J. Lopez-Perez

Experimental results on multicarrier MIMO HF communications

Achieving reliable communication over HF channels is known to be challenging due to the particularly hostile propagation medium. To address this problem, diversity techniques were shown to be promising. In this paper, we demonstrate through experimental results the benefits of different diversity strategies when applied to multi-input-multi-output (MIMO) multicarrier systems. The performance gains of polarisation, space and frequency diversities are quantified using different measurement campaigns.

Validation of a HF spread spectrum multi-carrier technology through real-link measurements

SUMMARYThe HF band data communication link has been traditionally desired by many of the large rangetransmission systemsalthough it is associated tounfavourable performancesas low transmission rate, largedelayandlowconfidenceintermsoflinkestablishmentandmaintenance.Althoughtransmissionrates maybe highenough totransmit digitalvoice, delay,usuallyover severalseconds, has beenthe mainhandicap tolet the systems provide interactive digital voice links. Indeed, there is no unclassified equipment with thiscapability.Themainachievementofthisproposalisthatweareabletoguaranteedigitalvoicetransmissionwith low latency, around 135ms (modemþcodec), providing a full interactive digital voice link.Performances of two new 2460bps HF modems are presented versus the 39-tone 2400bps MIL-STD-188-110A modem, working over an ITU-R moderate channel. Furthermore, these results are corroborated byreal tests carried out in a 1800Km link in the 18MHz band. Copyright # 2006 AEIT. 1. INTRODUCTION1.1. The HF channelIn the HF band (3–30MHz), long distance communica-tions are feasible, thanks to the use of the ionosphere aspassive reflector. However, the atmospheric nature of thisreflector makes the systems face a very hard communica-tionenvironment.Multipatheffects arealways present andhave to be considered in depth, besides the very fast time-varying channel characteristics.The main parameters used to measure channel behaviourare frequencycoherence ( f

Multi-Carrier Techniques Performance on Ionospheric Channel for Delay-Sensitive Applications

Multi-carrier modulations are widely employed in HF communications, and particularly OFDM, mainly because their ease of generation by means of DFT and also their appealing properties that can turn a selective fading channel into a set of flat channels. In order to cope with deep nulls in the channel traditional approach has been the use of channel coding and interleaving, thus causing an increase in communication delay. For delay-sensitive applications, spreading schemes over OFDM, such as OFDM-CDM can be applied. If CSI is known at transmitter, system performance can be improved by BER-optimum power loading and channel matrix SVD decomposition of OFDM-CDM signal. These techniques are well suited to delay-sensitive applications as they incur in no further delays.