WLL Architectures | Wireless Local Loop

WLL systems come in several architectures: a PSTN-based Direct Connect network, a Mobile Telephone Switching Office/Mobile Switching Center (MTSO/MSC)-based network, and proprietary networks.

PSTN-Based Direct Connect

There are several key components of the PSTN Direct Connect network:

§  The PSTN-to-Radio Interconnect system, which provides the concentration interface between the WLL and the wireline network

§  The System Controller (SC), which provides radio channel control functions and serves as a performance monitoring concentration point for all cell sites

§  The Base Transceiver Station (BTS), which is the cell site equipment that performs the radio transmit and receive functions

§  The Fixed Wireless Terminal (FWT), which is a fixed radio telephone unit that interfaces to a standard telephone set acting as the transmitter and receiver between the telephone and the base station

§  The Operations and Maintenance Center (OMC), which is responsible for the daily management of the radio network and provides the database and statistics for network management and planning

MTSO/MSC

An MTSO/MSC-based network contains virtually the same components of the PSTN Direct Connect network, except that the MTSO/MSC replaces the PSTN-to-Radio Interconnect system.

The key components of an MTSO/MSC-based network are:

§  Mobile Telephone Switching Office/Mobile Switching Center (MTSO/MSC), which performs the billing and database functions and provides a E1 or T1 interface to the PSTN

§  Cell Site equipment including the Base Transceiver Station (BTS)

§  Fixed Wireless Terminal (FWT)

§  Operations and Maintenance Center (OMC)

For digital systems such as GSM and CDMA, the radio control function is performed at the Base Station Controller (BSC) for GSM or the Centralized Base Site Controller (CBSC) for CDMA.

In GSM systems, there is a Base Station System Controller (BSSC), which includes the Base Station Controller (BSC) and the transcoder. The BSC manages a group of BTSs, acts as the digital processing interface between the BTSs and the MTSO/MSC, and performs GSM-defined call processing.

In CDMA systems, there is a Centralized Base Site Controller (CBSC), which consists of the Mobility Manager (MM) and the transcoder subsystems. The MM provides both mobile and fixed call processing control and performance monitoring for all cell sites as well as subscriber data to the switch.

As in PSTN-based networks, the FWT in MTSO/MSC-based networks is a fixed radio telephone unit that interfaces to a standard telephone set acting as the transmitter and receiver between the telephone and the base station.

Operations and maintenance functions are performed at the OMC. As in PSTN-based networks, the OMC in MTSO/MSC-based networks is responsible for the day-to-day management of the radio network and provides the database and statistics for network management and planning.

The PSTN Direct Connect network is appropriate when there is capacity on the existing local or central office switch. In this case, the switch continues to provide the billing and database functions, the numbering plan, and progress tones. The MTSO/MSC architecture is appropriate for adding a fixed subscriber capability to an already existing cellular mobile network or for offering both fixed and mobile services over the same network.

Proprietary networks

While MTSO/MSC-based and PSTN Direct Connect networks are implemented using existing cellular technologies, proprietary WLL solutions are designed specifically as replacements for wireline-based local loops. One of these proprietary solutions is Nortel's Proximity I, which is used in the UK to provide wireline-equivalent services in the 3.5-GHz band. The TDMA-based system was designed in conjunction with U.K. public operator Ionica, which is the source of the "I" designation. The I Series provides telecommunications service from any host network switch, providing toll quality voice, data, and fax services. The system is switch independent and is transparent to DTMF tones and switch features.

The Proximity I system architecture consists of the following main elements:

§  Residential service system (RSS), which is installed at the customer premises and provides a wireless link to the base station

§  Base station, which provides the connection between the customer's RSS and the PSTN

§  Operations, Administration, and Maintenance system, which provides such functions as radio link performance management and billing

Residential Service System (RSS). The RSS offers two lines, which can be assigned for both residential and home office use, or for two customers in the same 2-km area. Once an RSS is installed, the performance of the wireless link is virtually indistinguishable from a traditional wired link. The wireless link is able to handle high-speed fax and data via standard modems, as well as voice. The system supports subscriber features such as call transfer, intercom, conference call, and call pick-up.

The RSS has several components: a transceiver unit, residential junction unit (RSU), network interface unit, and power supply. The transceiver unit consists of an integral 30 cm octagonal array antenna with a radio transceiver encased within a weatherproof enclosure. The enclosure is mounted on the customer premises and points toward the local base station.

The RJU goes inside the house where it interfaces with existing wiring and telephone equipment. The Proximity I system supports two 32-Kbps links for every house, enabling subscribers to have a voice conversation and data connection for fax or Internet access at the same time. At this writing, work is under way to develop systems that can handle ISDN speeds of 64 Kbps and beyond. Further developments will result in RSSs that can handle more lines per unit for medium-sized businesses or apartment blocks.

The network interface unit, mounted internally or externally, is a cable junction box that accepts connections from customer premises wiring. The unit also provides access for service provider diagnostics and contains lightening protection circuitry.

The power unit is usually mounted internally and connects to the local power supply (110/220V AC). The power unit provides the DC supply to the transceiver unit. A rechargeable battery takes over in the event of a power failure and is capable of providing 12 hours of standby and 30 minutes of talk time.

Base station. The base station contains the radio frequency equipment for the microwave link between the customer's RSS and the PSTN, along with subsystems for call-signal processing, frequency reference, and network management. This connection is via radio to the RSS and by microwave radio, optical fiber, or wireline to the local exchange. The base station is modular and can be configured to meet a range of subscriber densities and traffic requirements. The base station has several components: transceiver microwave unit, cabinet, power supply, and network management module.

The base station's dual antenna transceiver microwave unit provides frequency conversion and amplification functions. Each unit provides three RF channels, the frequency of which can be set remotely. The unit can be configured for a maximum of 18 RF channels. The antennas are available in omnidirectional or sectored configurations, depending on population densities and geographical coverage. An omnidirectional system can support 600 or more customers, while a trisectored antenna can serve more than 2000 customers. Base stations in rural areas can be sited up to 20 km from a subscriber's premises.

The base station can be configured with either an internal or external cabinet. The internal cabinet is for location in an equipment room, while the external cabinet is weather-sealed and vandal-proofed for outside locations. Both types of cabinets house the integrated transceiver system, transmission equipment, optional power system, and batteries. A separate power cabinet provides DC power to the base station from the local 110/220-V AC source. This cabinet may include battery backup with battery management capability and power distribution panel that provides power for technicians' test equipment. The network management module is the base station polls individual RSS units to flag potential service degradation. Reports include link bit error rate (BER), signal-to-noise ratio, power supply failure, and the status of the customer standby battery.

The connection from the base stations to the local exchange on the PSTN is via the V5.2 open standard interface. In addition to facilitating interconnections between multivendor systems, this interface enables operators to take full advantage of Proximity I's ability to maximize spectrum utilization through allocation of finite spectrum on a dynamic per-call basis, rather than on a per-customer basis. Concentration allows the same finite spectrum to be shared across a much larger number of customers, producing large savings in infrastructure, installation, and operations costs for the network operator.

Operations, administration, and maintenance. OA&M functions are implemented through an element manager accessed through a field engineering terminal. In Nortel's Proximity I, the element manager is built around Hewlett Packard's OpenView. Communications with the network of base stations and customer equipment is done through the Airside Management Protocol, which is based on the OSI Common Management Information Protocol (CMIP). The field engineering terminal can operate in a remote operations center, but is primarily intended for use by on-site maintenance engineers who are responsible for the proper operation of the base stations.

All the applications software in the customer premises equipment is downloadable from the element manager. This software provides the algorithms which convert analog voice signals into 32-Kbps digital ADPCM, which provides toll quality voice transmission. Other applications software includes algorithms for controlling the draw of battery-delivered power, in the event of a 110/220-V AC power failure.

Via the Air Interface Protocol, the customer equipment is able to provide the element manager with information about its current status and performance, the most useful of which are measurements taken during the transmission of speech. This allows the management system to flag performance degradation for corrective action.

WLL Operation | Wireless Local Loop

As in a cellular network, a wireless local loop consists of cells that cover a specific region. Each cell contains a Base Transceiver Subsystem (BTS). The subscriber is normally in an office or home where a fixed subscriber station is installed to communicate wirelessly with the nearest BTS. The subscriber station appears and functions much like a wireline phone. The information received by the antennas of the BTS is processed and is then sent through T1/E1 land lines to the Base Station Controller (BSC) for connection to the Public Switched Telephone Network (PSTN).

Each cell can be further divided into sectors to achieve greater capacity. In the case of WLL, up to nine sectors can be accommodated, dramatically increasing system capacity. Various antenna configurations can be arranged to serve sectors with varying sizes in line with traffic demands. In doing so, coverage is adjusted for unevenly distributed subscriber populations surrounding the BTS. The BTS is strategically placed where it can cover the largest geographic area, forcing issues such as terrain to be taken into account during the planning stage. In the case of shopping centers, office buildings, or apartment buildings, coverage can be enhanced by using a remote antenna or alternative indoor products.

The BSC manages the signals arriving from different BTSs and connects the wireless network to the local central office switch for access to the PSTN. The BSC and the central office switch communicate through interconnecting T1/E1 links.

The base stations in a WLL system are deployed as needed to provide the necessary geographic coverage, with each base station connected back to the telephone network, typically either by wire or microwave links. In this way, a WLL system resembles a mobile cellular system: each base station supports a cell or several sectors of coverage, servicing subscribers within the coverage area and providing the link back to the PSTN. The extent of the coverage area is determined by the transmit power and the frequencies at which the base station and subscriber terminal radios operate, by the associated local propagation characteristics as a function of local geography and terrain, and by the radiation patterns of the base station and subscriber terminal antennas. In WLL systems that do not support user mobility, some reductions in cost can be obtained by optimizing the base station design and its site coverage patterns to best serve the known fixed subscriber locations.

The number of base stations that must be deployed depends on the anticipated traffic to be supported, on the capacity of the system, on the availability of base station sites, on the range of coverage provided by the system and by local propagation characteristics, and on the bandwidth that is available for use by the WLL network. Different systems offer different degrees of spectral efficiency in terms of users supportable per unit bandwidth, providing an advantage to high capacity systems in regions with high subscriber densities or with severely constrained access to bandwidth. In general, though, the greater the available bandwidth, the greater the capacity of the deployed network.

As noted, subscribers to a WLL system are linked via radio to a network of radio base stations which, in turn, are tied by a backhaul network to the PSTN. The WLL system's interface to the telephone network is then supported either by its own switch or through direct connection to the local exchange.

WLL systems are available that incorporate their own switch or that only connect to one or more specific switches. In part, this approach to WLL system architecture has reflected the difficulty of supporting direct connection to the wide variety of switches globally deployed. This is changing, however, with the adoption of V5.2 standard interfaces. It may also reflect a given WLL system's heritage as an adaptation of cellular technology designed for the support of mobile services. Because of the history of mobile services as competitive independent networks distinct from wireline service providers, cellular systems have been developed for use with specific mobile switches.

Direct connection, either through analog or digital interfaces, to the central office switch can allow the use of existing but underutilized switching resources. Such capacity left idle while waiting for full deployment of the local loop represents an inefficient and costly use of resources that effectively increases the switch's contribution to the overall cost per subscriber.

Analog two- or four-wire interfaces are necessary for copper line local loops, but they represent a cumbersome and relatively expensive interface when used with WLL technology. Digital interfaces, on the other hand, can be more convenient and less expensive, but compatibility between a specific switch and WLL system cannot always be ensured.

Technological Approaches | Wireless Local Loop

Wireless local loop technology falls into four general categories: analog cellular, digital cellular, proprietary fixed wireless, and cordless telecommunications.

First-generation wireless local loops are based on analog cellular technologies, which have a solid track record of performance, support a relatively wide coverage area, and provide economies of scale for infrastructure and handsets. The key drawback with analog cellular is that it is optimized for mobility rather than local loop service; since user bit rates are low, wireline voice quality is elusive.

Digital cellular offers greater capacity and better voice quality than analog cellular, but it too is geared toward mobile applications. Coverage areas are usually smaller than with analog and there is a profusion of industry standards. Even when there is general agreement on standards—as there is with GSM, for example—the frequencies over which it operates may differ from country to country.

Proprietary systems—such Nortel's Proximity I Series—are designed from the start as alternatives to the copper-based local loop. They operate at higher frequencies, usually 3.5 GHz or above, where the spectrum is less crowded. They provide high-quality voice at 32 Kbps via Adaptive Differential Pulse Code Modulation (ADPCM)—versus 8 Kbps or 13 Kbps used in most WLL systems—and support voice-band data modems and high-speed fax transmission at up to 28.8 Kbps. These systems are interoperable with the PSTN and are aimed at new operators in competitive markets, where the challenger must be able to match the advanced service offerings of the wired incumbent to have any chance of success. The key disadvantage with these proprietary approaches is that they do not usually support mobility.

Some WLL systems make use of the standardized cordless telecommunications systems—including CT2, DECT, PACS, and PHS. While all are well suited for deployment in dense urban areas, and offer higher quality voice at 32 Kbps and data services up to 28.8 Kbps, each has its strong points. CT2 (Cordless Telephone Two) makes a good pair-gain system for countries with an existing, but insufficient, feeder network infrastructure. DECT (Digital Enhanced Cordless Telecommunications) is a proven technology widely used in Europe in wireless PBX implementations, while PHS (Personal Handyphone System) has been successful in Japan and is headed toward becoming a pan-Asian standard.

PHS offers high-quality, low-cost mobile telephone services using a fully digital system operating in the 1.9-GHz spectrum. Originally developed by NTT, the Japanese telecommunications giant, PHS is based on GSM technology. In addition to personal communications, PHS is being used in wireless PBX and wireless local loop applications.

PACS (Personal Access Communications System) is based on Bellcore's Wireless Access Communications System (WACS) and on Japan's PHS. Operating in the 1.9-GHz licensed PCS band, PACS provides an approach to PCS that is fully compatible with the local exchange telephone network and interoperable with existing cellular systems. PACS supports mobility better than the other standards—at vehicular speeds at over 65 miles per hour—and it also can be used for pedestrian venues, commuting routes, and indoor wireless. Although some vendors are now getting behind PACS in the United States, they face an uphill battle for market acceptance, if only because they got off to a late start—the major service providers having already committed to their WLL strategies.

All of these technologies work well enough and the costs are very attractive. In fact, WLL technology is now far more economical than copper-based local loops. Wireline local loops now cost anywhere from $1000 to $2000 per subscriber to provision, depending on the distance of the subscriber to the central office switch, while wireless local loops are down to about $500 per subscriber, regardless of the distance of the subscriber to the local switch.

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.

Figure 9.1: The fixed wireless terminal is installed at the customer location. It connects several standard terminal devices (telephone, answering machine, fax, computer) to the nearest cell site Base Transceiver Station (BTS).

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.