An uplink non-orthogonal multiple-access communication system is developed for heterogeneous non-terrestrial networks (NTNs) that employ both low-Earth orbit (LEO) satellite constellations and high-altitude platforms (HAPs) as relays with hybrid automatic repeat request capabilities. The system is designed as a random-access and interference-resistant network to support low-rate users. A common waveform and frequency band is used for all uplink transmissions (whether the transmitter is a user terminal or a HAP). Both theoretical and simulation-based performance analysis results are provided as a function of user transmit power and packet arrival rates for channels with interference. The use of HAP relays is shown to significantly improve throughput and reduce queue size relative to direct transmission to the LEO satellites, especially for the case of low-power users in high interference environments.

If a frequency band has primary and secondary users, then the cognitive radios of the secondary users must monitor the band and be prepared to cease their transmissions if a primary user's radio begins to transmit. Traditional spectrum sensing requires the secondary radios to refrain from communicating while they check for the emergence of primary signals. We propose and evaluate methods by which the secondary radios can continue their communications while simultaneously monitoring the band to detect any transmissions that are initiated by the primary radios. Our methods for spectrum monitoring supplement traditional spectrum sensing and improve the communications efficiency of the secondary radios.

We consider wireless multicasting over lossy links and explore the benefit of cooperative strategies in which multicast receivers exchange messages. A key feature of the problem considered here is that the source downlink channel has a higher loss rate than the channels between pairs of receivers; this feature implies that completion time may be reduced by offloading transmissions to receiver-receiver links and taking advantage of their higher reliability. Three strategies are analyzed and compared: a strategy in which all transmissions are carried out by the original source node; a strategy in which the original source transmits until it is able to designate a proxy-source among the receiver nodes to complete the multicast; and a strategy in which the original source transmits the minimal number of packets possible and cooperative transmissions among receivers are used to complete the multicast. These strategies are compared in terms of the number of packet transmissions needed to complete the multicast.

Information-theoretic limits on resource consumption are employed to obtain analytical methods for conducting tradeoffs between power, bandwidth, and time for cognitive radio transmissions in ad hoc dynamic spectrum access networks. A quantitative measure of resource consumption is applied to the design and evaluation of protocols for adaptive modulation and coding. The application of information theory to the minimization of resource consumption in cognitive radio networks is illustrated for binary phase-shift-key modulation with coherent demodulation.

In many dynamic spectrum access networks, it is necessary for secondary users to monitor the frequency band in which they are communicating so that they can determine if the primary user has begun transmission. Traditional sensing methods require the transmitters of the secondary users to be silent while spectrum monitoring is performed. Statistics that are derived easily in a communications receiver have the potential to permit a level of spectrum monitoring while the receiver is demodulating and decoding a packet, so that it is not necessary to silence the secondary transmitters. The proposed techniques for spectrum monitoring can supplement existing methods and reduce the amount of time that secondary users must refrain from communicating.

We describe similarities and differences between complementary-code-key (CCK) modulation and modulation that is derived from biorthogonal signals, and we present performance results and other information that may be useful to those who have applications for CCK modulation that do not require IEEE 802.11b compliance. The properties and performance of the highrate IEEE CCK 802.11b modulation formats are investigated and compared with the properties and performance of alternative modulation formats that are based on biorthogonal signals. Several complementary properties are derived for the full-rate (11 Mb/s) CCK signal set, the half-rate (5.5 Mb/s) CCK signal set, a full-rate signal set obtained from biorthogonal signals, and a half-rate biorthogonal signal set. Each signal set is a complementary set, but each also has stronger complementary properties. We evaluate the performance of IEEE 802.11b standard CCK modulation, CCK with certain modifications that depart from the IEEE standard, and modulation that is derived from biorthogonal signals. Performance comparisons are presented for additive white Gaussian noise (AWGN) channels and for channels with specular multipath. In particular, for AWGN channels, we provide an accurate analytical approximation for the frame error probability for full-rate CCK modulation.

Results are presented on code performance and capacity bounds for channels with Gaussian, bi-level, block interference for binary and nonbinary modulation, noncoherent demodulation, and soft-decision decoding. Each transmitted block is corrupted by additive white Gaussian noise that has one of two power spectral densities. The smaller of the two is the power spectral density of the thermal noise, and the difference between the two spectral densities is the power spectral density of the interference noise. The average power in the interference is constant, so the power spectral density of the interference is inversely proportional to the average fraction of the blocks that have interference. This block interference channel has been used to model partial-band interference in frequency-hop communications and to model pulsed interference in a number of communication systems. Throughput results for turbo-product and low-density parity-check codes that are used on Gaussian block interference channels are compared with the capacity bounds for such channels. For some code rates, we show that the throughput of the codes and the capacity bounds are both nonmonotonic functions of the fraction of the blocks that have interference. Applications to adaptive-rate coding are discussed.

Cognitive radios that are employed in a network with dynamic frequency assignments must operate efficiently in the presence of uncertainties and variations in the propagation characteristics of the network's communication links. A low-complexity adaptive transmission protocol is described and evaluated for use in cognitive radio networks whose links have unknown and possibly time-varying propagation losses as a result of such phenomena as slow fading or variations in shadowing. The cognitive radios are required to derive only simple statistics in the receivers in order to provide the information that is needed by our protocol; no estimates or measurements of received power or channel gain are used. The protocol's primary mechanism for responding to changes in propagation loss is to adjust the modulation and coding. Because of disruptions that can be caused by higher levels of interference to other radios in the network, the transmitter power is increased only if the most powerful combination of coding and modulation is inadequate. We employ finite-state Markov models for slowly varying channels, and we demonstrate that for such channels our protocol performs nearly as well as an ideal protocol that is told the exact value of the propagation loss for each packet transmission. Thus, the additional complexity that is required to enable cognitive radios to obtain precise channel-gain estimates is not justified and would lead to only negligible improvement in throughput. The throughput of our adaptive transmission protocol is compared with an upper bound that is derived from information theory for a hypothetical ideal protocol that is given perfect channel-state information, and some preliminary results on learning the adaptation decision intervals are included.

Two classes of complementary signal sets are compared in terms of their complementary properties and their error probabilities for channels with thermal noise and multipath interference. One class consists of the high-rate (11 Mbps and 5.5 Mbps) signals employed in the IEEE 802.11b standard, and the other class includes full-rate (11 Mbps) complex signals derived from biorthogonal sequences and half-rate (5.5 Mbps) biorthogonal signals. We examine several types of complementary properties of each class of signals and give performance comparisons for the signals when employed on channels in which thermal noise is the only disturbance and channels with thermal noise and multipath interference. For standard IEEE 802.11b complementary-code-key (CCK) modulation, we find the performance is strongly dependent on the differential multipath delay. For systems that employ binary error-control coding, the signals that are based on biorthogonal modulation are superior to the two IEEE 802.11b CCK signal sets.

High-rate direct-sequence (DS) spread spectrum is a modulation technique in which most or all of the spreading is provided by nonbinary data modulation. For applications to mobile ad hoc wireless networks, the limited processing gain of high-rate DS spread spectrum gives only modest protection against multiple-access or multipath interference, which limits the applicability of the modulation technique to fairly benign channels. In this paper, we explore the increased interference-rejection capability that can be obtained from convolutional coding with Viterbi decoding, Reed–Solomon coding with errors-and-erasures decoding, and block product coding with iterative decoding. For channels with multiple-access or multipath interference, performance results are given for several soft-decision decoding metrics, the benefits of adaptive-rate coding are illustrated, and the accuracy and utility of the Gaussian approximation are described. We also show how to use the bit-error probability for a system without error-control coding to determine which modulation method will give the best packet-error probability in a system with error-control coding.

High-rate direct-sequence spread spectrum is a modulation technique in which most or all of the spreading is provided by nonbinary data modulation. For applications to mobile ad hoc wireless networks, the limited processing gain of high-rate direct-sequence spread spectrum gives only modest protection against multiple access or multipath interference, which limits the applicability of the modulation technique to fairly benign channels. In this paper, we explore the increased interference-rejection capability that can be obtained from convolutional coding with Viterbi decoding, Reed-Solomon coding with errors-and-erasures decoding, and block product coding with iterative decoding. For channels with multiple access or multipath interference, performance results are given for several soft-decision decoding metrics, the benefits of adaptive-rate coding are illustrated, and the accuracy and utility of the Gaussian approximation are described. We also show how to use the bit-error probability for a system without error-control coding to determine which modulation method will give the best packet-error probability in a system with error-control coding