Thomas C. Royster

Properties and performance of the IEEE 802.11b complementary-code-key signal sets

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 802.11 b compliance. The properties and performance of the highrate IEEE CCK 802.11 b 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 802.11 b 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.

Using DVB-S2 over asymmetric heterogeneous optical to radio frequency satellite links

The DVB-S2 coding standard has seen widespread use in many radio frequency (RF) communications applications. The availability of commercial-off-the-shelf (COTS) intellectual property (IP) that can be used to rapidly prototype and field communications systems makes this well-performing, standards-based approach to forward error correction (FEC) coding extremely attractive. In this paper, we evaluate the application of the DVB-S2 coding standard to an asymmetric satellite communications channel. The uplink comprises a fading optical link employing binary differential phase-shift keyed (DPSK) modulation, while the downlink comprises an RF link employing 16-ary amplitude and phase shift keyed (16-APSK) modulation. To simplify the payload implementation, hard-decision uplink demodulation is considered with uplink channel state information transmitted on the downlink for soft-decision decoding in the ground-based receiver. Additionally, we outline many of the tradeoffs in the overall system design, and some performance results of a baseline design are presented.

Jitter-aware time-frequency Resource Allocation and packing algorithm

One of the main components of the next generation protected military satellite communication systems is Dynamic Bandwidth Resource Allocation (DBRA). A centralized DBRA algorithm on the satellite dynamically grants terminals time and frequency resources as their traffic demands and channel conditions change, leading to significant increase in the overall system throughput.

Current designs for fast frequency hopping with MFSK

The performance of many existing communications systems can be improved simply by replacing the currently used forward error correction (FEC) code with a modern iteratively decodable FEC code. The potential improvements of adding modern FEC are even more pronounced for systems that do not currently employ any FEC. In this paper, we compare the performance of fast frequency hopping with simple diversity schemes using only symbol repetition and diversity schemes that also employ convolutional codes, short-block-length iteratively decodable codes, and long-block-length iteratively decodable codes. Both partial-band noise and multi-tone jamming channels are considered. The most powerful codes-namely, the long block length iteratively decodable codes-are shown to greatly improve link robustness while at the same time allow larger data rates than the traditional diversity approaches. In addition, it is demonstrated that these codes enable a system to realize the full benefit of independent tone hopping.

Cooperative Multicast Strategies under Heterogeneous Link Loss Rates

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.

Performance considerations for protected wideband satcom

There is increased interest in using transponded payloads to provide jam-resistant links to users. In the type of system we consider, multiple user terminals transmit to a satellite using a coordinated frequency hopping waveform, and the received signals are relayed to a ground processing “hub” terminal for baseband processing. In this paper, expressions for computing end-to-end signal-to-noise ratios for such systems are presented, and some example results are provided. In particular, modeling of effects such as signal suppression, intermodulation noise, and gain setting is discussed. By considering each component of the end-to-end link performance separately, the factors limiting performance are more apparent, enabling the system designer to identify which improvements would have the greatest impact.

Phase and power estimation for per-hop multi-user detection in frequency-hopping systems

Per-hop multi-user detection (PH-MUD) is a general class of techniques for frequency-hopping systems that enable multiple interfering hops to be simultaneously demodulated. Such techniques have the potential to increase the number of supportable users and increase per-user throughput. To realize these gains in actual systems, however, channel estimation capabilities sufficient for successful PH-MUD must be developed. In fact, the fundamental feature of such systems, hopping, requires that estimation be performed on a perhop basis, which precludes training amortization. We describe potential estimation techniques for PH-MUD and evaluate the resulting overhead, energy requirements, and performance. Practical estimation techniques with reasonable overhead are seen to provide good PH-MUD performance; thus PH-MUD remains a promising capability for frequency-hopping systems.

Digital construction and characterization of noise-like spread spectrum signals

A new method for generating digital noise-like spread spectrum signals is proposed. A standard binary keystream is used to generate a sequence of chips according to a Gaussian-like chip amplitude distribution for spreading sequences. The properties of these spreading signals are investigated as a function of the number of discrete amplitude levels and number of chips per symbol. The similarities between the generated signals and random Gaussian signals are evaluated based on higher-order moments. Implementation considerations, such as peak-to-average power ratio and amplifier backoff are also considered.