Timothy M. Schmidl

Robust frequency and timing synchronization for OFDM

A rapid synchronization method is presented for an orthogonal frequency-division multiplexing (OFDM) system using either a continuous transmission or a burst operation over a frequency-selective channel. The presence of a signal can be detected upon the receipt of just one training sequence of two symbols. The start of the frame and the beginning of the symbol can be found, and carrier frequency offsets of many subchannels spacings can be corrected. The algorithms operate near the Cramer-Rao lower bound for the variance of the frequency offset estimate, and the inherent averaging over many subcarriers allows acquisition at very low signal-to-noise ratios (SNRs).

High rate space-time block coded scheme: performance and improvement in correlated fading channels

In this paper, we study the performance of space-time block coding, also known as space-time transmit diversity (STTD) scheme for high data rate applications with 4 transmit antennas. The scheme consists of two STTD units at the transmitter and an interference-resistant detector at the receiver, termed the double-STTD (DSTTD) scheme. We compare the performance of DSTTD to general multi-input multi-output (GMIMO) signaling for a given data rate and under different channel correlation profiles. It is demonstrated that DSTTD outperforms GMIMO in all cases. In addition, additional performance gain can be attained for DSTTD in correlated channels by selectively ordering the transmit antennas to minimize the average interference terms.

Capacity analysis of frequency-selective MIMO channels with sub-optimal detectors

In this paper, a capacity analysis of multi-input multi-out (MIMO) channels with multipath assuming different types of sub-optimal detectors is presented. The results are instrumental for evaluating the potential of multipath MIMO channels when sub-optimal detectors are utilized. It is demonstrated that for some types of detectors, an increase in the number of paths may result in some performance loss, especially at low transmit power. In addition, it is shown that a type of iterative detector can attain near-capacity performance when highly reliable decision feedback is used.

A comparison of the open loop transmit diversity schemes for third generation wireless systems

Multiple transmit antennas giving rise to diversity (transmit diversity) have been shown to increase the downlink (base station to the mobile) capacity in cellular systems. The third generation partnership project (3Gpp) for WCDMA FDD has chosen space time transmit diversity (STTD) as the open loop transmit diversity technique for two transmit antennas. On the other hand, the CDMA2000 has chosen space time spreading (STS) and orthogonal transmit diversity (OTD) as the open loop transmit diversity techniques. In this paper, we compare the open loop transmit diversity techniques of 3Gpp and CDMA2000. We show that STS involves a QPSK to non-QPSK constellation transformation followed by an STTD encoding of the resulting symbols. Thus, the performance of STS is the same as STTD, while the peak to average ratio (PAR) and complexity of STS is higher than STTD. We also show theoretically that the performance of STTD is 0.3-0.5 dB better than OTD. Further, STTD has another distinct advantage over OTD and STS in its application to the broadcast channels. During the power on and possibly during soft handoff, the mobile does not know the presence/absence of the diversity antenna on the primary/soft handoff base station. In this case, a blind detection of the diversity antenna is possible for STTD with a very small degradation in performance. This is not possible for OTD and STS. We thus show that STTD is a better open loop transmit diversity technique than OTD and STS.

The use of iterative channel estimation (ICE) to improve link margin in wideband CDMA systems

Wideband CDMA (WCDMA) systems employ coherent detection by periodically inserting pilot symbols in the data stream. Because of the small number of the pilot symbols available, the channel estimates are not perfect, and there is a loss in link margin. Iterative channel estimation (ICE) is a novel method that uses both pilot and data symbols for channel estimation. Link margin gains of 0.3 to 1.3 dB are obtained for data rates of 32 to 64 KSPS, with larger gains at higher Doppler frequencies. The extra complexity to implement ICE is very modest at 2.56 and 5.12 MIPS for 32 and 64 KSPS channels, respectively, which is easily accommodated in todays DSP technology and represents a small fraction of the total power consumed by the mobile handset. Reducing the required E/sub b//N/sub o/ by even 0.4 dB will allow the system to support 10% more voice users. This paper gives the theoretical analysis of ICE, and Monte Carlo simulations are used to compare the performance of ICE against channel estimation methods which only use pilot symbols.

Interference cancellation using the Gibbs sampler

This paper introduces a new method of interference cancellation for the base station in a CDMA system that is based on the Gibbs sampler. This interference cancellation method greatly reduces the degradation caused by multiuser interference from other users in a cell. The performance of the Gibbs sampler interference canceller is compared to the performance of a three-stage partial parallel interference canceller and is shown to have a gain of 0.6 dB at a raw BER of 10% (vehicular environments) and a gain of 2.8 dB at a raw BER of 1% (pedestrian environments) in an AWGN channel. This will result in capacity gains of 15% and 90%, respectively. Using the Gibbs sampler interference canceller a performance within 0.5 dB of the single user bound can be achieved.

Software defined radio for smart utility networks

In this paper, we describe a software defined radio (SDR) platform that can be used to implement the IEEE 802.15.4g standard for low data rate wireless smart utility networks (SUN). The SUN standard supports multiple wireless band plans, defines multiple physical (PHY) layers: FSK, OFDM and O-QPSK, each supporting a large number of data rates. Because of the flexibility allowed in the standards, an SDR approach provides the greatest flexibility in implementing the standard. We propose to use the SDR platform based on Texas Instruments C28x 32-bit MCU platform and a sub-1GHz CC1260 low power integrated radio. This platform can may be used for both the smart e-meter as well as the data concentrator. We begin the paper by providing an overview of the different PHY layers and enumerating the common elements across all PHYs for both the transmitter and receiver. Next, we discuss a partitioning between software and hardware that provides the flexibility without sacrificing efficiency. Finally, we provide the details of a hardware accelerator incorporated into the C28x architecture (VCU II) that can simplify many of the complex tasks associated with these PHYs.