LMDS is a broadband wireless line-of-sight point-to-multipoint communication system that operates typically in the 24-31GHz bands. LMDS systems use a cellular-like network architecture similar to mobile networks, except that LMDS delivers network connectivity to stationary buildings and not to mobile users. Current LMDS systems are able to offer data rates of up to 622Mbps at the expense of dedicating a large chunk of allocated spectrum (100-112MHz) to a single subscriber. However, in general, service providers will offer data rates of around 10Mbps because of the need to maximise the number of subscribers given a finite band of spectrum.

Because of its Point-to-Multipoint nature, LMDS has been touted as a cost-effective last mile solution that service providers can adopt to connect their subscribers to a high speed Internet backbone. Not only can it deliver relatively high data rates, but it can also be deployed more rapidly and cheaply than fibre. In addition, LMDS systems are scalable - service coverage can be extended by adding more base stations or by subdividing an existing cell to deal with increasing customer demand. However, network planning and expansion is complicated by frequency interference issues, which affect the placement of base stations and the range of frequencies usable in adjacent and nearby cells.

There are two ways of separating the uplink connection (from the subscriber to the base station) from the downlink connection (from the base station to the subscriber). With Time Division Duplexing (TDD), the subscriber and the base station take turns talking to each other. At any time, both parties will use the entire spectrum allocated for that link. On the other hand, with Frequency Division Duplexing (FDD), the uplink and the downlink use different frequency bands separated by a large guard band to avoid interference (e.g. ETSI recommends a separation of 1008MHz for the 24.5-26.5GHz band that Singapore tried to auction).

Now, since one base station needs to communicate with several sets of Consumer Premises Equipment (CPE), there has to be a way of partitioning the uplink or the downlink frequency band (for the FDD case) / transmission duration (for the TDD case) among all the subscribers served by the base station. The Time Division Multiple Access (TDMA) approach separates the transmissions to the various CPEs in time such that at any instance the base station communicates with only one CPE. Alternatively, using Frequency Division Multiple Access (FDMA), each CPE is allocated a small slice of the spectrum allocated to the uplink or downlink, and transmits simultaneously along with the other CPEs, i.e. their transmissions are separated in frequency.

The radius of an LMDS cell can range from 2.5km to 10km depending on the modulation scheme used and the climate of the region in which it is deployed. The modulation schemes available differ primarily in how efficient they are at using available spectrum. Quadrature Phase Shift Keying (QPSK) provides only 1.5 bits per second per Hz, whereas 64-QAM (Quadrature Amplitude Modulation) pumps out 5 bits per second per Hz. However, higher bandwidth efficiencies require correspondingly higher carrier-to-interference (C/I) ratios to work, which mean smaller cell sizes. Furthermore, one must realise that LMDS transmission is most severely attenuated by rain. In a heavy rain region like Singapore, carrier-grade reliability is achievable up to a range of only about 1.5km using QPSK. 16-QAM and 64-QAM require cell sizes that are too small to be practical.

Last updated on: 12 Jun 2019

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