Jan 1, 2012 12:00 AM, By Robert C. Shapiro, P.E., Julius Catral and Bryan K. Jackson

Despite their profound differences, it is possible to layer P25 and LTE systems, especially when an RF planning tool is employed

With the advancement of land-mobile radio (LMR) and public-safety wireless technologies, there is a need to layer services onto common infrastructure in order to lower capital and operational expenses. One example would be to layer Long-Term Evolution (LTE) broadband services operating in the 700 MHz band onto new or existing Project 25–compliant digital radio systems operating on 700 MHz or 800 MHz airwaves.

Traditionally, P25 systems have utilized tall radio towers or rooftops because the need for coverage outweighs the need for capacity in most public-safety scenarios. In contrast, LTE networks typically require increased site density in order to meet additional capacity requirements. So, while there is a convergence in the network architectures, the sites are not necessarily identical and careful planning is required to minimize costs.

Link budgeting, antenna systems, tower heights and availability — as well as backhaul requirements — create a need for radio-frequency propagation software that can take these factors into consideration and support multiple design criteria within a harmonized platform. Some of the questions that should be considered are as follows:

• What are the challenges and opportunities that arise with the advent of layered P25 and LTE systems?
• What are the similarities and differences in how antennas are used in P25 and LTE systems?
• How do RF planning tools help operators devise better P25/LTE networks?
• What is the best method for comparing P25 and LTE site requirements?

The first thing to understand is that the RF link budgets associated with outdoor mobile-based P25 and LTE systems are quite different. The link budgets for P25 systems are based on received signal strengths, delivered audio quality (DAQ), and bit error rates (BER). In contrast, LTE systems are based on the achievable end-user connection data rates and system capacity as defined by a target service level agreement, or SLA. One key challenge then is to produce meaningful side-by-side comparisons of these two very different systems.

In order to accomplish this, the target data rate must be converted into a target carrier-to-noise ratio, which can be determined using the formula C/(N+I). At the cell edge of an LTE system that is delivering a target data rate of 100 kbps per user, this value is roughly 23 dB, which is greater than DAQ 4.0 — at this level speech is easily understandable with little noise or distortion. In contrast, typical P25 systems are designed for DAQ 3.4 at the cell edge — C/(N+I)= 17.7 dB — which means that speech is understandable but some noise or distortion is present. 

As a point of reference, for every 3 dB difference in the link budget, the radiated power is cut in half. This leads to a substantial reduction in the coverage footprint per cell for LTE systems compared with P25 systems; as a result, the site count for an LTE system will be considerably higher. Indeed, on average, expect a 3 to 1 ratio between the LTE and the P25 site count, assuming a balanced uplink and downlink system. For data rates greater than 100 kbps (up to 500 kbps) at the cell edge, the disparity is even wider. Indeed, for an LTE system that delivers data rates at the cell edge of 500 kbps per user, the site ratio increases to 4 to 1 or more. 

After reviewing the link budget differences and similarities, the next components to consider are the antennas. P25 systems typically use omnidirectional antennas that are mounted on radio towers, rooftops or water towers. In contrast, LTE sites typically use directional, or “sectorized,” antennas. The range of such antennas is limited compared with omnidirectional antennas, but they offer good signal consistency and data rates. To increase coverage area, sectorized antennas typically are clustered at a site, and usually are tilted to some degree in order to prevent interference with neighboring systems. Modeling of the antenna system is a key consideration in the evaluation of various system design scenarios.

The most effective way to make the RF link budget and antenna system comparisons in a real-world environment is to use radio-frequency planning software, as each deployment will differ based on local geographies, network topology, and the level of interference in the network. 

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© 2012 Penton Media, Inc.