The HF bands from 2-110Mhz are all currently under review by the FCC. There are several interesting bands already available that are underutilized: 27Mhz 49Mhz for instance.
There is the possibility of using some of these bands for low-bandwidth messaging services in 3rd world as well as 1st world. Data rates will be really low, 100-300bps.
- These services would most likely be simplex or half-duplex.
- Transmitter and receiver not working at the same time.
- Quite possibly no separation at all.
The basic SDR would use the cheap consumer grade sound chips for D/A and A/D (96khz). I demonstrated a working system for you using your laptop's sound card. Most voice, FM, AM, SSB, and many low speed data protocols can be handled with these cheap chips. It is possible that somebody will want to do higher speed such as video. They will have to use faster converters.
Here are some of the possible uses of the SDR/SDT:
- broadcast band AM520-1610khz (10khz)
- broadcast band FM 89-110Mhz (200khz) RECEIVE/TRANSMIT (send your MP3s to car and home stereo)
- RFID 13.56Mhz, 125-150Mhz, 865-868Mhz RECEIVE/TRANSMIT
- garage door openers 300-400Mhz TRANSMIT
- GMRS/FRS 400Mhz (Used by our metro-messaging system.) RECEIVE/TRANSMIT (25khz)
- Original CB 27Mhz RECEIVE/TRANSMIT (10khz)
- Alternative CB, toys, baby monitors, RC 49Mhz RECEIVE/TRANSMIT or TRANSMIT only (20khz)
- RC 72Mhz TRANSMIT (50khz)
- White Space various 50-700Mhz RECEIVE/TRANSMIT
- Discontinued analog TV 50-700Mhz (for third world use, will require additional hardware to support) RECEIVE
- Cordless phones 902-928Mhz (30khz)
- 2G cellular (30khz)
- PCS cellular 901-940 (50khz)
Below 27Mhz has mostly been abandoned for VHF. The band has advantages of obstacle penetration, not completely line of sight limited, and freedom from interference. Particularly useful in third world and developing countries.