Mark J. Miller

Remote non-linearity detection via burst power dithering and EM based SNR Estimation

A new approach to detect the onset of saturation (1dB output compression point) at remote terminals is proposed in this paper. The proposed approach dithers the transmit power of periodic ranging bursts by a known amount and then estimates the received difference in Signal to Noise Ratio (SNR) and uses this metric to indicate whether the operating point of the Power Amplifier is in the linear range or not. The Expectation-Maximization (EM) Algorithm is used to achieve the ML estimate of the received SNR and is compared to the Cramer-Rao Lower Bound (CRLB). Application of this algorithm to a commercial satellite broadband system as well as simplifications to the algorithm to make it easier for implementation on FPGAs, are also discussed.

An innovative synchronization preamble for UHF MILSATCOM

4-ary, multi-h continuous phase modulation (CPM) has been added to MIL-STD-188-181B for dedicated mode UHF SATCOM operation to provide greater throughput capacity for this power- and bandwidth-restricted communications network. The physical layer of the waveform is designed to occupy the entire channel bandwidth at all data rates, optimizing power and spectral efficiency, but presenting challenges for acquisition. As an example, at low data rates the waveform has a 4 dB processing gain relative to binary phase shift keying (BPSK). Hence this waveform is expected to be acquired successfully at an E/sub b//N/sub 0/ 4 dB lower than that of BPSK. To acquire the CPM waveform, the modem must be capable of acquiring at low signal-to-noise, in the presence of large unknown frequency and timing offsets, and with the band-limiting and hard-limiting channel impairments introduced by the satellite. To promote interoperability and reliability, the Joint Interoperability and Engineering Organization (JIEO) requested a synchronization method that would provide reliable acquisition at an E/sub b//N/sub 0/ corresponding to a 1/spl times/10/sup -3/ bit-error-rate and also provide automatic data rate and waveform parameter detection. The resulting preamble achieves the aforementioned objectives and, in addition, is short enough in length to allow it be implemented as the burst preamble for future incorporation of the CPM waveform into the MIL-STD-188-182 and -183 demand assigned multiple access modes of operation. In this paper the design philosophy and implementation of the preamble are presented.

Two-way global broadcast system architecture

The global broadcast system (GBS) implemented by the U.S. Department of Defense utilizes commercially-based digital video broadcast (DVB) technology to enable high-rate (10s of Mb/s), one-way, data dissemination to a large number of users. Currently, however, user terminals are limited to receive-only operation. For many users, a receive-only configuration is not adequate and a low to medium-rate (10s to 100s of kb/s) return channel is required for true network-centric operations and enhanced mission effectiveness. This paper describes a frequency re-use architecture that provides such return channel connectivity to a large number of users without requiring additional satellite bandwidth and with minimal interference to current receive-only users.

Pre-distortion techniques and bandwidth efficient modulation for military satellite communications

In bandwidth limited satellite channels, higher order modulations can be used to increase data throughput (as measured by bit/s/Hz). This paper compares two modulations with 4 bit/s/Hz efficiency, 16-QAM and 16-amplitude phase shift keying (16-APSK). It is shown that 16-APSK offers better performance in nonlinear satellite channels with adjacent channel interference (ACI). The nonlinearity of the channel is due to the high power amplifier (HPA) at the transmitter. The nonlinearity can be reduced by using larger output back-offs (OBO) at the transmit HPA. However, it is desirable to keep the OBO as low as possible in order to lower the costs associated with the HPA. Pre-distorting the transmitted signal suitably can lower the distortion due to the HPA, without a large OBO. When the HPA characteristics are completely known, the transmitter can pre-distort the signal based on the instantaneous transmitted power. This is called open-loop predistortion. Typically, the amplifier characteristics are usually not completely known. The existing methods in the technical literature estimate the pre-distortion required by assuming that the transmitter has access to the HPA output. However, in military satellite communications such a feedback path is also typically not present, e.g., onboard ships. This paper develops a closed-loop pre-distortion method when the transmitter cannot access the output of the HPA and the link is full-duplex. Here, the receiver estimates the pre-distortion required and communicates it to the transmitter. The performance of the closed-loop predistortion technique is characterized by simulations.

Detection of CPFSK signals using per survivor processing

The use of per survivor processing (PSP) is considered in the detection of a continuous phase frequency shift keying (CPFSK) signal. It is shown that PSP can enable the use of a trellis structure for the demodulation that is different from the trellis structure used to generate the CPFSK signal. This allows one to select a receive trellis structure with fewer phases, resulting in a reduced state sequence estimation algorithm. Results are presented using a receive trellis of 3 states to receive binary CPFSK signals with a modulation index, h, of 7/10. The complexity of the receiver is reduced by a factor of 3.3 with minimal performance degradation. An application of this PSP technique is also discussed which enables one to perform maximum likelihood sequence estimation (MLSE) of a digital FM signal, where the modulation index is only approximately known. Results presented for the binary case show this technique to provide a significant detection efficiency advantage over conventional techniques such as limiter-discriminator detection or non-coherent detection.

Novel approach for shipboard antenna handover without data loss

Shipboard antenna handover causes loss of demodulator synchronization, bit count integrity, frame timing, and cryptographic synchronization as well as TCP throughput degradation. This paper describes a novel approach for mitigating the effects of antenna handover which prevents data loss while minimizing handover processing delay. Ship-to-shore handover uses data buffers in the ship and shore modems, and shore-to-ship handover employs interleaving combined with minimal overhead handover encoding. The proposed technique works with any modulation and FEC coding (or no coding) schemes. The only control signal required is a handover signal at the shipboard modem to indicate the start of handover.