> How does nbems fare in weak signal conditions compared to other modes > such as olivia and mt63?
NBEMS uses the PSK modes for simplicity of tuning, narrowness, and speed. Other, wider modes, like Olivia and MT63, work further down under the noise threshold than the PSKmodes. This makes them better for casual communicating on HF, over longer distances, where the QSB fading is often very deep, fading 10 dB or more. NBEMS is designed primarily for emcomm messaging over distances up to 100 miles, and wherever possible, 2m SSB (using digital modes) is recommended so there is almost no fading to contend with and antennas are smaller and more portable. On 2m, once you achieve a detectable signal on the waterfall (which is needed for tuning and therefore has to at least be visible in the noise), it generally stays at that level (up to 100 miles), so the ability to continue copy far down under the noise threshold is not needed. In hilly regions, where 2m VHF will not work due to shadowing by the hills, the alternative is to use single-hop HF, with NVIS antennas, on 80m or 40m, and the fading is less than on multi-hop paths, but not as good as on VHF. For very difficult HF conditions, NBEMS had been designed to also support MFSK16 for ARQ transfers. Transfer times are much longer compared to the PSK modes, but the message does eventually get through without any errors. Since few disasters, requiring emergency communications, span more than 100 miles, narrowband PSK modes work just fine for the purpose, just as PSK31 works well enough for casual operating, even though wider modes work better under fading conditions. It is desirable, in order to be seen by as many potential message forwarding operators as possible, to have all the NBEMS stations appear on the waterfall at one dial setting, just like most PSK31 stations on HF do now. In order to do this within the 2500 Hz-wide receiver passbands most people have, the narrowband modes need to be used. For most short emcomm messages, the speed of PSK63 is quite sufficient, up to 25 PSK63 stations can share 2500 Hz of spectrum, and all can be seen at the same time, thereby maximizing the number of potential forwarding stations. The reason that NBEMS is described as a "system" is that it integrates several elements, such as particularly using VHF 2m, particularly using NVIS for HF, and using narrowband modes, instead of wider modes, for the most effective and reliable emergency messaging over distances up to 100 miles. 73, Skip KH6TY NBEMS Development Team -------------------------------------------------------------------------------- No virus found in this incoming message. Checked by AVG Free Edition. Version: 7.5.516 / Virus Database: 269.21.2/1305 - Release Date: 2/29/2008 6:32 PM
