Centrex or PBX: Wireless Local Loop

A wireless local loop (WLL) is a generic term for an access system that uses a wireless link to connect subscribers to their local exchange in place of conventional copper cabling. Wireless local loop—also known as fixed wireless access (FWA), or simply fixed radio—entails the use of analog or digital radio technology to provide telephone, facsimile and data services to business and residential subscribers. Depending on the existing telecommunications infrastructure, demand for services, and local market conditions, this technology can be both a substitute and a complement to copper wire in the local loop. WLL systems can help eliminate the backlog of orders for telephone service, which is estimated at over 50 million lines worldwide.

There are many WLL technologies operating in several radio frequencies and which adhere to different wireless standards. Most of them operate in a similar manner as cellular telephony, but WLL is fixed, not mobile. WLL systems provide rapid deployment of basic phone service in areas where the terrain or telecommunications development makes installation of traditional wireline service less attractive and less cost-effective. WLL systems can be easily integrated into the wireline public switched telephone network (PSTN) and can usually be deployed within a month of equipment delivery, far more quickly than traditional wireline installations which can take several months for initial deployment and years to grow capacity to meet the continually growing demand for communication services.

WLL systems also offer increased implementation and design flexibility. They can be used to provide first-line communication services in areas where there is no wireline infrastructure, or they can be implemented selectively as alternatives for wired feeder, distribution, or drop, as well as in competitive situations where there is convergence of the fixed and mobile markets.

WLL systems require minimal planning and can be deployed quickly, offering first-line telephone service to thousands of subscribers in a matter of months, instead of years. This is because operators can avoid having to deal with frequent wired local loop build-out issues which can be capital intensive. With WLL systems, construction costs are minimal and there is no need to arrange for rights-of-way for buried cable, both of which can dramatically slow down first-line service growth.

With WLL systems, operators can deliver service where it is needed, when it is needed—helping to reduce financial risk by ensuring faster payback on capital investments, especially if the system adheres to industry-accepted standards and protocols. Open standards and protocols allow operators to create efficient multiple-vendor systems, basing their technology planning and purchasing decisions on quality, effectiveness, and value without being locked into a single equipment vendor.

WLL technology is also generally compatible with existing operations support systems (OSS), as well as existing transmission and distribution systems. WLL systems are scalable, enabling operators to leverage their previous infrastructure investments as the system grows.

WLL solutions include analog systems for medium-to-low-density and rural applications. For high-density, high-growth urban and suburban locations, there are WLL solutions based on the digital standard optimal for wireless local loop use, Code Division Multiple Access, or CDMA. TDMA (Time Division Multiple Access) and GSM (Global System for Mobile telecommunications) systems are also offered. In addition to being able to provide higher voice quality than analog systems, digital WLL systems are able to support higher-speed fax and data services.

Although WLL systems are often based on mobile wireless technology, it is principally a fixed service. With the location of the subscribers known, a WLL system deployment can be tailored to provide user coverage at less cost than a comparable mobile system. However, WLL vendors such as Ericsson, Lucent Technologies, Motorola, Nokia, and Nortel offer complete network solutions that can serve both WLL and mobile cellular subscribers. The difference between fully mobile and WLL subscribers is the tariffing and numbering arrangements. WLL customers are typically charged using wireline tariffs and the numbering plan is similar to the wireline numbering plan. Mobile subscribers are charged according to mobile tariffs and they may have a different numbering space.

WLL subscribers receive phone service through a radio unit linked to the PSTN via a local base station. The radio unit consists of a transceiver, power supply, and antenna. It operates off AC- or DC-power and may be mounted indoors or outdoors, and it usually includes battery back-up for use during line power outages. On the customer side, the radio unit connects to the premises wiring, enabling the customer to use existing phones, modems, fax machines, and answering devices (Figure 1). The use of a cordless phone can provide mobility within the home or office.

The WLL subscriber has access to all the usual voice and data features, such as caller ID, call forwarding, call waiting, three-way calling, and distinctive ringing. Some radio units provide multiple channels, which are equivalent to having multiple lines. The radio unit offers service operators the advantage of over-the-air programming and activation to minimize service calls and network management costs.

The radio unit contains a coding and decoding unit that converts conventional speech into a digital format during voice transmission and back into a nondigital format for reception. Many TDMA-based WLL systems use the 8-Kbps Enhanced Variable Rate Coder (EVRC), which became a published Telecommunications Industry Association (TIA) standard (IS-127) in January 1997. EVRC provides benefits to both network operators and subscribers.

For operators, the high-quality voice reproduction of the EVRC does not sacrifice the capacity of a network nor the coverage area of a cell site. An 8-Kbps EVRC system, using the same number of cell sites, provides network operators with greater than 100 percent additional capacity than the 13-Kbps voice coders that are deployed in CDMA-based WLL systems. In fact, an 8-Kbps EVRC system requires at least 50 percent fewer cell sites than a comparable 13-Kbps system to provide similar coverage and in-building penetration.

For subscribers, the 8-Kbps EVRC uses a state-of-the-art background noise suppression algorithm to improve the quality of speech in noisy environments typical of urban streets where there is heavy pedestrian and vehicular traffic. This also is an advantage compared with traditional landline phone systems which do not have equivalent noise suppression capabilities.

Depending on vendor, the radio unit may also include special processors to enhance call privacy on analog WLL systems. Voice privacy is enhanced through the use of a Digital Signal Processor (DSP)-based speech coder, an echo canceler, a data encryption algorithm, and an error detection/correction mechanism. To prevent eavesdropping, the low bit rate encoded speech data is encrypted using a private key algorithm, which is randomly generated during a call. The key is used by the DSPs at both ends of the communications link to decrypt the received signal.

The use of a DSPs in the radio units of analog WLL systems also provides subscribers with other benefits, such as improved fax and data transmission.