Sumanth Jagannathan

The effect of time synchronization errors on the performance of cooperative MISO systems

We consider a wireless sensor network scenario where closely packed nodes can he grouped into clusters. Although each node might have only a single antenna, nodes within a cluster can cooperate during transmission or reception, thereby forming a cooperative multiple input multiple output (MIMO) system. Much of the work in the area of cooperative MIMO has assumed perfect synchronization between the nodes in the network and the associated benefits have been investigated. We examine this assumption in a system where a cluster of nodes cooperatively transmits to a single receive node over a flat fading channel. We evaluate the effect of clock jitter between nodes on the performance of this cooperative multiple input single output (MISO) system. The clock jitter at the transmit nodes results in the lack of a reference clock at the receive node and causes inter-symbol interference (ISI). This leads to a performance degradation in the system. However, simulation results indicate that jitters as large as 10% of the bit time do not have much effect on the BER performance of the system. We also show that when the channel is not undergoing fading, the power penalty due to the clock jitter is independent of the number of transmit nodes and the penalty increases as the SNR increases.

Greener Copper with Dynamic Spectrum Management

Power-saving benefits of Dynamic Spectrum Management (DSM) result from its increased robustness to channel and noise changes, as in this paper. A proposed intermittent noise model enables evaluation of the performance of DSM algorithms. Polite Level-1 DSM algorithm consumes much less power than non-polite DSL management algorithms in the presence of intermittent noise. Secondly, a proposed robust level- 2 DSM algorithm compares favorably with methods that change the PSDMASK only upon modem initialization or retraining. The proposed robust level-2 DSM algorithm significantly reduces power consumption. This DSM reduction of DSL transmit power can be even more significant when considering the additional loss of robustness and stability of non-DSM methods.

Common-mode data transmission using the binder sheath in digital subscriber lines

This paper presents the use of the common-mode signal in digital-subscriber-line (DSL) transmission. The DSL binders metallic sheath is used as a common reference to obtain reliable channels for the common-mode signals. The direct and crosstalk channels for the common-mode signal are obtained using multi-conductor transmission line theory, and the data-rate improvement by exploiting these extra dimensions is presented. The lines in the binder are coordinated to cancel differential-mode crosstalk and also to avoid interference caused by radiation from the common-mode within the binder. The high level of common-mode noise is countered by exploiting the correlation between the differential- and common-mode noises. Numerical results based on the derived models show data-rate improvements of up to 100% compared to using only differential-mode transmission.

Distributed Adaptive Bit-Loading for Spectrum Optimization in Multi-User Multicarrier Systems

This paper presents a discrete bit-loading algorithm for multi-user multicarrier systems with application to spectrum balancing in digital-subscriber-lines. The algorithm can be implemented in a distributed manner using limited information that is sent to modems by a spectrum management center (SMC). The SMC first classifies users as strong and weak. Strong users then form a rate-penalty table that is used along with the Levin-Campello bit-loading algorithm to limit the interference to weak users. Simulation results show that the proposed algorithm can achieve near-optimal performance. Moreover, the structure of the algorithm enables easy implementation of polite bit and gain adaptation procedures without the need for executing the entire bit-loading algorithm again.

A Polite Cross-Layer Protocol for Contention-Based Home Power-Line Communications

In typical home power-line communication (PLC) networks using contention-based access methods, providing Quality-of-service (QoS) to high-priority users often comes at the expense of reducing the throughput of low-priority users. This paper proposes a cross-layer protocol which involves interaction between the Physical (PHY) and the Medium-access-control (MAC) layers, for ensuring politeness of the high-priority users toward the low-priority users for uplink transmission in home PLC networks. This protocol modifies the contention-based CSMA protocol of the MAC layer to exploit the cyclostationarity of the noise in home PLC networks. The PLC noise spectrum has been shown in literature to be periodic with the period of AC line cycle. Using this periodicity, the proposed protocol allows longer medium-access times for low-priority users in every AC line cycle, while meeting the high throughput requirements of the high-priority users. The proposed cross-layer protocol, termed Opportunistic CSMA, improves the throughput of the low-priority users by as much as 300% compared to the current CSMA protocols in home PLC networks.

Multiuser OFDMA Resource Allocation Algorithms for In-Home Power-Line Communications

This paper reformulates the multi-user OFDMA resource allocation problems in broadcast and multiple-access channels by defining an equivalent interference channel. The resulting problems are then solved by employing low-complexity algorithms for spectrum optimization in the interference channel. The re-formulation averts the high-complexity integer programming problem that otherwise results from an OFDMA system that allocates each user to separate subchannels. Simulation results indicate that the proposed algorithm achieves near-optimal performance and is suitable for PLC resource allocation applications.

Multi-User Joint Subchannel and Power Resource-Allocation for Powerline Relay Networks

This paper investigates Orthogonal Frequency-Division Multiple-Access (OFDMA) resource-allocation schemes for two-hop relays in a home Powerline Communication (PLC) network. Unlike wireless channels, which can have noticeable path loss and shadow effects to bring a substantial relay gain, the powerline channel usually observes a quality of the relay channel on par with the direct link so that simple fixed relay configurations often lose performance instead of gaining any. Besides, the quasi-cyclostationary powerline channel benefits little from diversity combining, a scheme often found in cooperative wireless relay protocols to obtain performance improvements on fast fading channels. To address these special and challenging features of the powerline channel, Home-PLC relay protocols that jointly allocate subchannels and power to the source and relay nodes are proposed. Simulation results show that significant improvements of data rates can be achieved by jointly optimizing the subchannel and power allocation of Home-PLC relay networks.

Full length article: Distributed adaptive bit-loading for spectrum optimization in multi-user multicarrier systems

This paper presents a low-complexity discrete bit-loading algorithm for multi-user multicarrier systems with application to spectrum balancing in digital-subscriber-lines. The algorithm can be implemented in a distributed manner using limited information that is sent to modems by a spectrum management center (SMC). The SMC classifies users in the network as strong or weak, based on their channel and noise conditions. Weak users execute a discrete bit-loading algorithm approximating water-filling, such as the Levin-Campello (LC) algorithm, while strong users use a rate-penalty table along with a modified LC algorithm to limit their interference to weak users. Simulation results show that the proposed algorithm can achieve near-optimal performance. In addition, polite bit and gain adaptation algorithms are developed using the structure of the proposed bit-loading algorithm, which enables easy and fast adaptation to channel and noise variations.

Per-tone margin optimization in multi-carrier communication systems

This paper investigates the unequal allocation of margin across tones in a multicarrier communication system. Using empirical statistics of the channel signal-to-noise-ratio variation, an algorithm is developed for modems to optimally distribute the available margin across tones instead of allocating equal per-tone margins. Such unequal per-tone margin allocation results in better stability of the line in terms of the outage probability especially when bit and energy adaptation mechanisms are not available or respond slowly to changes in the channel or noise spectrum.

CTH06-1: Optimality of FDMA in Gaussian Multiple-Access Channels with Non-zero SNR Margin and Gap

This paper investigates the effect of non-zero SNR margin and coding gap on the maximum sum rate and achievable rate region of the Gaussian multiple-access channel (MAC). Frequency-division multiple-access (FDMA) is shown to be the only sum-rate-maximizing scheme for a scalar Gaussian MAC when the margin/gap is not zero. Furthermore, successive decoding is shown to be strictly sub-optimal from a sum rate perspective. With fixed probability-of-error, single-user codes with finite gap are sufficient to achieve the maximum sum rate. The achievable rate region of the scalar Gaussian MAC with non-zero margin/gap is also characterized. Unlike the capacity region of a scalar MAC, the optimal achievable rate region with non-zero margin/gap is not a pentagon. Finally it is shown that when there is non-zero margin/gap, the optimality of the FDMA scheme can be generalized to a Gaussian MAC with inter-symbol interference (ISI).