Daniel L. Noneaker

The effect of automatic gain control on serial, matched-filter acquisition in direct-sequence packet radio communications

The performance of a noncoherent serial acquisition technique is evaluated for direct-sequence spread-spectrum packet communications. The acquisition technique that is considered uses threshold crossing of a matched-filter output to detect a fixed-length preamble at the start of each packet. The analysis accounts for frequency mismatch between the transmitter and the receiver due to oscillator inaccuracies and mobility-induced Doppler shifts. It also accounts for the effects of automatic gain control (AGC) in the receiver. The role of the AGC system in determining the acquisition performance is examined. In addition, selection of the optimal acquisition threshold is considered, and a simple method for selection of a good suboptimal threshold is presented. It is shown that use of this threshold results in performance close to that obtained with the optimal threshold over a wide range of channels.

Distributed and adaptive TDMA algorithms for multiple-hop mobile networks

Time-division multiple-access (TDMA) protocols can support contention-free link-level broadcast transmissions in mobile, multiple-hop wireless networks. However, it is typically difficult to adapt TDMA assignments to dynamic topologies. The paper defines and discusses two algorithms for adjusting TDMA slot assignments to the dynamic changes of a mobile network. Our new protocols offer three significant advantages: (1) the information required by the protocols is gathered exclusively from a terminals local neighborhood; (2) global coordination is not required; (3) changes to the state information at a terminal do not need to be distributed to all terminals in the network. The operation of the algorithms is evaluated with an extensive simulation study using a large number of randomly generated networks in which the diameters, degrees, and number of nodes are varied. Results show that the efficiency of the algorithms, determined as the average number of transmission slots assigned per terminal, is better under static conditions than two cases chosen for reference; the efficiency does not degrade significantly under dynamic conditions for which the algorithms continually adjust to added and deleted links. For the networks considered, the algorithms typically converge in times on the order of several hundred slots and a convergence bound approximately four times larger is established. Convergence time is shown to increase in almost a linear fashion with terminal radius and it is relatively insensitive to the number of terminals.

Error probability bounds for M-PSK and M-DPSK and selective fading diversity channels

A method is described for obtaining tight closed-form bounds on the probability of error for M-ary phase-shift keying (M-PSK) and M-ary differential phase-shift keying (M-DPSK) on fading diversity channels. The channels exhibit doubly selective fading and have specular components. In addition, the random impulse responses of the diversity channels may be correlated; and the probability distributions for the fading on different diversity channels need not be the same. Error probability expressions are given for binary DPSK, 8-DPSK, and 16-DPSK modulation as examples of the application of the general method described. >

An adaptive transmission-scheduling protocol for mobile ad hoc networks

Transmission-scheduling protocols can support contention-free link-level broadcast transmissions and delay sensitive traffic in mobile, multiple-hop packet radio networks. Use of transmission-scheduling protocols, however, can be very inefficient in mobile environments due to the difficulty in adapting transmission schedules. The paper defines a new adaptive and distributed protocol that permits a terminal to adapt transmission assignments to changes in topology using information it collects from its local neighborhood only. Because global coordination among all the terminals is not required and changes to transmission assignments are distributed to nearby terminals only, the protocol can adapt quickly to changes in the network connectivity. The two key parameters that affect the ability of the protocol to adapt to changes in connectivity are the rate of connectivity changes and the number of terminals near the connectivity changes. Using simulation, we determine the ranges for these parameters for which our adaptive protocol can maintain collision-free schedules with an acceptable level of overhead. The stability of the protocol is also characterized by showing that the protocol can quickly return to a collision-free transmission schedule after a period of very rapid changes in connectivity. Our channel-access protocol does not require a contention-based random-access phase to adapt the transmission schedules, and thus its ability to adapt quickly does not deteriorate with an increase in the traffic load.

Optimal combining for RAKE reception in mobile cellular CDMA forward links

Two methods for multipath combining are compared for RAKE reception of a CDMA cellular forward-link traffic channel. Combining based on estimation of the signal-to-interference ratio of the output of each tap of the RAKE receiver is shown to provide significant performance improvement in comparison with the more commonly considered combiner that is based on measurement of the signal strength. The performance of RAKE receivers using the two methods of combining is considered for several fading, multipath channels.

The receiver blocking problem in a DS mobile ad hoc network with directional antennas

In this paper we examine the effect of the receiver blocking problem on the performance of a direct-sequence ad hoc network in which a subset of the nodes employ multiple directional antennas. We present a packet transmission scheduling algorithm and a media access control (MAC) protocol that are designed to exploit the capabilities of the nodes with multiple directional antennas. It is shown that the presence of directional antennas in the network increases the networks vulnerability to the receiver blocking problem with a correspondingly severe impact on network performance. We examine the reason for this phenomenon, and we present a modified MAC protocol that mitigates the problem.

The Effects of Sequence Selection on DS Spread Spectrum with Selective Fading and Rake Reception

Error probabilities are evaluated for direct-sequence spread-spectrum communications and Rake reception over channels with doubly selective fading. The error probability for such a system depends on the spreading sequence, the autocorrelation function of the fading process, the received signal-to-noise ratio, and the number of taps in the Rake receiver. The focus of the paper is on the effect of the spreading sequence on the performance of each of two systems. One system employs noncoherent detection of differentially-encoded binary direct-sequence spread-spectrum signals and a post-detection diversity-combining Rake receiver which uses equal-gain combining. The other system employs coherent detection of binary direct-sequence spread-spectrum signals and a post-detection diversity-combining Rake receiver with perfect gain estimates for the channel. A simple sequence selection criterion is introduced, and the sensitivity of the performance of the system to the choice of the spreading sequence is examined. It is shown that significant performance differences result from different choices of the spreading sequence. It is also shown that, given a moderate range of delay spreads, sequences can be found that yield low bit error probabilities over that range. These are found to be robust with respect to the delay spectrum for the channel, the number of taps in the Rake receiver, the Doppler spread, and the signal-to-noise ratio.

Analysis of a serial acquisition scheme for unslotted distributed direct-sequence packet radio networks

This paper investigates the problem of acquisition in unslotted distributed direct-sequence packet radio networks. A noncoherent serial acquisition scheme is considered that uses passive matched filters to detect packet headers. A new Gaussian approximation is derived to model the effects of multiple-access interference during acquisition. This approximation takes into account oscillator inaccuracies and Doppler shifts, and its validity depends only on the presence of a sufficient number of chips in the acquisition header. In particular, the accuracy of the approximation is not contingent on the presence of a large number of equal-power interferers. False alarm processing is modeled and a simple expression for the probability of acquisition is derived. The problem of threshold selection is considered and it is shown that a single threshold can be chosen to provide acceptable performance in a wide range of channel conditions.

Distributed formation of broadcast transmission schedules for mobile ad hoc networks

In a mobile ad hoc packet radio network using transmission scheduling, when a terminal boots and is ready to join the network, it has no knowledge of the networks current state. In order to send and receive packets without interference, the new terminal must have some means of notifying other terminals of its presence and exchanging information with them so that it can form a collision free broadcast transmission schedule. We demonstrate the performance of a distributed protocol designed to allow a new terminal to acquire sufficient information about its local environment to achieve collision free transmissions. Furthermore, we show that changes to the network are limited to the new terminals local neighborhood. As a result, the size of the overall network has very little effect on the performance of the protocol. The protocol is not only effective in adding a new terminal; it can also organize a group of terminals into a new network. This is demonstrated for the extreme case of all terminals powering up simultaneously. The overall performance of the algorithm depends primarily on the density of the network and rate at which changes occur in the network.

Evaluation of a split-connection mobile transport protocol

A performance evaluation is presented for a split-connection protocol for wireless Internet access which is denoted Split-Connection Mobile Tranport Protocol (SCMTP). It uses the general approach of the previously introduced Mobile End Transport Protocol (METP) but with differences that include a wireless-link channel-access protocol better matched with current cellular networks and more general ARQ methods for error control in the wireless link. In common with METP, SCMTP uses a standard TCP protocol on the wire-line connection and isolates the data flow in the wire-line network from the effect of wireless packet errors.Performance is considered for the important case of a single SCMTP split connection between a fixed host and a mobile host with heavy downlink traffic from the fixed host to the mobile host. It is shown for these conditions that if the wire-line packet-error rate is small, a steady state is reached in which the connections data flow in the wire-line network remains under the control of the receiver-advertised window of the TCP entity at the base station. Performance is evaluated for the steady-state operation of the SCMTP protocol, and relationships are established between the key properties of the split connection and the end-to-end performance of the connection.It is shown that for heavy downlink traffic, the delay in the wire-line part of the connection does not affect steady-state throughput if the receiver buffer is sized appropriately. It is also shown that use of the go-back-N ARQ protocol on the wireless link yields better performance than the stop-and-wait ARQ protocol, although the performance with go-back-N ARQ is more sensitive to the characteristics of the wireless channel. It is shown that under a broad range of conditions, SCMTP with go-back-N ARQ provides nearly optimal utilization of the capacity of the wireless link.