Practicalities of private LTE

Long term evolution (LTE) is the standard for wireless communication and it has been a global driver behind current technological and social change. In this article, Tait Communications research leader Dr Clive Horn examines the standard and offers practical advice when considering merging LTE with existing narrowband radio technology.

Rich mobile data applications running on ever ‘smarter’ phones are a vehicle for global social and economic change. Mobile broadband is the key technology that enables devices such as the iPhone to change the world with their ever more impressive data applications. LTE is at the heart of providing that mobile broadband.

Figure 1 illustrates how the demand for commercial data is increasing and overtaking commercial voice.

The adoption of any new technology requires careful analysis prior to deployment, including prediction of the impact on procedures and integration with back-office systems. For this reason, Tait sees the demand for LTE in our industry following at some distance behind commercial demand. However, once its value is fully understood, a rapid uptake of the technology is expected.

Having worked on LTE and P25 for some time, Dr Horn has listed some of the practical things needed when considering adding LTE to current narrowband radio technology.

A solution for data

High-quality reliable voice coverage remains one of the fundamental cornerstones of the industry, and technologies such as P25 are designed for exactly that. The emerging model for mission-critical voice and data networks is one of parallel systems: mission-critical voice with parallel mission-critical data. LTE represents a solution for data.

Directional antennas

Within land mobile radio (LMR) systems, it’s common to operate with single omnidirectional antennas at a high site yielding wide-area coverage. However, to get useful range from broadband technology, directional antennas are generally needed.

Figure 2 shows a test-bed site with the di-pole antennas at the top for P25 and the directional panel antennas for broadband below. In this case, a microwave link for backhaul was used.

Backhaul capability

The data rates at a broadband site can be very high. This has a knock-on effect on the design of the backhaul capability. The system designer needs to look carefully at whether any current backhaul technology can handle the potential data rates from broadband.

Data rates

In P25, the data rate is essentially fixed over the very wide-area coverage. However, with LTE the data rate is related to signal strength.

Figure 3 shows how the data rate decreases with range. This is commonly referred to as “the wedding cake problem”. Further, because the data rate available on the channel is shared amongst all users, the system design must ensure a minimum data rate at the cell edge.

Operational procedures will play a part in controlling the available data rate to each user. For example, the back office may decide they only need to receive streaming video from one cell-edge user and switch off other sources at the cell edge.

Multiple-in, multiple-out

Multiple-in, multiple-out (MIMO) is not usually used in narrowband systems. However, it is a term used widely in broadband. To be more precise, this is referring to spatial multiplexing, where multiantennas are used for both the transmitter and receiver. For example, 2x2 MIMO uses two transmitting antennas and two receiving antennas to double the peak data rate, compared with a single antenna system. Generally MIMO requires good signal strength to operate. It’s also important to note that the parallel transmitted signals traverse the ether via different paths (diversity). If they fail to do this, it may not be possible to unravel the signals at the terminal.

The practical upshot of this is that because MIMO works well close to the cell centre in a multipath environment, it is reliable in an urban environment but may not be suitable in a rural environment.

Beam forming

Beam forming using multiple antennas is another topical technology we see in LTE that is not used in P25 systems. Simply put, it means transmitting a pair (or more) of signals from the antennas so that when the signals arrive at a location, they combine to raise the signal energy in the target area. Remember, the more signal power available, the better the data rate.

However, the peak of energy may cover only a small area. Beam forming is useful to boost the signal to a cell-edge user, but may not improve the signal for geographically spread users.

Diversity

Public safety systems have used diversity for many years. Systems that use voting on the uplink are implicitly using a form of diversity - in this case, selection diversity.

Spatial diversity (multiple antennas) is great news for data transmission in a mobile environment. Technically, it means if the same signal is transmitted via more than one path to a user, the receiver can combine the signals to get better detection.

For diversity to work well, the two paths need to experience different fading. In an urban or suburban environment, the paths are highly likely to be different because there are so many to choose from. LTE employs diversity, for instance using the multiple antennas at a base site to transmit the same signal down to a user.

However, in a rural environment with a high-site base station, the benefits may not be so great. In this scenario, the two paths are practically the same (not sufficiently diverse), hence only a modest benefit other than the usual summing of signal powers. In a rural environment LTE may not offer the benefits expected, at least not from a single base site.

LTE is a good technical solution for high-speed data; however, like anything, expert engineering design is essential to make the solution perform to expectations.