LMDS: It's all in the plan

By interfacing with new or existing wireline infrastructure, local multipoint distribution service can provide a last-mile solution for broadband service delivery, as well as a wireless local loop alternative.

The LMDS licenses auctioned in March give the successful bidder the exclusive right to use a large chunk of spectrum-150, 1150 or 1300 MHz, depending on the license-in the 28 and 31 GHz bands. It does not stipulate the specific technological solution, nor the range of services that an operator may offer. (LMDS is a broadband wireless access method capable of delivering data, Internet, voice, video and multimedia services over the air at millimeter wave frequencies.)

For broadband wireless operators such as those using LMDS spectrum, determining a compelling business case means planning for the network's successful deployment, rollout and evolution.

These steps include service, cell and network planning. Although these planning functions are interdependent, it is helpful to look at each individually to better understand LMDS' specific requirements.

Service planning drives not only the service provider's ability to generate revenue, but also the efficient operation of the network. It affects both cell planning and choice of network architecture.

Planning incorporates a range of considerations, including the demographics of the coverage area, multiple access methodologies, bandwidth requirements for specific services, appropriate wireline connectivity at both the base station and the customer premises, and future service provisioning opportunities.The large amount of bandwidth associated with millimeter wave fre quency bands distinguishes broadband wireless from other access options, providing increased flexibility in service delivery options.

This effectively opens the market for broadband wireless operators to choose a service set that complements their core competencies and addresses market opportunities.

Demographic data on coverage areas will help the network operator determine an appropriate mix of dedicated and shared access services. These should be coupled with network implementation concepts that allow highly flexible service deployments using both shared and dedicated access channels.

Dedicated access generally is achieved using frequency division multiple access (FDMA) combined with high order modulations such as 16/64 quadrature amplitude modulation. Shared access typically is implemented using time division multiple access (TDMA) combined with lower order modulation schemes such as quaternary phase shift key.

Each of these modulation schemes has varying spectral efficiencies that affect their bandwidth requirements and susceptibility to interference.

Once bandwidth issues are addressed, the operator needs to look at options vis-a-vis specific service types. These will determine connectivity requirements at the base station and the type of network interface unit required in the customer premises.

Two key considerations will determine how the operator proceeds. First is the transport standard adopted for over-the-air transmission. While asynchronous transfer mode is widely favored for its multiservice capabilities, quality of service features and dynamic bandwidth functionality, the potential use of Internet protocol (IP) also has generated some interest.

The second consideration is the ability of a given architecture to provide the required connectivity for current and future services, as well as its ability to integrate with multivendor solutions to ensure flexible and scalable service options.

Typically, large- and medium-sized business connectivity-including teleworking connectivity-is targeted at market entry, because these tend to be less cost-sensitive customers.

In addition, multiple dwelling units (MDUs) and multiple business units (MBUs) present good business cases, provided the in-building wiring solutions are cost-effective. In the MDU and MBU scenario, other technologies such as digital subscriber line or hybrid fiber/coax often are integrated into the customer in-building solution to address the wiring issue.

More cost-sensitive markets for LMDS connectivity are residential and small office/home office (SOHO). These tend to require Ethernet-based Internet access in combination with telephony such as basic service or voice over IP.

Generally, the broadband nature of LMDS licensing, in combination with the varied service connectivity required to address the market, drive the need for a multiservice capable network solution.

ATM provides optimal flexibility in this area (Figure 1). In addition to flexible access and transport, the network implementation requires highly flexible connectivity to other networks such as Internet service providers or the public switched network.

When considering an ATM-based network, there are two basic topologies:

* Centralized switching networks using distributed ATM multiplexer-based radio base stations, and

* Distributed switching networks using ATM base stations with integrated wireless access capabilities. The distributed ATM switching topology offers numerous benefits (Table 1).

Avoid airborne land mines Although the services available over LMDS and other broadband wireless multipoint networks are fixed, the architecture is based on a cellular approach. Effective cell planning considers cell layout, size and sectorization.

It is important to design the cells to accommodate the network's existing and future service requirements because increasing a network's capacity is not simply a matter of adding another cell within the layout.

Because an LMDS network can be built out as demand warrants, there is no increased financial burden inherent in designing with future requirements in mind.

Three key issues inherent in millimeter wave transmissions are considered at the cell planning stage: rainfall effects, line-of-sight requirements and intercell/intracell interference suppression.

Millimeter wave signals associated with transmission in high-frequency bands such as LMDS can be affected by rainfall, causing a reduction in the signal level. While engineered solutions involve dynamic power adaptation and advanced modulation schemes, a higher probability of rainfall in a given area generally will be reflected in smaller cell sizes to ensure adequate coverage and uninterrupted service.

A reduced cell size results in a greater number of cells required to service a given coverage area and increases the number of base stations and associated equipment.

Line-of-sight requirements must be addressed to account for topography and obstructions.

This is one reason operators have shifted their focus from using LMDS as a residential video delivery system to one of servicing business customers-in addition to the obvious business case advantages inherent in serving this more lucrative market.

Adapting a customer premises tower mount on a commercial building to accommodate clear line of sight simply may be a case of increasing the height of the mount. Local bylaws can make this more difficult to achieve in a residential setting.

The highly directionalized nature of antennas designed for use at the customer premises also lets the installer choose the optimal orientation to ensure an unobstructed signal.

Interference issues must be addressed from multiple perspectives, especially where cells are sectorized to maximize frequency reuse. In the absence of detailed cell planning, intracell interference can occur with signals generated from adjacent sector transmitters or as a result of transmitter overshoot.

A requirement for in-band intercell links provides additional interference planning challenges.

Although these issues may pose a daunting challenge for broadband wireless operators, RF design tools are available to address the cell planning issues integrated into the overall network planning function.

Detailed mapping data-including buildings, topography and foliage data-derived from airplanes and imaging satellites provides the input that is processed to arrive at accurate cell layout models for propagation assessments.

The cell layout also needs to address an issue that has a significant real estate component: the availability of suitable tower locations or rooftops as base station sites. Suitable sites often have high associated lease rates, unsuitable infrastructure elements or poor service access performance-a low percentage of potential customer roof-tops within range and in clear sight.

Network planning Many related network planning elements need to be considered carefully during the cell planning process. Cell planning is just one part of a larger, inclusive network planning exercise. In contrast with low-frequency cellular telephony networks, many backbone issues need to be assessed when deploying a broadband wireless network. Among these inter-related network design factors are:

* Cost of backhaul/backbone interconnection

* A potential large recurring expense that can be minimized through planning alternate/backup intercell linking strategies. It can be capitalized through procurement/use of point-to-point wireless linking, which requires clear line of sight and careful range planning

* Backup or alternate intercell linking solutions that give the LMDS operator some degree of freedom to pursue the lowest costs

* The network product deployed should allow for the exploitation of any intercell linking possibility (wireline, fiber or wireless)

* Connections to other networks or points of presence (POPs)

* Planning base station locations to minimize networking needed to connect the LMDS network to these other clouds

* Competitive connection capability that gives the LMDS system operator key bargaining tools to secure the lowest costs* The network product deployed shoul d allow for flexible connectivity to any other network growth strategies

* Often the initial build-out is only a partial coverage of a large city

* How do these first cells fit into the larger coverage plan?

* Does the polarization and frequency plan extend from these first cells into the larger coverage plan?

* What are the costs of adding back-haul/backbone interconnect capacity?

Operators need to understand how future cells affect the network capacity needs. Factors such as intercell interference can severely affect frequency reuse performance and reduce the benefits of future network expansions.

The network product deployed needs to include management of polarization, frequency and sectorization plans.

It is important to allow for various connection options to POPs, as well as various intercell linking scenarios (Figure 2).

Planning for point-to-point wireless intercell linking requires consideration of clear line of sight and radio ranges between connection points.

The outputs of the network planning function include a number of elements needed for full network management, including nodal views of the network, types and locations of equipment, equipment configurations, frequency plans, polarization plans and address data related to connectable customer sites.

Although the issues inherent in providing broadband wireless access to customers may vary from other access options, the planning process is similar. Each technology offers challenges, but the bottom line remains: The customer doesn't care how the services are delivered as long as they get there. LMDS is another way to get there.