John E. Kleider

Pilot design for OFDM with null edge subcarriers

Pilot symbol assisted modulation (PSAM) orthogonal frequency division multiplexing (OFDM) has proven to be a popular technique for high-speed communication through multipath fading channels. In this paper we examine PSAM pilot design optimization in OFDM systems that employ edge null subcarriers for spectral shaping. Specifically, we show that the commonly used even pilot spacing design is suboptimal in terms of symbol estimate mean squared error (MSE) performance when a sufficient number of null subcarriers are present. We pursue a parametric design of the pilot spacings and use convex optimization techniques in order to find a pilot design that results in near-optimal symbol estimate MSE performance. Finally, we present several example PSAM OFDM pilot designs including one example based on the IEEE 802.16 standard to demonstrate performance improvements over the conventional even-spacing pilot design when null edge subcarriers are present.

Radio frequency watermarking for OFDM wireless networks

In this work, we apply watermarking to the physical layer of the wireless baseband modulation waveform, with the motivation to improve flexibility and efficiency of authentication processes in a secure wireless network. We present two baseband watermarking methods, called constellation dithering (CD) and baud dithering (BD), applied to orthogonal frequency division multiplexing (OFDM). We provide the watermark detection and capacity performance attributes in an additive white Gaussian noise (AWGN) channel. Both watermarking techniques allow interoperability with uninformed systems (such as receivers in the 802.11 WLAN commercial standard). Results indicate that, while the BD method provides higher detection robustness and capacity, the CD method exhibits more performance flexibility and is easily modified to the desired user characteristics.

Preamble and embedded synchronization for RF carrier frequency-hopped OFDM

In this work, we apply RF carrier frequency hopping to orthogonal frequency-division multiplexing (FH-OFDM). Achievable hop rate and bandwidth efficiency are determined based on signal acquisition/synchronization and data demodulation performance in the presence of unknown time-frequency offsets, and channel gain/phase perturbations. We compare performance using two different data-aided synchronization approaches. The first method sends synchronization information in a preamble before the OFDM payload symbol, whereas the second method embeds the synchronization information directly into the OFDM symbol stream. In the embedded technique, superposition of the synchronization information causes interference onto the OFDM data information. Thus, the sync information must be removed before satisfactory bit-error rate (BER) performance can be achieved. Consequently, embedded interference cancellation (EIC) is utilized which requires accurate estimation of the synchronization offsets and channel perturbations. Using coherent quadrature phase-shift keying-OFDM modulation, performance comparisons are presented using the COST207 multipath fading channel model. Fading channel BER performance results indicate that the embedded technique incurs only a slight signal-to-noise ratio penalty (less than 1 dB) compared with the preamble method. However, the embedded method offers the potential for improved hop rate and bandwidth efficiency because no dedicated slot is required for a synchronization field.

Timing Synchronization in Distributed Mobile MISO Rayleigh Fading Channels

This paper proposes a novel acquisition technique to improve single-antenna receiver detection performance of signals from multiple transmitters that are distributed spatially. We assume that the distributed transmissions occur simultaneously, but the start time of each of the respective transmissions are not synchronous due to imperfect coordination and non-synchronized clocks across distributed transmitters. First, we propose a robust combining method in mobile channels when there is no feedback or channel knowledge between the receiver and the distributed transmitters. Next, we propose a power control technique, given feedback (and partial channel knowledge) between the receiver and distributed transmitters, to improve the receiver operating characteristic (ROC) performance when re-acquiring multiple distributed transmitters. The ROC performance improvements are demonstrated for multiple distributed transmitter signal detection in a mobile Rayleigh fading channel. Analytical solutions are derived to predict the ROC performance when no channel knowledge is assumed, which is corroborated via simulation. For the power control method, simulation is used to demonstrate performance improvement (over the no feedback case) when the channel is estimated with error and feedback latency. The probability of missed detection is improved by at least a factor of 10 and 100, respectively.

Synchronization for RF carrier frequency hopped OFDM: analysis and simulation

In this work, we apply RF carrier frequency hopping to orthogonal frequency division multiplexing (FH-OFDM) and study the hop rate capability using realistic performance of signal acquisition/synchronization of the unknown time and frequency offset parameters. OFDM synchronization is a challenging task without RF carrier hopping due to OFDMs sensitivity to time and frequency offsets. Given sufficient synchronization estimation performance is achieved, our objective is to determine the hop rate capabilities for OFDM using two different data-aided synchronization approaches. The first method is based on sending information in a preamble before the OFDM payload symbol. The second method embeds known synchronization information directly into the OFDM symbol stream. Using differential QPSK-OFDM modulation and spectral efficiency greater than 1 bps/Hz, results indicate that sufficient synchronization performance can be achieved for hop rates greater than 6 and 1 khops/sec for the preamble- and embedded-based synchronizers, respectively.

Synchronization for broadband OFDM mobile ad hoc networking: Simulation and implementation

Broadband wireless services are typically considered usable in limited mobility conditions (fixed access or at pedestrian speeds). In this paper, we present a method for time and frequency synchronization of broadband OFDM for mobile ad hoc networking systems, where each node randomly and rapidly changes location. The proposed method is robust in Rayleigh fading and is computationally tractable for low-power digital signal processor (DSP) implementations. The synchronization method uses a pseudo-noise (PN) preamble sequence, provides low probability of false detection, low probability of missed detection, and exhibits a low peak-to-average power ratio (PAPR). We show that the power spectral density (PSD) of the PN-based preamble, after the digital up-converter shaping filter, fits into a spectral mask similar to that of the OFDM traffic signals PSD. Efficient implementation is demonstrated using frequency-domain equivalent processing on a fixed-point Texas Instruments DSP and a floating-point Motorola G4 PowerPC with Altivec.

OFDM Pilot Design for Channel Estimation with Null Edge Subcarriers

Wireless communication systems are increasingly adopting orthogonal frequency division multiplexing (OFDM) to enable efficient high-data rate transmissions. Such systems often employ pilot symbols to estimate wireless channels, and null subcarriers on band edges to reduce adjacent channel interference. Recent work has focused on designing the pilot sequence to improve channel estimation performance. In this paper, we present a new pilot design for OFDM systems based on a arbitrary-order polynomial parameterization of the pilot subcarrier indices. We show our design achieves better performance than existing methods. Theoretical models and simulated performance demonstrate the increased accuracy of our design.

MISO joint synchronization-pilot design for OFDM systems

In this paper we apply a new joint synchronization-pilot sequence (JSPS) optimization design technique to multiple transmitter OFDM systems. One objective in the design of the JSPS for MISO transmissions is to eliminate the requirement for the different preamble fields used for coarse and fine synchronization and channel training. In this work, independent JSPSs are designed for each transmitter in a multiple antenna system, but are transmitted simultaneously from each antenna. Consequently, JSPSs can potentially reduce the overhead of multiple antenna preamble transmissions (improving bandwidth efficiency). The objective of this paper is to determine the performance advantages of joint carrier frequency offset (CFO) and channel estimation using only the pilot portion of the JSPS. By jointly optimizing the position and power of each pilot across the transmitter antennas in Rayleigh fading channels with CFO, we show > 15 dB SNR improvement in the performance of the channel estimator.

Superimposed Training for Channel Shortening Equalization in OFDM

In orthogonal frequency division multiplexing (OFDM) systems, the cyclic prefix (CP) is required to be greater than the length of the channel impulse response to avoid inter-block interference. However, a long CP decreases power and bandwidth efficiencies. Recently channel shortening equalizers (CSEs) have been introduced to enable the use of a shorter CP, but they either require perfect channel knowledge, or demand high complexity and long decoding delay when the channel is unknown. In this paper, we propose a low complexity CSE for OFDM with unknown channels by using superimposed training. Our unique design achieves bandwidth efficient channel estimation, and low complexity channel shortening equalization as well as symbol decoding. Simulation results demonstrate the effectiveness of the proposed scheme

Adaptive Channel Shortening Equalization For Coherent OFDM Doubly Selective Channels

In this work we propose a new adaptive channel shortening technique for doubly selective (time-varying frequency-selective) orthogonal frequency division multiplexing (OFDM) channels. OFDM is considered to be a very bandwidth efficient wireless transmission technique (due to its tightly packed orthogonal subcarrier structure) for multi-path channels. Transmissions in multi-path channels exhibiting long delay spread channels, however, require an excessively long cyclic prefix (CP) to prevent intersymbol interference (ISI) with OFDM, and thus undesirably reduce the bandwidth and power efficiency of the information transmission. Channel shortening equalization (CSE) is used to shorten the channel delay spread to a certain length (less than the CP length) so that ISI is minimized. Our technique adapts to channel variation and provides a significant bit error rate (BER) improvement in performance over non-adaptive techniques. We show that the adaptive technique improves the demodulated BER by greater than a factor of 10, with diversity order 1, in a mobile long delay spread (frequency-selective) channel, while the non-adaptive method exhibits a BER floor at 10-1