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Explain 4 major technology of WLL system. What are the advantages and disadvantages of this approach?

Subject: Wireless Technology

Topic: Wireless local loop

Difficulty: Medium

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The WLL system can be based on 4 major technologies. They are:

Satellite-Based Systems:

These systems provide telephony services for rural communities and isolated areas such as islands.

Satellite systems are designed for a Gaussian or Rician channel with K factor greater than 7 dB.

These systems can be of two types:

Technology designed specifically for WLL applications and Technology piggy backed onto mobile satellite systems as an adjunct service of these.

The former offers quality and grade of service comparable to wireline access, but it may be expensive.

The latter promises to be less costly but, due to bandwidth restrictions, may not offer the quality and grade of service comparable to plain old telephone service (POTS).

An example of a satellite based technology specifically designed for WLL is the HNS telephony earth station (TES) technology.

Advantages:

• Low bit rate for voice and data.

• Low cost mobile terminals.

• It provides quality and grade of service for WLL applications.

Disadvantages:

• The number of satellites and propagation delay put restrictions on the system design.

• Handover capability is needed e.g. LEO,MEO satellites are in motion relative to the earth’s surface, so they need handover capability for all fixed and cellular applications

Cellular-Based Systems:

These systems provide large power, large range, median subscriber density, and median circuit quality WLL services. Cellular WLL technologies are primarily used to expand the basic telephony services.

This approach offers both mobility and fixed wireless access from the same cellular platform.

For relatively sparsely populated rural and even urban settings, WLL technologies based on existing cellular systems can be economical and rapidly deployable . They include much sophisticated technology (and therefore overhead bandwidth) not necessarily required for the WLL application. The resultant limited user bandwidth represents a fundamental limitation of such systems for WLL.

Advantages: • They provide fixed wireless access and mobility.

• They can be rapidly deployed in rural and urban areas.

• They provide large power and operating range.

• They provide medium circuit and medium subscriber density.

Disadvantages:

• They are not recommended for deployment indoors and in picocells.

• Air interface is complex.

• The user bandwidth is limited.

Fixed Wireless Access Systems

These systems are proprietary radio systems designed specifically for fixed wireless applications, which may or may not be extensible to PCS or cordless.

The primary disadvantage of the cellular approach is its limitation on toll quality voice (new toll-quality vocoders designed for cellular technologies may eliminate this problem), and signaling transparency.

The primary disadvantage of low-tier PCS and microcellular approaches is their range.

FWA systems for zonal areas are designed to cover the local telephone area directly from the PSTN switches. The systems for rural areas provide connection at the remote ends of rural links to the end users.

Advantages:

• Less expensive

• It can be easily installed

• The installation time need is less.

Disadvantages:

• Limitation on toll-quality and signaling transparency.

Low-Tier PCS or Microcellular-Based Systems:

These systems provide low power, small range, high subscriber density, and high circuit quality WLL services. These technologies are considered to facilitate rapid market entry and to expand the capacity of the existing infrastructure.

They are typically operated at 800 MHz, 1.5 GHz, 1.8 GHz, and 1.9 GHz frequency bands

Compared with the cellular-based WLL, more base stations are required to cover the same service area. Operators may consider low-tier WLL technologies when an existing infrastructure is in place to support backhaul.

For densely populated urban environments, WLL technologies based on existing low-tier PCS radio technologies.

Advantages:

• High Subscriber density.

• Low power.

• High circuit quality.

Disadvantages:

• Transmission cost is more.

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