Polar OFDM (P-OFDM) is an OFDM format that can double the spectral efficiency of existing real-valued and unipolar optical OFDM intensity-modulation with direct detection (IM/DD) optical communication systems. P-OFDM modulates the amplitude and phase of a complex OFDM signal to allow a real unipolar signal. In this paper, we investigate the effect of power allocation to the phase and the amplitude components of P-OFDM time-domain signal on (1) the effective signal-to-noise ratio (SNR) at the receiver (2) the peak-to-average power ratio (PAPR) under transmit power constraint and (3) the induced clipping noise under optical-source's dynamic-range and transmit power constraints. In addition to a 4 dB reduction in PAPR from ACO-OFDM, we demonstrate the improvement of throughput for P-OFDM over other optical OFDM methods when the signal power is limited.

Underwater Acoustic Sensor Networks find use in critical time-sensitive applications such as disaster prevention and coastline protection. The sensor nodes used in such networks are powered by batteries that are difficult to recharge or replace. Hence it is imperative that routing algorithms used in such networks be very energy efficient while also satisfying necessary delay constraints for time-sensitive applications. Unlike the radio frequency medium, underwater acoustic channels have low bandwidth, large propagation delays and long multipath delay spreads. While energy-efficient routing is an actively researched area for terrestrial radio frequency networks, results from those studies generally do not apply to underwater acoustic networks due to vast differences in channel characteristics. In this paper we explore delay-constrained energy optimization for routing in underwater acoustic sensor networks. Specifically, we propose an offline Mixed Integer Linear Programming based routing algorithm that enables computation of delay constrained energy efficient routes.

We evaluate the performance of three direct-detection orthogonal frequency-division multiplexing (OFDM) schemes in combating multipath distortion in indoor optical wireless links, comparing them to unipolar M-ary pulse-amplitude modulation (M-PAM) with minimum mean-square error decision-feedback equalization (MMSE-DFE). The three OFDM techniques are DC-clipped OFDM and asymmetrically clipped optical OFDM (ACO-OFDM) and PAM-modulated discrete multitone (PAM-DMT). We describe an iterative procedure to achieve optimal power allocation for DC-OFDM. For each modulation method, we quantify the received electrical SNR required at a given bit rate on a given channel, considering an ensemble of 170 indoor wireless channels. When using the same symbol rate for all modulation methods, M-PAM with MMSE-DFE has better performance than any OFDM format over a range of spectral efficiencies, with the advantage of (M-PAM) increasing at high spectral efficiency. ACO-OFDM and PAM-DMT have practically identical performance at any spectral efficiency. They are the best OFDM formats at low spectral efficiency, whereas DC-OFDM is best at high spectral efficiency. When ACO-OFDM or PAM-DMT are allowed to use twice the symbol rate of M-PAM, these OFDM formats have better performance than M-PAM. When channel state information is unavailable at the transmitter, however, M-PAM significantly outperforms all OFDM formats. When using the same symbol rate for all modulation methods, M-PAM requires approximately three times more computational complexity per processor than all OFDM formats and 63% faster analog-to-digital converters, assuming oversampling ratios of 1.23 and 2 for ACO-OFDM and M-PAM, respectively. When OFDM uses twice the symbol rate of M-PAM, OFDM requires 23% faster analog-to-digital converters than M-PAM but OFDM requires approximately 40% less computational complexity than M-PAM per processor.

Recently, we proposed a novel transmission scheme termed decomposed-quadrature optical single carrier frequency domain equalization (DQO-SCFDE) suitable for intensity-modulated direct-detection (IM/DD) diffuse optical wireless (DOW) communication. DQO-SCFDE has been shown to perform better than the conventional asymmetrically clipped optical OFDM (ACO-OFDM) when the non-linearity characteristics of the light emitting diodes (LED) are considered. In this paper we present a detail comparison of DQO-SCFDE and ACO-OFDM. We then propose a space-time coding scheme for DQO-SCFDE using two LED's at the transmitter to improve the bit-error rate performance.

This paper introduces a new constellation mapping scheme for asymmetrically-clipped optical OFDM (ACO-OFDM) that puts more bits on the low-frequency subcarriers. The lowpass nature of the wireless optical channel motivates the use of this generic bit-loading scheme. This scheme has no knowledge of the channel at the receiver; it performs significantly better than the traditional constant constellation mapping on all subcarriers in a variety of channels. The average bit-error rate (BER) curves for the new modulation scheme have up to a 5 dB improvement for signal-to-noise ratios (SNRs) where the BER is in the order of 10-3. In addition to the generic low-frequency bit-loading, this paper also proposes clipping the ACO-OFDM signal at the receiver to mitigate the additive electronic noise.

We present a new modulation scheme for asymmetrically-clipped optical OFDM (ACO-OFDM). We show that a generic bit-loading scheme that puts more bits on the low-frequency subcarriers is more efficient for wireless optical channels than the constant modulation scheme typically used in radio frequency (RF) OFDM schemes. We discuss the low- pass nature of the wireless optical channel that motivates the use of this generic bit-loading scheme. This scheme performs significantly better than the traditional constant modulation on all tones in a variety of channels. The average Bit-error rate (BER) curves for the new modulation scheme have up to a 5 dB improvement for Signal-to-Noise Ratios where the BER is in the order of 10-3.

Orthogonal frequency-division multiple access (OFDMA) is an attractive technique for exploiting multiuser diversity in the downlink of a cellular system. This paper addresses three problems in multiuser diversity for OFDMA systems. First, we propose a way to significantly reduce the amount of channel state information (CSI) feedback without sacrificing performance too much, by selective and adaptive feedback. Second, we propose a way to increase the cell throughput and fairness by applying an opportunistic beamforming scheme to orthogonal frequency-division multiplexing. This beamforming scheme increases the frequency fading rate, which increases the multiuser diversity effect. Thirdly, we deal with the issue of fairness and quality-of-service (QoS) in opportunistic systems by proposing a modified proportional fair (PF) scheduler for OFDMA. Key features in the scheduler are that it incorporates QoS classes into the PF scheduler and that it has a tunable fairness level. Extensive simulation results are presented to evaluate the performance of the proposed schemes. The opportunistic beamforming scheme performed well in comparison with several other schemes. The modified PF scheduler was able to give users different QoS, based on their requirements, while still exploiting multiuser diversity

In the above titled paper (ibid., vol. 55, no. 5, pp. 941-952, May 07), a change to equation (7), that was requested by the author, was omitted in the final version. The paragraph leading up to, and including the equation, is repeated here.

In opportunistic SDMA, the base station forms several random beams simultaneously and relies on fast but limited feedback for scheduling users on the beams. In this paper, opportunistic SDMA is applied to an OFDMA downlink by having independent beamformers run in parallel on different frequency bands (SD-OFDMA). A method for improving the performance of completely random beamforming is proposed and evaluated. It exploits the temporal channel correlation and the partial channel state information available at the transmitter to keep "successful" beams over longer periods of time than the default block duration. Extensive simulation results show a significant improvement in cell throughput compared to a completely random scheme. The results also show that in order to gain more from the multiple simultaneous beams than what is lost in additional interference, the system load must be high.

The allocation of available resources/subcarriers is an important design aspect for next generation wireless OFDM multiuser systems. The approaches considered usually assign a subset of subcarriers to each user and do not consider the loss of coding and diversity gains when a user is allotted very few subcarriers. We investigate an approach which exploits the multiplexing diversity available in the system in the absence of channel state information at the transmitter. This is achieved by spreading the data of all users over the complete set of subcarriers by using an appropriate combination of coding, multiplexing and interleaving. The performance of such a system is analyzed and the potential for substantial coding, interleaving and diversity gains is demonstrated. This paper also outlines a new approach to evaluate performance bounds for concatenated codes with higher order modulation on a fading multipath channel, by simplifying the description of pairwise error events using an ideal modulator assumption. Simulations are presented for a suboptimal decoding scheme, which does not rely on information from other users

Provides a listing of current committee members and society officers.

We present an algorithm that autonomously synchronizes all DMT-Zipper based VDSL modems in an unbundled access network, solving the problem with nonorthogonal NEXT that appears in systems with unsynchronized modems. The algorithm we present runs autonomously in each VDSL modem in the central office or in the street cabinet. We determine the other modems' relative frame offsets by exploiting their NEXT signal using the inherent cyclic redundancy found in DMT signals. By estimating the relative frame offsets of the other users, we can adjust a given user's own frame-timing relative to the mean of the others. With our method all modems in the network will be synchronized to within a small fraction of the total DMT frame-length, suppressing the nonorthogonal NEXT to a level far below the background noise-floor. This means that we can achieve the same performance in an unbundled access network without any master clock reference as in a system where all modems are perfectly synchronized using a master clock.

This paper presents a symbol time offset estimator for coherent orthogonal frequency division multiplexing (OFDM) systems. The estimator exploits both the redundancy in the cyclic prefix and available pilot symbols used for channel estimation. The estimator is robust against frequency offsets and is suitable for use in dispersive channels. We base the estimator on the maximum-likelihood estimator for the additive white Gaussian noise channel. Simulations for an example system indicate a system performance as close as 0.6 dB to a perfectly synchronized system.

We present an algorithm for self-synchronizing all modems in a discrete multitone (DMT)-based very high-speed digital subscriber line (VDSL) system using the Zipper duplex method. This solves the problem with non-orthogonal near-end crosstalk (NEXT) that appears in systems with unsynchronized modems. The algorithm we present runs autonomously in each VDSL-modem. It uses the auto-correlation of the DMT-signal to determine the frame-offset of other users, and adjusts the own frame-timing to be better aligned with the other users. With our method all modems will be self-synchronized to within a small fraction of the total DMT frame-length. This self-synchronization suppresses the NEXT to a level far below the background noise-floor. This means that our self-synchronized system has the same performance as a system where all modems are perfectly synchronized to a master clock.

OFDM (orthogonal frequency division multiplexing) is an effective technique for achieving reliable high-bit-rate transmission over wireless channels. Generally, for wireless applications, a guard interval, most often in the form of a cyclic extension of the OFDM data symbol, is inserted between OFDM blocks to minimize interblock interference. We quantify the penalties incurred when a zero-padded guard interval is used instead of a cyclic extension. Specifically, the resulting interchannel interference is computed and the degradation in the bit error rate is quantified.

We address the issue of high-level synthesis of low-power digital signal processing (DSP) systems by proposing switching activity models. In particular, we present a technology independent hierarchical scheme to compare relative power performance of two competing DSP systems. The basic building blocks considered for such system are a full-adder and a one-bit delay. Estimates of switching activity at the output of these building blocks is used to model the activity in different architectural primitives used for building DSP systems. This method is very fast and simple and simulations show accuracy within 4% of extensive bit-level simulations. Therefore, it can easily be integrated into current communications/DSP CAD tools for low-power applications. The models show that the choice of multiplier/multiplicand is important when using array multipliers in a data-path. If the input signal with smaller variance is chosen as the as the multiplicand, up to 20% savings in switching activity can be achieved. This observation is verified by analog simulation.

This paper presents a symbol time offset estimator for coherent OFDM systems. This estimator exploits both the redundancy in the cyclic prefix and the pilot symbols used for channel estimation. The estimator is robust against frequency offsets and is suitable for use in dispersive channels. We base the estimator on the ML estimator for the AWGN channel. Simulations of an example system indicate a system performance as close as 0.3 dB to a perfectly synchronized system. Compared to an estimator not using pilots, our estimator's performance could allow a shorter cyclic prefix and thus a more spectrally efficient system.

We present a multiuser synchronization scheme for tracking the mobile's uplink time and frequency offsets. It uses the redundancy introduced by the cyclic prefix and does not need additional pilots. We show performance results of an orthogonal frequency division multiplexing (OFDM)-based radio interface based on the universal mobile telecommunication system (UMTS) parameters. For a UMTS-typical mobile channel environment, the performance of a coherent system employing the scheme is virtually indistinguishable from the performance of a perfectly synchronized system. In a differentially modulated system, synchronization errors decrease the system performance by about 0.7 dB compared to a perfectly synchronized system.

We present a new duplex scheme, called Zipper, for discrete multitone (DMT)-based very high bit-rate digital subscriber line (VDSL) systems on copper wires. This scheme divides the available bandwidth by assigning different subcarriers for the upstream and downstream directions. It has high flexibility to divide the capacity between the up and downstream, as well as good coexistence possibilities with other systems such as ADSL. Simulation results show the high bit-rate performance in different environments such as mixed ADSL and VDSL traffic under radio frequency interference and with different background noise sources.

This paper derives probability density functions (PDFs) to describe the behavior of data demodulated with imperfect channel estimation. The PDFs apply to cases of flat Rayleigh-fading, multiple-diversity reception Rayleigh-fading, and flat Ricean-fading. These PDFs can be used to determine analytically both the symbol error rate and bit error rate for multi-amplitude constellations with in-phase and quadrature components, thus avoiding the need for extensive computer simulation.

We present and analyze low-rank channel estimators for orthogonal frequency-division multiplexing (OFDM) systems using the frequency correlation of the channel. Low-rank approximations based on the discrete Fourier transform (DFT) have been proposed, but these suffer from poor performance when the channel is not sample spaced. We apply the theory of optimal rank-reduction to linear minimum mean-squared error (LMMSE) estimators and show that these estimators, when using a fixed design, are robust to changes in channel correlation and signal-to-noise ratio (SNR). The performance is presented in terms of uncoded symbol-error rate (SER) for a system using 16-quadrature amplitude modulation (QAM).

We introduce a method for tracking a Rayleigh-fading channel with Orthogonal Frequency Division multiplexing (OFDM) so that multi-amplitude bit rate schemes such as 16 QAM may be used in a wireless channel. In addition, we derive a distribution to predict the symbol error rate of the modulation scheme. The modulation scheme is applied to an indoor wireless system operating at a rate of 25 Mb/s. The probability of error derived from simulation shows good agreement with the theoretically predicted probability of error. We keep a fairly large bit rate by using few training symbols; in simulation, the error propagation accounts for a slight increase in symbol error rate, but is not catastrophic due to the use of rotationally-invariant 16 QAM constellations.>