Multichannel Multipoint Distribution Service (MMDS) is a microwave technology that traces its origins to 1972 when it was introduced to provide an analog service called Multipoint Distribution Service (MDS). For many years, MMDS was used for one-way broadcast of television programming, but in early 1999, the FCC opened up this spectrum to allow for twoway transmissions, making it useful for delivering telecommunication services, including high-speed Internet access to homes and businesses.
This technology, which has now been updated to digital, operates in the 2- to 3-GHz range, enabling large amounts of data to be carried over the air from the operator’s antenna towers to small receiving dishes installed at each customer location. The useful signal range of MMDS is about 30 miles, which beats Local Multipoint Distribution Service (LMDS) at 7.5 miles and Digital Subscriber Line (DSL) at 18,000 feet. Furthermore, MMDS is easier and less costly to install than cable service.
With MMDS, a complete package of TV programs can be transmitted to homes and businesses. Since MMDS operates within the frequency range of 2 to 3 GHz, which is much lower than LMDS at 28 to 31 GHz, it can support only up to 24 stations. However, operating at a lower frequency range means that the signals are not as susceptible to interference as those using LMDS technology.
Most of the time the operator receives TV programming via a satellite downlink. Large satellite antennas installed at the head end collect these signals and feed them into encoders that compress and encrypt the programming. The encoded video and audio signals are modulated, via amplitude modulation (AM) and frequency modulation (FM), respectively, to an intermediate frequency (IF) signal.
These IF signals are up-converted to MMDS frequencies and then amplified and combined for delivery to a coaxial cable, which is connected to the transmitting antenna. The antenna can have an omnidirectional or sectional pattern. The small antennas at each subscriber location receive the signals and pass them via a cable to a set-top box connected to the television. If the service also supports highspeed Internet access, a cable also goes to a special modem connected to the subscriber’s PC.
MMDS sends data as fast as 10 Mbps downstream (toward the computer). Typically, service providers offer downstream rates of 512 kbps to 2.0 Mbps, with burst rates up to 5 Mbps whenever spare bandwidth becomes available. Originally, there was a line-of-sight limitation with MMDS technology. But this has been overcome with a complementary technology called Vector Orthogonal Frequency Division Multiplexing (VOFDM).
Because MMDS does not require an unobstructed line of sight between antennas, signals bouncing off objects en route to their destination require a mechanism for being reassembled in their proper order at the receiving site. VOFDM handles this function by leveraging multipath signals, which normally degrade transmissions.
It does this by combining multiple signals at the receiving end to enhance or recreate the transmitted signals. This increases the overall wireless system performance, link quality, and availability. It also increases service providers’ market coverage through non-line-of-sight transmission.
MMDS equipment can be categorized into two types based on the duplexing technology used: Frequency Division Duplexing (FDD) or Time Division Duplexing (TDD). Systems based on FDD are a good solution for voice and bidirectional data because forward and reverse use separate and equally large frequency bands. However, the fixed nature of this scheme limits overall efficiency when used for Internet access. This is so because Internet traffic tends to be “bursty” and asymmetric.
Instead of preassigning bandwidth with FDD, Internet traffic is best supported by a more flexible bandwidth allocation scheme. This is where TDD comes in; it is more efficient because each radio channel is divided into multiple time slots through Time Division Multiple Access (TDMA) technology, which enables multiple channels to be supported.
Because TDD has flexible timeslot allocations, it is better suited for data delivery—specifically Internet traffic. TDD enables service providers to vary uplink and downlink ratios as they add customers and services. Many more users can be supported by the allocation of bandwidth on a nonpredefined basis.
MMDS is being used to fill the gaps in market segments where cable modems and DSL cannot be deployed because of distance limitations and cost concerns. Like these technologies, MMDS provides data services and enhanced video services such as video on demand, as well as Internet access.
MMDS will be another access method to complement a carrier’s existing cable and DSL infrastructure, or it can be used alone for direct competition. With VOFDM technology, MMDS is becoming a workable option that can be deployed cost-effectively to reach urban businesses that do have lineof- sight access and in suburban and rural markets for small businesses and telecommuters.
