PDF of this tutorial- 2 MDS channels 2150–2162 MHz
- 16 ITFS channels A through D Group 2500–2596 MHz
- 8 MMDS channels E and F Group 2596–2644 MHz
- 4 ITFS G Group interleaved with 3 MMDS H1/2/3 channels 2644–2686 MHz
In many other countries, a similar amount of downstream spectrum has been assigned for MMDS use within the range of 2 GHz to 3 GHz.
A major change is occurring in the MMDS industry with the transition to digital-video compression and transmission. The digital technology enables compression of at least five video streams of similar resolution to NTSC analog video into one 6-MHz RF channel. In the digital environment, an operator who has access to most of the downstream channels listed above can offer a selection of program streams that can aggressively compete with either direct broadcast satellite (DBS) or CATV entertainment video-delivery systems with a few channels to spare.
With the spectral efficiencies of digital-video compression, a few RF channels per MMDS system can be dedicated to provide broadband (>10–Mbps) high-speed data service to Internet users and provide an additional revenue stream for MMDS operators. This is already authorized by the FCC for the case of RF downstream with telco line upstream. To meet the FCC guidelines, the downstream transmission must be substantially similar to one of the presently authorized digital-video modulation formats for cochannel and adjacent-channel interference reasons. A single 6-MHz RF channel using 64–QAM can deliver a raw data rate of 30–Mbps or 27–Mbps after forward error correction (FEC).
Several operators have applied for developmental licenses for various cities to test data access schemes with RF return path from subscriber to hub. A consortium of several MMDS industry participants has been formed and is funding theoretical studies and conducting lab and field tests. These activities are intended to support a formal request to the FCC for rulemaking to authorize standard rules to permit RF two-way data access via MMDS.
Assuming that most of the MMDS RF channels are dedicated to an entertainment video-delivery service, only a limited number of channels are available for downstream data. A conversion to digital video may be an attractive way to increase the capacity of systems and the channels available for data. It is an alternative to antenna sectorization.
MMDS/WCS transmitter systems for Internet access include channelized transmitters, channel combiners, automatic backup, and network management equipment. Digital transmitters accept a 44-MHz centered QAM IF then upconvert and amplify the signal. Transmitters are available with 5 W to 100 W of average output power. Crystal oscillators are used to provide excellent phase noise. Feedforward amplifiers and equalization techniques provide good gain and phase linearity while minimizing power consumption. To improve system availability, an automatic backup system with primary and backup transmitters can be used. Spectral shaping filters are required for WCS systems to meet stringent FCC specifications for out-of-band power. Channel combiners utilize waveguide directional filters to combine either non-adjacent or adjacent microwave channels for transmission. Transmitter systems are supplied with signaling network-management protocol (SNMP) capability for remote configuration and fault management.
Figure 6. A Typical MMDS/WCS Transmitter Block Diagram
Figure 7. A Typical Transmitter Rack with Two Transmitters
*This module provided courtesy of the ITS division of ADC.
