Wireless local loop (WLL) is a generic term for an access system that uses wireless links rather than conventional copper wires to connect subscribers to the local telephone company’s switch. Also known as fixed wireless access (FWA) or simply fixed radio, this type of system uses analog or digital radio technology to provide telephone, facsimile, and data services to business and residential subscribers.
WLL systems provide rapid deployment of basic phone service in areas where the terrain or telecommunications development makes installation of traditional wireline service too expensive. WLL systems can be easily integrated into a 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 pent up demand for communication services.
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 Code Division Multiple Access (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 can support higher-speed fax and data services.
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 subscribers receive phone service through a radio unit linked to the PSTN via a local base station.7 The radio unit consists of a transceiver, power supply, and antenna. It operates off ac or dc power, may be mounted indoors or outdoors, and usually includes battery backup for use during line power outages. On the customer side, the radio unit connects to the premise’s wiring, enabling customers to use existing phones, modems, fax machines, and answering devices (Figure 142).
WLL subscribers can access 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.
Figure 142
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 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), a Telecommunications Industry Association (TIA) standard (IS-127). EVRC provides benefits to both network operators and subscribers.
For operators, the high-quality voice reproduction of 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 more than 100-percent additional capacity than the 13-Kbps voice coders 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, 8-Kbps EVRC systems use 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 is also an advantage compared to traditional land-line phone systems, which do not have equivalent noise-suppression capabilities.
Depending on vendor, a radio unit may also include special processors to enhance call privacy on analog WLL systems. Voice privacy is enhanced through the use of a speech coder based on Digital Signal Processor (DSP) technology, an echo canceler, a data encryption algorithm, and an error detection/correction mechanism. To prevent eavesdropping, the speech data (encoded with a low bit rate), 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 improves fax and data transmission.
WLL Architectures
WLL systems come in several architectures: a PSTN-Based Direct Connect network, a Mobile Telephone Switching Office/Mobile Switching Center (MTSO/MSC) network, and proprietary networks.
PSTN-BASED DIRECT CONNECT There are several key components of a PSTN Direct Connect network:
PSTN-TO-RADIO INTERCONNECT SYSTEM Provides the concentration interface between the WLL and wireline network
SYSTEM CONTROLLER (SC) Provides radio channel control functions and serves as a performance monitoring concentration point for all cell sites
BASE TRANSCEIVER STATION (BTS) Is the cell site equipment that performs the radio transmit and receive functions
FIXED WIRELESS TERMINAL (FWT) 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 CENTER (OMC) 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 T1/E1 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 with 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 UK. 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 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 functions such as radio link performance management and billing.
RESIDENTIAL SERVICE SYSTEM (RSS) The RSS offers two lines that 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 (RJU), network interface unit, and power supply. The transceiver unit consists of an integral 30-cm octogonal 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, allowing 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 lightning-protection circuitry.
The power unit is usually mounted internally and connects to the local power supply (110/220 volts 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 by either 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 2,000 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-volt 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 usage 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 operation 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. The network of base stations and customer equipment communicate 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 that convert analog voice signals into 32-Kbps digital ADPCM, which provides toll-quality voice transmission. Other application software includes algorithms for controlling the draw of battery delivered power, in the event of a 110/220-volt ac power failure.
Via the Air Interface Protocol, customer equipment can 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.
Last Word
Wireless local loops eliminate the need for laying cables and hard-wired connections between a local switch and the subscriber’s premises, resulting in faster service startup and lower installation and maintenance costs. And because the subscriber locations are fixed, the initial deployment of radio base stations need provide coverage only to areas where immediate demand for service is apparent. Once the WLL system is in place, new customers can be added quickly and easily. Such systems support standard analog as well as digital services, and provide the capability to support the evolution to new and enhanced services as the needs of the market evolve.
