Frame synchronization for PSAM in AWGN and Rayleigh fading channels

Pilot Symbol Assisted Modulation (PSAM) is a good method to compensate for the channel fading effect in wireless mobile communications. In PSAM, known pilot symbols are periodically inserted into the transmitted data symbol stream and the receiver uses these symbols to derive amplitude and phase reference. One aspect of this procedure, which has not received much attention yet, is the frame synchronization, i.e. the method used by the receiver to locate the time position of the pilot symbols. In this study, two novel non-coherent frame synchronization methods are introduced in which only the magnitude of received signal is used to obtain the timing of the pilot symbol. The methods are evaluated for both AWGN and frequency non-selective slow Rayleigh fading channels. One synchronization technique is derived by standard maximum likelihood (ML) estimation formulation, and the other is obtained by using maximum a Posteriori probability (MAP) with a threshold test. Signal processing in the receiver uses simplifying approximations that rely on relatively high signal-to-noise ratio (SNR) as consistent with the reception of 16-QAM. Computer simulation has been used to test the acquisition time performance and the probability of false acquisition. Several lengths and patterns of pilot symbol sequences were tested where every 10th symbol was a pilot symbol and all other symbols were randomly selected data symbols. When compared with the other published synchronizers, results from this study show better performance in both AWGN and fading channels. Significantly better performance is observed in the presence of receiver frequency offsets

Software Defined Radio Implementation of Carrier and Timing Synchronization for Distributed Arrays

The communication range of wireless networks can be greatly improved by using distributed beamforming from a set of independent radio nodes. One of the key challenges in establishing a beamformed communication link from separate radios is achieving carrier frequency and sample timing synchronization. This paper describes an implementation that addresses both carrier frequency and sample timing synchronization simultaneously using RF signaling between designated master and slave nodes. By using a pilot signal transmitted by the master node, each slave estimates and tracks the frequency and timing offset and digitally compensates for them. A real-time implementation of the proposed system was developed in GNU Radio and tested with Ettus USRP N210 software defined radios. The measurements show that the distributed array can reach a residual frequency error of 5 Hz and a residual timing offset of 1/16 the sample duration for 70 percent of the time. This performance enables distributed beamforming for range extension applications.Comment: Submitted to 2019 IEEE Aerospace Conferenc

Design of pilot signal for MC-CDMA uplink systems

Coherent demodulation needs to maintain the symbol timing synchronization and to estimate the channel response in multicarrier code division multiple access systems. The use of inverse DFT (IDFT) is often considered to generate a pilot signal for this propose. However it requires a large implementation complexity, making it impractical for real applications. In this paper, we consider the design of a novel pilot signal for both symbol synchronization and channel estimation. By controlling the design parameters, the pilot signal can be generated to provide a desired signal-tointerference noise power ratio, while maintaining an acceptable peak-to-average power ratio. Simulation results show that the use of the proposed pilot signal can improve the detection performance compared to the use of conventional IDFT-based pilot signal, while significantly reducing the implementation complexity

Superimposed training low probability of detection underwater communications

This paper proposes a superimposed training method for low probability of detection underwater acoustic communications. A long pilot sequence was superimposed to the message for equalization and synchronization purposes. A fast Hadamard transform (FHT) estimated the channel impulse response and compressed the pilot energy. A Wiener filter performed equalization. The interference signal was removed using hyperslice cancellation by coordinate zeroing. An inverse FHT decompressed the remaining sequence energy and the message was retrieved. Results from a shallow water experiment presented bit error rates <10-2 for signal-to-noise ratios <-8 dB.info:eu-repo/semantics/publishedVersio

Synchronization of OFDM at low SNR over an AWGN channel

This paper is based on Extended Symbol OFDM (ES-OFDM) where symbols are extended in time. This way ES-OFDM can operate at low SNR. Each doubling of the symbol length improves the SNR performance by 3 dB in case of a coherent receiver. One of the basic questions is how to synchronize to signals far below the noise floor. An algorithm is presented which is based on the transmission of pilot symbols. At the receiver, the received signal is cross correlated with the known pilot symbol and the maximum magnitude is determined. The position of the maximum value within the cross correlation function indicates the time difference between transmitter and receiver. The performance of the algorithm in case of an Additive White Gaussian Noise (AWGN) channel, is assessed based on a theoretical approximation of the probability of correct detection of the time difference. The theoretical approximation matches with simulation results and shows that synchronization can be achieved for low (negative) SNRs

Synchronization in wireless communications

The last decade has witnessed an immense increase of wireless communications services in order to keep pace with the ever increasing demand for higher data rates combined with higher mobility. To satisfy this demand for higher data rates, the throughput over the existing transmission media had to be increased. Several techniques were proposed to boost up the data rate: multicarrier systems to combat selective fading, ultra wide band (UWB) communications systems to share the spectrum with other users, MIMO transmissions to increase the capacity of wireless links, iteratively decodable codes (e.g., turbo codes and LDPC codes) to improve the quality of the link, cognitive radios, and so forth

AirSync: Enabling Distributed Multiuser MIMO with Full Spatial Multiplexing

The enormous success of advanced wireless devices is pushing the demand for higher wireless data rates. Denser spectrum reuse through the deployment of more access points per square mile has the potential to successfully meet the increasing demand for more bandwidth. In theory, the best approach to density increase is via distributed multiuser MIMO, where several access points are connected to a central server and operate as a large distributed multi-antenna access point, ensuring that all transmitted signal power serves the purpose of data transmission, rather than creating "interference." In practice, while enterprise networks offer a natural setup in which distributed MIMO might be possible, there are serious implementation difficulties, the primary one being the need to eliminate phase and timing offsets between the jointly coordinated access points. In this paper we propose AirSync, a novel scheme which provides not only time but also phase synchronization, thus enabling distributed MIMO with full spatial multiplexing gains. AirSync locks the phase of all access points using a common reference broadcasted over the air in conjunction with a Kalman filter which closely tracks the phase drift. We have implemented AirSync as a digital circuit in the FPGA of the WARP radio platform. Our experimental testbed, comprised of two access points and two clients, shows that AirSync is able to achieve phase synchronization within a few degrees, and allows the system to nearly achieve the theoretical optimal multiplexing gain. We also discuss MAC and higher layer aspects of a practical deployment. To the best of our knowledge, AirSync offers the first ever realization of the full multiuser MIMO gain, namely the ability to increase the number of wireless clients linearly with the number of jointly coordinated access points, without reducing the per client rate.Comment: Submitted to Transactions on Networkin