Multi-hop relaying is an economically efficient architecture for coverage extension and throughput enhancement in future wireless networks. OFDM, on the other hand, is a spectrally efficient physical layer modulation technique for broadband transmission. As a natural consequence of combining OFDM with multi-hop relaying, the allocation of per-hop subcarrier power and per-hop transmission time is crucial in optimizing the network performance. This paper is concerned with the end-to-end information outage in an OFDM based linear relay network. Our goal is to find an optimal power and time adaptation policy to minimize the outage probability under a long-term total power constraint. We solve the problem in two steps. First, for any given channel realization, we derive the minimum short-term power required to meet a target transmission rate. We show that it can be obtained through two nested bisection loops. To reduce computational complexity and signalling overhead, we also propose a sub-optimal algorithm. In the second step, we determine a power threshold to control the transmission on-off so that the long-term total power constraint is satisfied. Numerical examples are provided to illustrate the performance of the proposed power and time adaptation schemes with respect to other resource adaptation schemes.

This paper is concerned with the end-to-end transmission outage in an OFDM based wireless multi-hop network. The network consists of a source node, a destination node, and a set of serial relay nodes. Transmission outage occurs when the end-to-end transmission rate is less than a target rate. Our goal is to find an optimal power and time control policy to minimize the outage probability at a target end-to-end data rate under a long-term total power constraint. The problem is solved in two steps. First, for any given channel realization, we derive the minimum total power required to meet the target rate by varying the transmission power and time allocated to each hop. This is formulated as a convex optimization problem and is solved efficiently. We then compare the required minimum total power for each channel realization with a power threshold. If the threshold is exceeded, the transmission is turned off. The optimal threshold is chosen to meet the long-term total power constraint. Two suboptimal power and time control policies are also presented and compared with the optimal policy by simulation.

Ultra-wideband (UWB) communication systems are expected to operate in a highly frequency-selective multipath fading environment. To exploit multipath diversity gains in a multiuser scenario, we developed a differentially-encoded, di-symbol time-division multiuser impulse radio (d2TD-IR) system with delay-sum autocorrelation receivers. In traditional time-division multiple access systems, each user transmits a single pulse during a symbol duration in a pre-assigned chip which is longer than maximum excess delay of the channel. However, due to the exponential decay property of UWB channel, we proposed the use of much shorter chip duration, which significantly increases the transmission rate. Because dense pulse transmission will induce multiuser interference, two time-hopping access sequences, which alternately encode the odd- and even-index symbols, are employed with delay-sum autocorrelation receivers to maximally suppress the interference. It was shown that when the chip duration is properly chosen, the proposed system outperforms the conventional time-hopping impulse radio system at high signal-to-noise ratio. This paper also proposed a method to estimate the optimal chip duration when only the average power decay profile of the UWB channel is known.

For transmitted-reference impulse radio, it has been experimentally shown that multiple transmit antennas can provide an energy boost in the received signal. Here, we propose a 2-transmit and Q-receive differential space-time block coded impulse radio with autocorrelation receiver. A mathematical model for predicting the system bit-error-rate (BER) performance is derived for intersymbol interference-free transmission over an ultra-wideband channel, and validated with computer simulated results. From the results, it is observed that in migrating from 1-transmit to 2-transmit antennas, a signal-to-noise ratio gain of 3 dB, 2 dB and 1 dB at BER = 10-5can be achieved, respectively, for the case of 1-, 2- and 4-receive antennas.

For most wireless services with variable rate transmission, both average rate and rate oscillation are important performance metrics. The traditional performance criterion, utility of average transmission rate, boosts the average rate but also results in high rate oscillations. We introduce a utility function of instantaneous transmission rates. It is capable of facilitating the resource allocation with flexible combinations of average rate and rate oscillation. Based on the new utility, we consider the time and power allocation in a time-shared wireless network. Two adaptation policies are developed, namely, time sharing (TS) and joint time sharing and power control (JTPC). An extension to quantized time sharing with limited channel feedback (QTSL) for practical systems is also discussed. Simulation results show that by controlling the concavity of the utility function, a tradeoff between the average rate and rate oscillation can be easily made.

Average transmission rate and rate oscillation are two important performance metrics for most wireless services. Both are often needed to be optimized in multi-user scheduling and resource management. In this paper we introduce a utility function that increases with average rate but decreases with rate variance. It is capable of facilitating resource allocation with flexible combinations of the two performance metrics. A generalized gradient scheduling algorithm (GGSA) is then developed to maximize the proposed utility. It is shown that the best scheduler should maximize the sum of concave functions of instantaneous transmission rates in order to maximize the utility of average rate and rate oscillation. The scheduler reduces to the traditional gradient scheduling algorithm when the rate variance term in the new utility function is omitted. We analyze the dynamics of average transmission rates and rate variances using ordinary differential equation and show that GGSA is asymptotically optimal under the condition that the transmission rate vector, after an appropriate scaling, converges to a fixed vector as time goes into infinity.

The problem of non-cooperative power control is studied for wireless ad hoc networks supporting data services. We develop a maximum throughput based non-cooperative power control game (MT-NPG), where each transmitter node requires only theeffectiveinterferencemeasurement from its corresponding receiver. An expression for the optimal power control to maximize the average transmission rate for each individual link is derived. Then, we design a distributed iterative power updating algorithm to approximate the optimal solution. The existence and uniqueness of Nash equilibrium for the proposed game is investigated. Simulation results show that the proposed MT-NPG yields a significant improvement in the sum of average achievable rates when compared with existing approaches.

We study transmission schemes for lifetime maximization in wireless sensor networks. Specifically we consider the network where all the sensors observe uncorrelated signals and forward the quantized observations to a common fusion center. We demonstrate how the minimum node lifetime is maximized under individual estimation accuracy constraints through adapting transmission times and powers assigned to different sensor nodes. A partially distributed algorithm is proposed to implement joint time sharing and power control, for which each sensor node only needs local information and little common information. In addition, we obtain a necessary and sufficient condition for convergence of the algorithm. Simulation results show a significant lifetime gain over existing schemes, especially when the sensing environment becomes more heterogeneous and the number of nodes increases.

For most of wireless services with variable rate transmission, both average rate and rate oscillation are important performance metrics. One often needs to decide how much rate oscillation the service can tolerate to obtain a higher average rate. Service satisfaction for each user is quantified by an increasing and concave utility function of instantaneous transmission rate. It is capable of facilitating the resource allocation with flexible combinations of average rate and rate oscillation. Particularly, we are interested in maximizing the time-average aggregate utility by scheduling user transmissions in a time-shared wireless network. A resource allocation policy is developed, namely, time sharing (TS), to exploit the concavity of utility function and the fluctuation of channel gain. This is formulated as a constrained convex optimization problem. Our analysis shows that in the TS policy the optimal scheduler allows multiple users with relatively better channel conditions to share a same time frame in an adaptive time-division manner. In addition, the more concave the utility function is, the higher the probability of time frame sharing is. An extension to quantized time sharing with limited channel feedback (QTSL) for practical systems is all studied. Simulation results show that, two to three bits of channel state information (CSI) are sufficient for the performance of QTSL scheme to approach that of the optimal TS policy when the number of time slots in a time frame is not less than the number of users, especially, in high SNR region.

In ISAR imaging, most works have assumed the target to be a rigid body, a body without any rotating, vibrating or moving parts. The rotation of structures in a target, such as the rotor of a helicopter or the turret of a tank target, may induce frequency modulation on the returned signals and generate sidebands about the center frequency of the target’s body doppler frequency, known as the micro-doppler phenomenon. In this paper we present a new method based on range grouping of the target’s range profile for the separation of the rotating or moving parts from the target’s body. The method is carried out on the Range Profile Image, different from other methods that do the separation at the signal level.

Differential impulse radio employing autocorrelation receivers (AcR) is an efficient, low-complexity technology for short range wireless ultra wide-band (UWB) communications. However, existing systems are based on single transmit and single receive antennas. To enhance system capacity, this paper extends the technique to multiple-input multiple-output (MIMO) systems. The resulting system is capable of exploiting not only multipath diversity, but also the spatial diversity. The proposed receiver inherits the merits of the AcR, which include neither local oscillators nor AD/DA converters. From both theoretical performance analysis and computer simulations, it was observed that the system employing 2 transmit and 2 receive antennas has about 5 ~ 7dB performance gain over the single-input single- output (SISO) system.

Explicit closed-form expressions of the bit error probabilities are obtained for space-time block codes based on generalized orthogonal designs with differential encoding and differential detection utilizing 2/sup b/-ary phase shift keying mapping. The frequency non-selective, block-wise constant Rayleigh fading channel is considered here. The results are applicable to any arbitrary Doppler spectrum and any number of transmit and receive antennas, where the number of transmit antennas is dictated by the available coding schemes.

Ultra wide-band (UWB) communication systems are expected to operate in a highly frequency-selective multipath fading environment. Multipath diversity gains can be collected to provide enhanced performance. To this end, we developed a differential time division multiple access (TDMA) impulse radio scheme that is tailored for multi-user UWB communications. In the system, the differential detector is adopted. The detector, when properly combined with the TDMA system, can fully exploit the multipath diversity of UWB channels. A novel delay-sum scheme is employed at the receiver to mitigate multi-user interference (MUI) and additive white Gaussian noise (AWGN). The proposed system shows better bit-error-rate (BER) performance than the conventional time-hopping impulse radio (TH-IR) system at high signal-to-noise ratio (SNR). Application of the system is promising because the receiver only needs to entail simple delay, integral and compare operations at the symbol rate.

Explicit closed-form expressions of the bit-error probabilities are obtained for space-time block codes based on generalized orthogonal designs with differential encoding and differential detection using 2/sup b/-ary phase-shift keying mapping. The frequency-nonselective, block-wise constant Rayleigh fading channel is considered here. The results are applicable to any number of transmit and receive antennas, where the number of transmit antennas is dictated by the available coding schemes.

Orthogonal frequency division multiplexing (OFDM) has established itself as a very appealing technique for high-rate data transmission over dispersive channels. The main disadvantage of OFDM is that the signal exhibits Gaussian-like time-domain waveform with high peak-to-average power ratio, which limits the efficiency of the transmitter amplifier and degrades the received signal to noise ratio. The companding technique can be employed to mitigate quantization noise and the peak-to-average-power ratio of OFDM system. The symbol error rate of the companded OFDM systems is investigated. The performances of the systems with and without companding are compared.

A new technique for inversed synthetic aperture radar (ISAR) ranging, which resembles the principle of the Vernier measuring system, is presented. In this technique, the transmitted ISAR pulse comprises a train of chirp subpulses with uniformly stepped up center frequencies. The return ISAR echo is first processed, using hardware, to determine a coarse estimate of the target range. Further refinements of the range estimate are achieved through software processing, consisting of two stages of discrete Fourier transform operation. The ranging accuracy can be increased without the need for increased bandwidth, but at the expense of a slight increase in computational complexity. Numerical evaluation shows that a noiseless system is capable of achieving high-ranging accuracy, of the order of millimeters, even in the presence of dispersion and target motion. From computer simulations, the proposed system is also found to be robust against additive system noise and frequency jitter under practical conditions.

This paper presents a fundamental approach for deriving the bit error probability of BDPSK and QDPSK over the nonselective Rayleigh fading channel for a receiver with an arbitrary IF filter, and for a fading process with an arbitrary Doppler spectrum with arbitrary Doppler bandwidth. The results generalize those published earlier which were restricted to matched filter reception and to a fading process with a small Doppler bandwidth compared to the symbol rate. This allows the error probability to be studied in the presence of varying degrees of ISI due to the bandlimitation of the received signal by the IF filter, and in the presence of fading fluctuations of various rates. The analytical approach presented is simple, and yet powerful in that it can handle the case of diversity reception. This is a great advantage over the alternative approach of using the distribution of the differential phase of the received signal over a symbol interval. The bit error probability results apply to both conventional BDPSK and QDPSK, as well as /spl pi//2-2DPSK and /spl pi//4-4DPSK, and allow the irreducible bit error probability as well as the SNR at which this irreducible value sets in to be studied as a function of the Doppler bandwidth and IF filter bandwidth. The computed results are applicable to the design of digital cellular mobile communication systems.

We present a new closed-form bit error probability formula for narrow-band FSK with limiter-discriminator-integrator detection in Rayleigh fading channels. This formula eliminates the tedious numerical integrations required in previous publications. The computed results are verified by experiment. We provide physical explanations, on how the system parameters such as fading, IF filter bandwidths and FM carrier frequency deviations affect the error performance, not previously available in the literature.>

Of the more band-efficient constant envelope digital modulations, duobinary FM is attractive in terms of its potential error performance, as indicated by the minimum distance of the signal structure. In this paper, the error performance of narrow-band duobinary FM, with discriminator detection in an additive Gaussian noise background, is evaluated for a range of system bandwidthsBTand frequency deviation ratiosh. The effects of intersymbol interference, due to the restricted bandwidth, are taken into account in the analysis. In addition, at the narrower bandwidths,0.5 \leq BT < 1.0, the filtered background noise is correlated, and these effects are included. It is shown that with the usual duobinary detector, "clicks," which are a major contributor to errors in conventional digital FM, do not contribute significantly to errors for the low frequency deviation ratios which are commonly used. The error rate theory agrees well with results obtained experimentally. The deviation ratios that give the best error performance for a given system bandwidth are presented. In a narrower bandwidth such asBT = 1/2, where duobinary FM is proposed, duobinary FM with a frequency deviation ratio ofh = 0.7performs better than binary FM.