Multiple choices for critical communications

As the expression goes, “When all you have is a hammer, every problem can look like a nail.” One technology does not optimally fit all applications. There are several new technology choices for critical communications, and each addresses the “three Cs” of critical communications: coverage, capacity and cross-agency interoperability.

Coverage

High-powered, high-profile radio sites have historically been the primary infrastructure alternative in land mobile radio. Typically, tower sites provide most of the coverage, but they are not a cost-effective way to reach the dead spots and critical building areas. Most public safety specifications require a 95% level of coverage. The last few percentage points of coverage can drive up the cost of a system exponentially unless there is a cost-effective alternative to traditional tower sites. Furthermore, as the public, local governments and environmental organizations make it more difficult to obtain high-profile tower sites, there is a greater need to use less-obtrusive, low-profile cell sites for local coverage on demand. Let’s examine the operations and benefits of low-profile cell sites and mobile repeater stations, alternative technologies that provide complementary coverage to traditional, high-profile infrastructure.

Cell sites

Cell sites (typically pole-mounted base stations, such as M/A-COM’s OpenSky system) are best suited for low-density applications where a low-power base station (a single channel with two to four talk paths) is required to provide spot or fill-in coverage. The cell site uses a leased telephone line, a microwave link or a spread-spectrum radio connection for network access. The cell site is self-contained, meaning the unit is self-cooled and no separate HVAC is required. The unit is contained in a weatherproof enclosure and equipped with a battery backup.

The cell site can also be used for selective in-building coverage. The primary benefit is that it is easy to deploy — on a telephone pole, a building or an existing tower — lowering the installed cost of coverage as compared to a traditional tower site.

A complete, high-powered, high-profile tower site can cost more than $500,000. This includes site access, power, microwave connectivity, the equipment shelter, the antenna system(s), the tower and lighting, plus the base-station RF equipment. A cell site, however, can be constructed for less than $50,000, making it possible to deploy 10 sites for the cost of one traditional tower site.

Vehicle tactical network (V-TAC)

Cellphone users know it is difficult to achieve ubiquitous, in-building coverage. Although the portable radios used in private wireless networks have five times the transmit power of commercial cellphones (3W vs. 0.6W), in high-loss buildings the possibility still exists that communications will be lost at a critical time. The OpenSky V-TAC is a new application of vehicle repeater technology that provides critical in-building communications. This vehicle tactical network is a time-division, multiple-access-based repeater installed in the vehicle. Historically, in-band vehicular repeaters have been difficult to implement because of self-interference and difficulty in coordinating multiple repeaters at the same scene. The tactical network avoids these issues by using frequency agility and the time separation of TDMA to ensure reliable operations without interference.

A vehicle-tactical network is automatically activated by a user’s portable device when he is unable to directly access the network. This ensures that a radio user entering a building will not lose network communications. Network services and all system features are maintained when the communications path is rerouted to the V-TAC.

Capacity

Spectrum is scarce and costly. In many existing mobile radio networks, current traffic exceeds the capacity of the network, and sufficient radio spectrum is not available for network expansion. New designs must provide a significant increase in capacity for voice and data applications. The following technologies increase capacity:

• Trunking enables multiple groups to independently access shared radio resources.
• Simulcast systems minimize the spectrum required for high-density applications.
• Combining voice and data into one IP network avoids installing separate infrastructures.
• Compression of voice messages using voice-over-Internet protocol increases network backhaul capacity.
• Spectrum efficient TDMA increases capacity without adding spectrum.
• Internet protocol technology increases channel sharing, reliability and redundancy.

Simulcast vs. multisite

A number of comparisons can be made between simulcast systems and multisite networks, but the primary consideration is capacity. Simulcast systems use the same frequencies at all sites in the system. They can be more efficient than multisite networks from a capacity standpoint if a channel at all sites is needed for all group calls. In many cases, however, particular group calls only require a channel at a subset of the sites, such that a multisite network is often a more efficient deployment of scarce spectrum resources and customer budgets. An example of a seven-site simulcast system is shown in Figure 1 on page 30.

Note that all sites repeat each call on the same RF frequency. If the three patrol cars and the ambulance shown in the figure are selected for the same talk group, then three of the sites will be transmitting even though no units on the talk group are within the coverage area of those sites. If this system were configured for multisite operation instead of simulcast, then only the required sites would repeat the group call, as shown in Figure 2 on page 32.

Where FDMA-simulcast fits

Traditionally, simulcast systems, such as Enhanced Digital Access Communications Systems, have been deployed in many medium-sized or large counties and cities where frequencies were insufficient to construct a multisite network. A typical simulcast system for a large city or county might involve more than five sites and more than 20 channels. When a majority of the talk groups on the system are expected to be dispersed across the entire coverage area, or where frequency constraints exist, simulcast may continue to make sense for these systems.

Where multisite fits

Multisite networks are ideal for covering large areas such as a major utility or a statewide system where ample frequencies are available. They will also cover large areas where available frequencies can be sufficiently reused. For cities and counties, multisite can be the best choice if their talk groups are small or stay in local areas, or if there are ample frequencies — which in the past was the primary issue. The enhanced capacity of TDMA has redefined the standard of what is considered an ample quantity of frequencies for building a multisite network. While simulcast still has its place in the system designer’s toolbox, the availability of TDMA technology will now cause some cities and counties to consider a multisite solution. Also, traditional multisite implementations can now achieve much greater capacity by the use of TDMA.

Spectrum-efficient TDMA

TDMA enables simultaneous calls on each RF channel, without dedicated control channels, offering more than a two-to-one cost-to-benefit ratio for capacity over a traditional trunked system. Compared to a traditional frequency-division, multiple-access radio system that requires three frequencies and three base stations (a control channel and two working channels), a two-slot TDMA system requires only one frequency and one base station to achieve the same call capacity. In this case, the TDMA system has three times the spectral efficiency. In a four-slot TDMA configuration, the spectral efficiency advantage increases to five to one. The resulting benefit is less equipment required and less cost per unit of traffic. Figure 3 on page 35 illustrates the effect of TDMA on capacity. The figure compares four-slot TDMA and two-slot TDMA (with control integrated within the working channel) to FDMA systems (with dedicated control channels). The voice users supported by each system are shown for a sample traffic profile to achieve a 1% busy-hour grade-of-service. Two curves are shown for FDMA systems to account for the reduced system capacity caused by the “hang-time,” which is typical for an EDACS system at the end of calls that are made in a message-trunked system.

The data show that to support about 1,000 users, an FDMA system that is not set up for message trunking (no hang time) requires 13 channels (12 working channels and one control channel). A message trunked system requires 16 channels. For the TDMA systems, only six channels and three channels are required for two-slot and four-slot, respectively. The power of TDMA is even more evident when comparing users supported by a fixed number of frequencies. Using the sample voice traffic profile for a trunked site with five frequencies, an FDMA system using message trunking supports about 116 users, increasing to about 155 users without message trunking. The two-slot TDMA system supports 772 users, and a four-slot TDMA system supports more than 2,000 users.

Cross-agency interoperability

Interoperability is crucial for interagency cooperation in public safety and public service applications. When faced with a serious threat to public safety, multiple public safety agencies are typically required to work across agency and jurisdiction boundaries to achieve a coordinated, integrated incident response. Responding agencies must be able to communicate with other cooperating agencies from within and from outside their normal area of jurisdiction.

Some vendors claim that interoperability can be achieved if everyone buys their technology. However, radios and infrastructure should support a range of interoperable solutions to enable interagency communications for coordinated emergency response. Multimode software-based user radios can communicate using various digital and analog modes. Infrastructure supports network interconnection with other analog and digital radio systems. M/A-COM radio users, for example, can take advantage of the following forms of interoperability:

• direct mode connections to conventional analog radios.
• direct mode connections to digital radios using a P25 common air interface as an interoperability standard.
• radio access to conventional mutual aid channels (NPSPAC).
• networked interoperability with users of analog conventional systems.
• networked interoperability with users on other analog and digital trunked systems.

Building a network

Customizing a design based on technology building blocks is the key to successful mission-critical communications systems. The best approach to meeting these communications needs is to first understand the available technologies and the associated performance and cost trade-offs, and then to select the optimum combination of technologies for a specific application. There are many technology choices in critical communications, so use a vendor that can act as an open “technology broker” in making sure that the right technology is applied to a customer’s requirement. With multiple tools in the toolbox, every problem does not have to look like a nail.

Bender is a technology marketing manager and Herther is the director of product integration for the M/A-COM Wireless Systems Business unit. M/A-COM, Inc., a unit of Tyco Electronics, has completed its acquisition of Com-Net Ericsson Critical Radio Systems. This new business is now part of M/A-COM’s Wireless Systems organization. For more information: www.macom-wireless.com or www.opensky.com.