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Explain the types of WLL Techniques. OR Explain LMDS and MMDS with the help of diagram. State its advantages and disadvantages

Subject: Wireless Technology

Topic: Wireless Local Loop

Difficulty: Low

1 Answer
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Local Multipoint Distribution Service (LMDS):

It is a broadband method used for delivering data, voice, internet including video services in 25 GHz frequency and frequencies greater than that.

The propagation characteristics are well defined in the spectrum.

The LMDS system has network configuration similar to the cellular networks.

In US a frequency of 1.3 MHz bandwidth is allocated for LMDS for delivering broadband services. The propagation characteristic of the signals reduces the potential coverage area at one cell site.

The range of LMDS transmitter can be upto 8 km in case of metropolitan regions. In LMDS the signals are transmitted by point to multipoint or broadcast methods. The service offered by LMDS method is dependent on the operator’s choice. Also, the return path from subscribers to base station is of point-to-point transmission method.

Advantages:

LMDS systems can be easily deployed.

The systems are cost effective. Network management, maintenance and operation are possible. The system data rate is very high.

According to customer demands, the architecture is scalable.

Disadvantages:

It needs a large number of base stations because of its short range of operation from the base stations

The LMDS services include voice, video, data signal as different combinations.

Multichannel Multipoint Distributed Service (MMDS):

The FCC has assigned five bands of frequency in U.S. in the range from 2.15 GHz to 2.68 GHz for fixed wireless access that uses multichannel multipoint distributed service (MMDS).

In 1970’s two licensed frequency bands were allocated for television broadcasting. They are called Multipoint Distribution Service (MDS). Then the FCC enhanced the frequency band allocations as allowed Multichannel MDS (MMDS) services. However, there is a constraint regarding the service area in km and transmitted power as the FCC did not allow the power transmitted of the base station (BS) serving and area that is more than 50 km. It is a drawback of the MMDS.

Advantages:

Because of the larger wavelength of the signal, the MMDS signals are susceptible to the train absorption.

Compared to LMDS signals, the wavelengths of the MMDS signals are larger. This allows the signals to travel a long distance any without power losses in the signal.

Base station cost needed for MMDS is lower as it consists equipment’s that can be operated at low frequencies.

Thus, the MMDS method has more advantages than LMDS in wireless Local Loop (WLL) technology.

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