Joon Ho Cho

Energy efficient broadcast in wireless ad hoc networks with Hitch-hiking

In this paper, we propose a novel concept called Hitch-hiking in order to reduce the energy consumption of broadcast application for wireless networks. Hitch-hiking takes advantage of the physical layer design that facilitates the combining of partial signals to obtain the complete information. The concept of combining partial signals using maximal ratio combiner [15] has been used to improve the reliability of the communication link but has never been exploited to reduce energy consumption in broadcasting over wireless ad hoc networks. We study the advantage of Hitch-hiking for the scenario when the transmission power level of nodes is fixed as well as the scenario when the nodes can adjust their power level. For both scenarios, we show that Hitch-hiking is advantageous and have proposed algorithms to construct broadcast tree with Hitch-hiking taken into consideration. For fixed transmission power case, we propose and analyze a centralized heuristic algorithm called SPWMH (Single Power Wireless Multicast with Hitch-hiking) to construct a broadcast tree with minimum forwarding nodes. For the latter case, we propose a centralized heuristic algorithm called Wireless Multicast with Hitch-hiking (WMH) to construct an energy efficient tree using Hitch-hiking and also present a distributed version of the heuristic. We also evaluate the proposed heuristics through simulation. Simulation results show that Hitch-hiking can reduce the transmission cost of broadcast by as much as 50%. Further, we propose and evaluate a protocol called Power Saving with Broadcast Tree (PSBT) that reduces energy consumption of broadcast by eliminating redundancy in receive operation. Finally, we propose an algorithm that takes advantage of both Hitch-hiking and PSBT in conserving energy.

Capacity of MIMO wireless channel with full-duplex amplify-and-forward relay

In this paper, a full-duplex amplify-and-forward (AF) relay is considered that suffers from self-interference in a MIMO wireless channel. We derive the optimal transformation matrix that maximizes the mutual information under average power constraint at the relay output. The capacity of the channel without a direct path is derived and it is shown that the relay maximizes the throughput as if there is no interference from the relay itself. A comparison is made to the channel with a half-duplex relay having the same average transmission power. In particular, a necessary and sufficient condition is derived in terms only of the signal-to-noise ratios (SNRs) for a full-duplex relay to perform better than a half-duplex relay in SISO cases. A simple rule of thumb shows that a full-duplex relay outperforms a half-duplex relay if the sum of the SNR in dB at the relay input and that at the destination input is greater than 0.75. Numerical results show that this result also holds for MIMO channels approximately.

An optimal signal design for band-limited asynchronous DS-CDMA communications

An optimal signal design for band-limited, asynchronous, direct-sequence code-division multiple-access (DS-CDMA) communications with aperiodic random spreading sequences and a conventional matched filter receiver is considered in an additive white Gaussian noise (AWGN) channel. With bandwidth defined in the strict sense, two optimization problems are solved under finite bandwidth and zero interchip interference constraints. First, a chip waveform optimization is performed given the system bandwidth, the data symbol transmission rate, and the processing gain. A technique to characterize a band-limited chip waveform with a finite number of parameters is developed, and it is used to derive optimum chip waveforms which minimize the effect of multiple-access interference (MAI) for any energy and delay profile of users. Next, a joint optimization of the processing gain and the chip waveform is performed, given the system bandwidth and the data symbol transmission rate. A sufficient condition for a system to have lower average probability of bit error for any energy profile is found, and it is used to derive some design strategies. It is shown that the flat spectrum pulse with the processing gain leading to zero excess bandwidth results in the minimum average probability of bit error. Design examples and numerical results are also provided.

Joint transmitter and receiver optimization for PAM in additive cyclostationary noise

A joint optimization of transmitter and receiver is considered for strictly band-limited linear modulations in additive cyclostationary noise. Under the assumptions that the period of the cyclostationarity of the noise is the same as the inverse of the symbol transmission rate and that the noise has a positive-definite autocorrelation function, the optimum transmitter and receiver waveforms that jointly minimize the mean-squared error at the output of the linear receiver are derived with the data sequence modeled as a wide-sense stationary (WSS) colored random process and the channel modeled as a linear time-invariant system with a general frequency-selective impulse response. Numerical results show that this joint optimization technique leads to a significant performance improvement over the systems with receiver-only optimization and the systems with no transmitter and receiver optimizations.

{ Asymptotic Optimality of Binary Faster-than-Nyquist Signaling}

In this letter, the asymptotic information rate of faster-than-Nyquist (FTN) signaling is examined when the data sequence consists of independent and identically distributed (i.i.d.) binary symbols. It is shown that, as the FTN rate tends to infinity, the information rate converges to that of the FTN signaling with i.i.d. Gaussian symbols. This leads to the optimality of the i.i.d. binary FTN signaling in the sense that the channel capacity can be asymptotically achieved by employing a transmit pulse that results in the same power spectral density as the water-filling solution.

An Optimal Full-Duplex AF Relay for Joint Analog and Digital Domain Self-Interference Cancellation

In this paper, a full-duplex (FD) amplify-and-forward (AF) relay is designed to compensate for the duplexing loss of the half-duplex (HD) AF relay. In particular, when there is no direct link between a source and a destination, joint analog domain self-interference suppression and digital domain residual self-interference cancellation is considered with an FD-AF relay having single receive antenna but multiple transmit antennas. Unlike previous approaches, a nonconvex quadratically constrained quadratic programming problem is formulated to find the optimal solution. The end-to-end spectral efficiency or, equivalently, the end-to-end signal-to-interference-plus-noise ratio from the source to the destination is chosen as the objective function to be maximized subject to the average transmit power constraint at the relay. In addition, an average power constraint is imposed on the output of the relays receive antenna to avoid the nonlinear distortion in the low noise amplifier and the excessive quantization noise in the analog-to-digital converter. Through the systematic reduction and the partitioning of the constraint set, the optimal solution is derived in a closed algorithmic expression and shows how it allocates the transmission power not only in the direction of maximal performance improvement but also in the orthogonal direction in order to balance the system performance and the amount of self interference. It is shown that the optimal FD-AF relay significantly outperforms the optimal HD-AF relay even with the hardware limitations in the RF chain of the relays receiver being well taken into account.

An orthogonal cognitive radio for a satellite communication link

In this paper, we consider overlaying a secondary-user signal over a satellite communication channel occupied by a primary user. The objective is to find the jointly optimal transmitter and receiver pair of a secondary system to minimize the mean squared error (MSE) at the output of the secondary receiver, subject to zero interference induced at the output samples of the primary receiver. It is assumed that both the primary and the secondary transmitters employ linear modulation and the receivers employ linear filter front-ends. Unlike prior work, the secondary user is allowed to transmit data symbols at a fraction of the primary users symbol rate. This rate reduction technique enables the secondary user to achieve the additive white Gaussian noise (AWGN) bound or the performance close to the AWGN bound even when the excess bandwidth of the primary user is very small but positive. Numerical results show that the proposed scheme significantly improves the spectral efficiency by exploiting the cyclostationarity of the primary-user signal.

Continuous-time equivalents of Welch bound equality sequences

The theory developed on the sequence optimization for equal-power code-division multiple-access (CDMA) systems is generalized in this paper. Unlike previous works, where discrete-time or vector channel models are predominantly employed, the overloaded channels are modeled as strictly band-limited, continuous-time multiple-access channels corrupted by additive white Gaussian noise (AWGN). After being posed as a multiple-input/single-output (MISO) joint transmitter and receiver optimization problem with the total mean-squared error (MSE) as the objective functional to be minimized, the variational problem is solved by using a frequency-domain approach. It is shown that there exist continuous-time equivalents of Welch bound equality (CTE-WBE) sequences as the jointly optimum transmit waveforms and that the matched filters are the jointly optimum receivers. The user capacity of the band-limited channels is characterized by a necessary and sufficient condition for the admissibility of users into the system in terms of the channel load, the received signal-to-noise ratio, and the signal-to-interference-plus-noise ratio (SINR) requirement. It is also shown that CTE-WBE sequences achieve the lower bound on the continuous-time equivalent of total squared correlation (CTE-TSC).

Capacity of Second-Order Cyclostationary Complex Gaussian Noise Channels

In this paper, we derive the capacity of a continuous-time, single-input single-output (SISO), frequency-selective, band-limited, linear time-invariant (LTI) channel, whose output is corrupted by a second-order cyclostationary (SOCS) complex Gaussian noise. By using a pair of invertible, linear-conjugate linear time-varying operators called a properizing FREquency SHift (p-FRESH) vectorizer and a p-FRESH scalarizer, it is shown that, whether the complex noise is proper or improper, the SISO channel can always be converted to an equivalent multiple-input multiple-output (MIMO) LTI channel whose output is now corrupted by a proper-complex vector wide-sense stationary noise. A variational problem is then formulated in the frequency domain to find the optimal input distribution that maximizes the throughput of the equivalent MIMO channel. It turns out that the optimal input to the SISO channel, obtained through a procedure similar to the water filling, is an SOCS complex Gaussian random process with the same cycle period as the noise. It is shown that this procedure, named cyclic water filling, significantly outperforms ordinary water filling by effectively utilizing the spectral correlation of the cyclostationary noise.

Asymptotically optimal low-complexity SC-FDE in data-like co-channel interference

In this paper, the design of a low-complexity linear equalizer is considered for single-carrier (SC) block transmission in the presence of inter-symbol interference (ISI) and data-like co-channel interference (CCI). Unlike the linear minimum mean-squared error (LMMSE) frequency-domain equalizer (FDE) designed to suppress ISI only, the LMMSE FDE suffers from high computational complexity due to the CCI component in the signal correlation matrix. Motivated by the fact that the double Fourier transform of the autocorrelation function of a wide-sense cyclostationary process consists of impulse fences with equal spacing, a low-complexity FDE is proposed that approximates the frequency-domain correlation matrix of the CCI plus noise by a block matrix with diagonal blocks. It is shown that the proposed FDE is asymptotically optimal in the sense that the average mean-squared error (MSE) converges to that of the LMMSE FDE as the block length tends to infinity. It is also shown that the proposed FDE is more numerically stable than the LMMSE FDE when the receive filter is matched to the transmit filter and its output is over-sampled to better capture the cyclostationarity of the CCI. Discussions and numerical results include SC block transmission systems with unique word instead of cyclic prefix, and systems with multiple receive antennas.