Performance of HF modems on high‐latitude paths using multiple frequencies

Abstract

Abstract. The high-latitude HF channel has been measured and characterized in terms ofDoppler spread• delay spread, and signal-to-noise ratio. The performance of data modems has been determined over a comprehensive range of simulated channel conditions. A comparison is made between the channel measurements and the modem characterizations. Thereby the percentage of time that the modem works satisfactorily (availability), during the measured channel conditions, is determined. Two 75 bits/s modems (compliant with STANAG 4415 and STANAG 4285), a 2400 bits/s modem (compliant with STANAG 4285), Morse code, and voice have been selected for analysis. The two 75 bits/s modems show 60-75% higher availability than the 2400 bits/s modem on the measured paths. The 2400 bits/s modem is degraded in performance during a geomagnetic disturbance whereas the 75 bits/s modems maintain approximately the same performance level. The paper also addresses the necessary frequency pool (at a specific time) required for an HF circuit to achieve the best possible communications availability. For the 75 bits/s modems, independent of path length, a one-frequency pool is all that is required during summer and winter when the ionospheric conditions are quiet. A four frequency pool is necessary during disturbed ionospheric conditions. For the 2400 bits/s waveform, three to four frequencies are needed on the 200 km west-east path, and four to six frequencies are required on the north-south 2000 km path. This applies to all seasons and geomagnetic conditions analyzed.

Radio communication via Near Vertical Incidence Skywave propagation: an overview

“…It uses an NVIS channel instead of a VHF link, which could not be used due to the accidented terrain. Several other studies were done to improve the effective bitrate of modems used in one-hop ionospheric propagation and NVIS propagation [107]. Johnson [110] details modem characteristics for a wireless mesh network for NVIS propagation, long-haul skywave and surface wave signals.…”

“…STANAG 4415, MIL-STD 118 110C and DRM) are designed to work for both NVIS and oblique incidence propagation, and are optimized for data transfer in the presence of fading, multipath and noise. In [107], the link availability of two modems (one compatible with STANAG 4415 and 4285, and the second only with STANAG 4285) is compared with Morse code and voice traffic. Davies [108] contains measurements of throughput in relation to SNR for STANAG 4539 [109] over several distances, to evaluate the effects of low SNR and variable channel conditions, taking into account advanced waveforms, coding and protocols.…”

Measurement and modeling of HF channel directional spread characteristics for northerly paths

“…From the perspective of communications systems, such large delay and Doppler spreads can result in a significant degradation in system performance (e.g., in achievable data throughput). Discussion of this aspect is given by Angling et al [1998] and Jodalen et al [2001] who report the results from an extensive measurement campaign over a number of high-latitude paths using the DAMSON system and relate their results to anticipated modem performance.…”

Near vertical incidence skywave

“…It uses an NVIS channel instead of a VHF link, as the latter could not be used due to the accidented terrain. Several other studies were done to improve the effective bitrate of modems used in one-hop ionospheric propagation and NVIS propagation [Jodalen, 2001]. Johnson [2006] [Digital Radio Mondial, 2009], are designed to work for both NVIS and oblique incidence propagation, and are designed for faster data transfer in the presence of fading, multipath and noise.…”

“…Johnson [2006] [Digital Radio Mondial, 2009], are designed to work for both NVIS and oblique incidence propagation, and are designed for faster data transfer in the presence of fading, multipath and noise. In [Jodalen, 2001], the link availability of two modems -one compatible with STANAG 4415 and 4285 [NATO, 1989], the second with STANAG 4285 only -has been compared with Morse code and voice. [Davies, 2003] contains measurements of throughput in relation to SNR for STANAG 4538 [NATO, 2009] over several distances, to test the effects of low SNR and variable channel conditions, taking into account advanced waveforms, coding and protocols.…”