Meeting the E911 mandate

The vast differences in wireless E911 Phase II location technology can influence an operator's entire business

In October, wireless service providers must declare their plans for compliance with the FCC's Wireless E911 Phase II mandate. The directive requires wireless carriers to provide a much more specific location of wireless E911 callers to public emergency service agencies - narrowing the area to within tens of meters rather than anywhere within a wireless base station coverage area.

In many cases where wireless operators have not implemented cost-recovery methods, even the current broad criteria for location accuracy is not in effect. For Phase II, however, wireless service providers must begin to comply with the new standard during 2001, even if cost-recovery methods are not available. Then cost-effectiveness will become an even greater issue.

According to FCC guidelines, the wireless network must forward the location and phone number of wireless E911 callers to public safety agencies (Telephony, April 3, page 86). The FCC mandate accommodates two fundamentally different wireless location technology approaches.

Network-based location systems include the capability to locate the E911 caller, which is based in or alongside the wireless network infrastructure. Wireless carriers must identify the location of wireless callers within 100 meters at least 67% of the time and within 300 meters in 95% of all cases.

With handset-based location systems, the wireless subscriber's handset must determine its own location and forward this location via the wireless network to public safety agencies. This approach requires current subscribers to obtain new, modified handsets. These solutions must locate E911 calls to within 50 meters 67% of the time and within 100 meters in 95% of all cases.

Wireless carriers have many alternative technologies and methodologies to best fit their network and business requirements. They can choose myriad network-based and handset-based location solutions and deployment options. Much of the rhetoric to date has focused on the technology aspects of E911 location alternatives, but that provides little scrutiny of these systems as integrated solutions to a business issue.

Sharing options

Shared solutions to the E911 Phase II requirement can be attractive: They can allow carriers to meet FCC requirements more efficiently and less expensively than individual carrier solutions. Furthermore, a location system that accommodates multiple users in the most effective manner can help accelerate cost recovery via value-added services.

Three types of shared solutions are available: third-party service bureaus that lease location services to carriers, contract arrangements in which an owner carrier leases location services to other carriers and consortiums of wireless service provider owners.

Handset-based location schemes are not suitable as shared solutions, but a properly designed network-based solution can perform the location function for a multitude of carriers, reducing costs for each. However, not all network-based location technologies are suited to sharing.

When evaluating individual and shared solutions, wireless carriers should ask the following six questions:

1. Does the technology meet the FCC requirements in terms of accuracy and availability?

2. Does it impinge on current operations and subscriber procedures?

3. Will the solution be cost-effective in the short and long term?

4. Will it work with present and future technologies and standards?

5. Does the product increase the risk of subscriber churn or other subscriber disruption?

6. Does the approach support cost-effective value-added services?

Moreover, operators should consider some possible consequences of wireless E911 solutions.

For example, subscribers should be protected from disruption and ensured of safety. Carriers choosing a handset-based approach must modify or replace in-service handsets to ensure that customers calling for help can be precisely located. Subscribers with old handsets won't be located accurately. Systems employing RF multipath "fingerprinting" methods can require frequent and laborious database updating to keep current with changing coverage area topology.

Some systems require duplicate receivers that must constantly "stare" at each individual access control channel - even if none are in use - for E911 calls or other location calls. If a base station site must monitor 10 channels to cover itself and neighboring base stations, it would need 10 location receivers. Systems based on control-channel location can have only a fraction of a second to identify the caller's location.

Furthermore, tracking of in-motion E911 callers is impossible without more redundant equipment to locate voice channels. Products that do not have flexible deployment methodologies, that cannot adjust to disparate operating environments and that lack scalability limit one's ability to optimize expenditures to operating realities.

Operators must consider still more factors for shared solutions. A location system should support multiple air interfaces and frequencies (PCS and cellular) without additional equipment at wireless base stations, and it should accommodate additional interfaces.

One carrier in a shared solution should be able to expand its coverage area without affecting the entire network. The system should lend itself to geographic and capacity expansion on a per-carrier basis. Finally, the system and its generated information should be able to be segregated to preserve proprietary customer information.

Aiming for transparency

An ideal E911 location system would operate transparently with wireless carriers and their customers, integrate seamlessly with existing wireless networks and require no action on the part of subscribers to obtain the new location capability.

Essentially, four different E911 Phase II technologies and methodologies are available for cellular and PCS providers: handset-based products, RF multipath fingerprint technologies, network-based monitoring products and independent location systems that operate on demand alongside the host network.

Handset-based approaches focus the E911 solution on wireless subscribers, requiring modifications to in-service handsets or replacement of these handsets with a new design. These products, which are not currently available and whose costs are unknown, may be capable of relatively accurate caller location once the handset has had sufficient time after powering up to determine its position. However, subscribers whose handsets are not converted or replaced cannot be located under the new FCC accuracy standards.

Carriers with established customer bases must evaluate the risk and costs associated with the replacement process, whether this process could endanger the retention of current subscribers and if the process could absorb valuable resources into customer service operations. They also must be aware that only customers with modified or new handsets could purchase new location-based value-added services.

RF multipath fingerprinting approaches are a type of network-based technology fixed on the principle that wireless handset radio signal fingerprints seen by a receiving base station change depending on the location of the transmitting wireless handset. This system requires entire wireless service areas to be accurately measured in small increments of distance (in meters) for RF characteristics. A database then is created with the different multipath fingerprints and their associated locations.

When a subscriber makes an E911 call, the system detects the calling handset's RF multipath fingerprint and compares it with its pre-established historical database of location characteristics. Points that are not sufficiently distinct can yield multiple position indications. Stationary callers are particularly challenging in this regard.

Unlike human fingerprints, RF fingerprints can change. Because few wireless calling areas remain static with respect to their RF characteristics (a new structure in a service area can change radio signals for miles around it), an RF fingerprint location system imposes the burden that entire service areas must periodically be re-measured and databases updated to maintain accurate operation.

The third type of E911 product uses network-based monitor, or stare, techniques in which location system radio receivers constantly monitor each wireless network control channel to detect attempts to place an E911 call and then locate the attempting handset during the approximately 100-millisecond period it is transmitting an E911 call attempt on the wireless network control channel.

Monitoring receivers must always be active on control channels, even those not involved with calls to E911, to detect E911 calls. This means that redundant detection equipment is required at all base stations. For example, if a base station site must monitor 10 channels to cover its own location and to assist neighboring base stations in establishing E911 locations, it would need 10 active location receivers, even if none were in use for E911 or other location calls.

This redundancy requires additional capital expense and physical space at base stations. Additional system redundancy is required to track in-motion calls.

Demanding independence

The most effective location system approach is a network overlay that operates independently of handsets and the host wireless networks, provides locations in a dynamic, real-time, as-required mode and functions alongside the host networks in response to its requirements for locations.

This approach is transparent to the wireless service providers and their subscribers; provides accurate location of in-service handsets with no modifications; does not cause user disruption; does not need service area RF measurement or database updating, network reconfiguration and redundant equipment.

Some overlay location systems use time difference of arrival (TDOA) techniques via small location sensors at base stations managed by a central controller located at the wireless switch (Figure 1).

These types of systems are activated when a wireless network receives an E911 call. When an E911 call is placed, the mobile positioning center function electronically notifies the central controller, which instructs the location sensors at the serving base station and nearby base stations to make time of arrival measurements on the voice channel assigned to the calling handset. Software-based digital signal processing is employed to compensate for multipath and other signal distortion.

Based on the time of arrival measurements, the central controller calculates hyperbolas of potential positions between base station pairs using TDOA location techniques. The intersection of these hyperbolas yields the caller's actual location, which then is reported electronically by the wireless service provider to the public safety answering point. The location and reporting process takes place in seconds. If needed, locations can be updated constantly to provide tracking of an in-motion E911 caller.

In a small number of rural settings, where cell sites are too far apart for "hear-ability" of the calling handset at three base station sites, the location sensors are deployed using a combination of TDOA and angle of arrival (Figure 2). This combination can provide the measurements necessary for location calculation with the desired accuracy from as few as two sites.

Multifrequency and multiple air interface support from common equipment enable shared configurations without requiring capital outlays for redundant equipment. In addition, the scalable nature of such a system enables carriers to expand location coverage areas to meet public safety answering position requirements with simple additions to their initial equipment deployment.

A significant advantage of a location system oriented toward the voice channel is that such a system does not need to know anything about any caller, except that the caller to be located is temporarily using a particular voice channel through a specific base station. Important subscriber information does not need to be shared among carriers, and subscriber privacy is maintained.

Shared solutions such as service bureaus and consortiums can offer highly cost-effective, reliable ways to meet the FCC's E911 Phase II mandate and provide for entry into value-added services.