Pierre R. Chevillat

A dynamic link adaptation algorithm for IEEE 802.11 a wireless LANs

We present a simple but powerful dynamic link adaptation mechanism for wireless LANs, which are based on the IEEE 802.11a standard. Using this method, a transmitter is able to detect whether the quality of the link is improving or deteriorating, and based on this information to switch to a higher or lower transmission rate, respectively. For determining the link quality, the transmitter employs only information that is available locally. Therefore, the proposed method can be implemented without changes or enhancements to the current IEEE 802.11 standard. The efficiency of our method is investigated and evaluated by means of simulations.

Rapid Training of a Voiceband Data-Modem Receiver Employing an Equalizer with Fractional-T Spaced Coefficients

Fast initial training of a voiceband data-modem receiver which employs an equalizer with fractionl- T spaced coefficients is presented. The modem receiver performs rapid gain acquisition, and monitors the equalizer delay line for the presence of a periodic training signal whose period is equal to the length of the equalizer delay line. Testing for periodicity is combined with estimating carrier-frequency offset. Once a full period of the training signal has been obtained, the equalizer coefficients are computed by spectral division, and coefficient centering is performed. The receiver then proceeds in a reference-directed mode until the end of the training signal is detected. In the paper, algorithms for rapid gain acquisition and the detection of a cyclic training signal in the presence of noise and carrier-frequency offset are presented. Cyclic training of an equalizer with fractional- T spaced coefficients is derived. It is shown that the equalizer coefficients can be computed efficiently by spectral division. Coefficient centering, carrier-phase and symbol synchronization, reference-directed training, and the transition to data mode are described. Finally, measurement results for an experimental 2400 baud modem using an equalizer with 64 T/2 -spaced coefficients are given. The modem achieves a startup time of 20 ms on a Bell 3002 unconditioned leased line.

Optimum FIR Transmitter and Receiver Filters for Data Transmission Over Band-Limited Channels

The paper deals with the design of digital transmitter and receiver filters with finite impulse response (FIR) for data transmission over band-limited channels. The filters are matched and satisfy a zero intersymbol interference constraint when cascaded. For baseband transmission, the filters achieve optimum spectral concentration in the frequency range [-(1+\beta)/2T, (1+\beta)/2T] . Mathematically, the filter design leads to a generalized eigenvalue problem which is solved numerically by a projected gradient Procedure. For transmission over bandpass channels by combined amplitude and phase modulation, the design technique is modified so that filters with complex-valued impulse response and optimum spectral concentration in the range of positive bandpass frequencies [f_{c} - (1+\beta)/2T, f_{c} + (1+\beta)/2T] are obtained. In addition, the complex formulation allows the design of impulse responses with enhanced spectral attenuation in the corresponding range of negative frequencies in order to minimize imageband interference. Results are shown in terms of filter coefficients, signal spectra, and spectral concentrations obtained. For example, filters designed for a voiceband data modem operating at a symbol rate of 2400 baud achieve a spectral concentration of 98.5 percent with 24 coefficients and \beta = 0.1 , and with only 0.001 percent of the total energy in the imageband region.

Noise-predictive partial-response equalizers and applications

The authors consider enhancing the performance of the traditional linear partial-response (PR) equalizer by using noise prediction techniques. The resulting noise-predictive PR equalizer consists of a forward linear PR equalizer followed by a linear predictor to whiten the noise and the residual distortion at the equalizer output. Assuming correct decisions and applying results from prediction theory, it is shown that the minimum mean-square error of a noise-predictive PR equalizer equals that of a decision-feedback equalizer which employs no PR shaping. This result shows that the performance of a receiver with a PR shaping equalizer can always be improved by attaching a noise-whitening predictor. Simulation results are presented for channels typically encountered in wire transmission and magnetic recording, i.e., channels with a spectral null at DC and strong high-frequency attenuation. The results show a substantial performance improvement when a noise-predictive PR equalizer is used.<<ETX>>

A Multiple Stack Algorithm for Erasurefree Decoding of Convolutional Codes

A new algorithm for erasurefree sequential decoding of convolutional codes is introduced which achieves low error probabilities at substantially higher decoding speeds than the Viterbi decoding algorithm. The algorithmic properties of the Multiple Stack Algorithm (MSA) are investigated and it is demonstrated that the MSA reaches a decision with an exponentially rather than Pareto distributed computational effort. The MSAs error probability on the binary symmetric channel is studied as a function of its parameters and its performance and complexity compared to that of the Viterbi algorithm. The MSA is seen to achieve equal and lower error probabilities with a significantly lower average decoding effort. The new algorithm can thus be considered an attractive alternative to the Viterbi algorithm where low error probabilities and high decoding speeds are required simultaneously.

Decoding of trellis-encoded signals in the presence of intersymbol interference and noise

A novel receiver for data transmission systems using trellis-coded modulation (TCM) is investigated. It comprises a whitened matched filter and a trellis decoder which combines the previously separated functions of equalization and TCM decoding. TCM encoder, channel, and whitened matched filter are modeled by a single finite-state machine with combined ISI (intersymbol interference) and code states. Using ISI-state truncation and the self-partitioning principles inherent in TCM, a systematic method is developed for reducing the state complexity of the corresponding combined ISI and code trellis. A modified branch metric is used for canceling those ISI terms which are not represented by the trellis states. The approach leads to a family of Viterbi decoders which offer a tradeoff between complexity and performance. An adaptive version of the receiver is discussed, and simulation results are presented for channels with severe linear distortion. It is shown that the adaptive receiver achieves a significant gain in noise margin.<<ETX>>

Dynamic Data Rate and Transmit Power Adjustment in IEEE 802.11 Wireless LANs

In this paper a novel link adaptation algorithm is proposed that is capable of adjusting the transmit power level and the data rate jointly to the radio channel conditions. The proposed method relies solely on link quality information available at the transmitter by employing the reception or non-reception of the acknowledgment frames as a measure of the channel quality with respect to the power level and data rate. The method is fully compatible with the 802.11 wireless LAN standard. In contrast to many other proposals, it neither relies on the RTS/CTS protocol nor requires a feedback channel to transmit link-quality estimates from the receiver to the transmitter. Different strategies for optimizing the data rate and power level are given. These depend on the scenarios considered, the number of active stations, and the service requirements. The two main strategies are either to drive the system towards the highest possible data rate and adjust the rate and power levels accordingly (“high-performance” mode) or to focus on power saving, possibly trading this for other performance criteria such as throughput or delay performance (“low-power” mode). Other special cases, such as power or rate only adaptation, are also discussed. It can be shown that in most cases the best choice for achieving low transfer times, maximizing throughput, and alleviating the hidden terminal problem is to transmit at the highest possible rates and with high power levels. This “high-performance” mode of operation also minimizes the transmission times, which in turn maximizes the time for putting idling components into a sleep mode, thereby minimizing the overall power consumption.

Architecture of a digital signal processor

A digital signal processor (DSP) is described which achieves high processing efficiency by executing concurrently four functions in every processor cycle: instruction prefetching from a dedicated instruction memory and generation of an effective operand, access to a single-port data memory and transfer of a data word over a common data bus, arithmetic/logic-unit (ALU) operation, and multiplication. Instructions have a single format and contain an operand, index control bits, and two independent operation codes called “transfer” code and “compute” code. The first code specifies the transfer of a data word over the common data bus, e.g., from data memory to a local register. The second determines an operation of the ALU on the contents of local registers. A fast free-running multiplier operates in parallel with the ALU and delivers a product in every cycle with a pipeline delay of two cycles. The architecture allows transversal-filter operations to be performed with one multiplication and ALU operation in every cycle. This is accomplished by a novel interleaving technique called ZIP-ing. The efficiency of the processor is demonstrated by programming examples.

A power-efficient wireless sensor network for continuously monitoring seismic vibrations

We present a novel power-efficient wireless sensor network for continuously monitoring and analyzing seismic vibrations with sensor nodes and forwarding the retrieved information with low-cost relay nodes to backend applications. The applied vibration sensing algorithms are derived from the DIN 4150–3 standard. All nodes in the network are battery-powered and equipped with an IEEE 802.15.4 compatible radio transceiver. The nodes communicate with each other by executing a novel power-efficient protocol stack, which provides all network functions required by the vibration-sensing application and uses a publish/subscribe messaging protocol for communicating between the network nodes and the backend applications. Results obtained in certification and field tests show that the proposed vibration-sensing solution is standard-compliant, and that the wireless vibration sensor network (WVSN) exhibits excellent performance in terms of packet delivery rate, latency, and power efficiency.

The SP16 signal processor

The SP16 processor executes typical signal-processing tasks efficiently by overlapping in each processor cycle four functions: instruction prefetching, data-memory access and transfer of one data word over a single data bus, ALU operation, and multiplication. Despite SP16 being a one-address machine with a single data bus, it can perform filter operations with a multiplication and ALU operation in every processor cycle.