consider parameter estimation and error reduction for orthogonal frequency-division multiplexing (OFDM) based high-speed wireless local area networks (WLANs). We devise or select algorithms that can provide benefit to the overall system performance and can be efficiently implemented in real-time. In particular, first, we give a channel model which is especially useful for assessing the channel parameter estimation methods devised for OFDM-based WLANs. Second, we provide a sequential method for the estimation of carrier frequency offset (CFO), symbol timing, and channel response by exploiting the structure of the packet preamble specified by the IEEE 802.11a standard. Finally, to correct the residue CFO induced phase error using the pilot tones, we consider maximum-likelihood phase tracking and least-squares phase fitting approaches; to improve the channel estimation accuracy using the decoded data, we present a semiblind channel estimation method; to mitigate the sampling clock induced time delay error, we provide a sampling clock synchronization approach that obviates the need of an automatic frequency control clock recovery circuit. The overall system performance of using our algorithms is demonstrated via several numerical examples.

Implementation of a MIMO OFDM-Based Wireless LAN System

The combination of multiple-input multiple-output (MIMO) signal processing with orthogonal frequency division multiplexing (OFDM) is regarded as a promising solution for enhancing the data rates of next-generation wireless communication systems operating in frequency-selective fading environments. To realize this extension of OFDM with MIMO, a number of changes are required in the baseband signal processing. An overview is given of the necessary changes, including time and frequency synchronization, channel estimation, synchronization tracking, and MIMO detection. As a test case, the OFDM-based wireless local area network (WLAN) standard IEEE 802.11a is considered, but the results are applicable more generally. The complete MIMO OFDM processing is implemented in a system with three transmit and three receive antennas, and its performance is evaluated with both simulations and experimental test results. Results from measurements with this MIMO OFDM system in a typical office environment show, on average, a doubling of the system throughput, compared with a single antenna OFDM system. An average expected tripling of the throughput was most likely not achieved due to coupling between the transmitter and receiver branches.

Peak-to-average power control for multiple-antenna HIPERLAN/2 and IEEE802.11a systems

A combination of multi-input multi-output (MIMO) signal processing with orthogonal frequency division multiplexing (OFDM) is regarded as a promising solution for enhancing the performance of next generation wireless local area network (WLAN) systems. This paper evaluates the peak-to-average power ratio (PAR) performance in a MIMO OFDM-based WLAN system using a space-time block code (STBC). The investigated PAR control schemes, which are originally based on the selective mapping (SLM) and partial transmit sequence (PTS) approaches, select the transmitted sequence with the lowest average PAR over all transmit antennas and retrieve the side information (SI) very accurately at the expense of a slight degradation of the PAR performance. The low probability of false SI can improve the overall detection performance of the MIMO-WLAN system encountered with erroneous SI compared to the ordinary SLM and PTS approaches, respectively.