Verifying talk-back coverage

Last month we talked about methods for conducting drive-test surveys with emphasis on talk-out, or downlink measurements. We neglected to mention talk-back, or uplink performance. The purpose of this column is to examine two methods for measuring uplink performance: (1) direct measurement of the uplink and (2) estimating uplink performance from downlink measurements.

Let’s first consider direct measurement. Although not common, direct measurement of the uplink is feasible today using modern computer-controlled test receivers and GPS. A direct uplink measurement requires a test transmitter in the drive-test vehicle. For convenience, the same drive-test vehicle can serve as the uplink transmitter and the downlink receiver, but one must be careful to not desense the downlink receiver with the uplink transmitter.

A test receiver must be installed at each repeater site to continuously collect and record measurements. For large networks, the number of required test receivers may make the direct uplink test impractical.

Unlike the downlink drive-test survey, vehicle location is not available at the receiver — it is available at the transmitter. Thus, measurements must be post-processed and time-synchronized using GPS time from each test station, so latitude and longitude can be applied correctly to each receiver measurement.

The observant reader will recall that each measurement is actually a linear average of at least 50 subsamples over a distance of at least 40 wavelengths. It is unlikely that the receiver will know in real time when the transmitter has traveled 40 wavelengths, so the subsamples must be stored locally and averaging is done as part of post-processing. Thus, the storage requirements for the uplink test are more demanding than they are in the downlink drive-test survey.

Furthermore, we must remember that each receiver tells us the performance at just one repeater site. If the network employs voting receivers, further processing must be done to simulate the voting algorithm and pick the strongest signal. (Voters actually select the best receiver based on some measure of signal-to-noise ratio, not signal strength, so this is an approximation.)

At this point, we might be a bit discouraged. So is there an alternative? Yes, we can estimate uplink performance from downlink measurements if we employ two important principles:

• Antenna reciprocity: The principle of antenna reciprocity guarantees that the antenna pattern is the same whether the antenna is receiving or transmitting.

• Local mean is nearly constant between uplink and downlink. Although instantaneous measurements of the uplink and downlink in Rayleigh fading are completely uncorrelated because of frequency separation (except for simplex systems), the local mean amplitude of the uplink signal is nearly identical to the downlink signal (for the same effective radiated power, or ERP) for the most widely used bands [1].

So uplink performance follows downlink performance. But we’re not done yet. There are still important differences between the uplink and downlink paths. The downlink transmitter typically operates with more power than the uplink transmitter (e.g., 100 W versus 3 W) and the uplink receiver system typically has better sensitivity than the downlink receiver because of the tower-top amplifier (e.g., -126 dBm at the antenna port versus -118 dBm).

What we need at this point is a performance metric common to the uplink and the downlink. We’ll use this metric to scale the downlink measurements to quantify uplink performance. The metric of choice is the maximum path loss, or L_max, which is the path loss that results in a received signal exactly equal to the receiver’s static sensitivity. For example, the 12 dB SINAD sensitivity of a typical 800 MHz portable radio is -118 dBm. If the ERP of the repeater site is 57 dBm (500 W) and the receive antenna gain is 0 dBd, then the maximum path loss, using Equation 1, is 179.2 dB.

Now, let’s compute the maximum path loss on the uplink. We’ll assume the receiver system sensitivity referenced to the antenna port is -126 dBm and the portable radio operates with an ERP of 35 dBm (3 W). The receive antenna gain, G_r, is 10 dBd. We’ll also assume that the repeater site uses a duplexed antenna or equivalently, that the transmit and receive antennas are identical and at the same tower height. Using Equation 1, we find that the maximum path loss for the uplink is 175.2 dB.

The link imbalance is the difference between the downlink maximum path loss and uplink maximum path loss and can be found by applying Equation 2. A positive value of link imbalance indicates a system that favors the downlink, while a negative value indicates a system that favors the uplink. A perfectly balanced system has a link imbalance of 0 dB.

For our duplexed antenna case, the link imbalance is ▸=179.2-175.2=4 dB. To estimate uplink coverage for this case, we should scale each of our downlink measurements downward by 4 dB.

Typically, downlink performance is measured by computing the service area reliability for a particular service threshold, say -101 dBm. The service threshold is only valid for a certain value of receiver sensitivity, which, as we have noted, is quite different for the uplink and the downlink. Thus, when we scale the downlink measurement by 4 dB, we are not saying that the signal at the repeater site is 4 dB less than at the portable. Instead, we are saying that the signal is 4 dB closer to the receiver noise floor.

An imbalance favoring the downlink is typical of land mobile radio systems, but system designers are aware of this imbalance, and they usually apply the following techniques to help correct it:

• higher gain receive antenna at the repeater site,

• antenna diversity,

• voting receivers.

A higher-gain receive antenna is the simplest solution, but it will have a narrower vertical beamwidth and will therefore perform poorly near the repeater site. There also are practical limitations to antenna gain. For an omnidirectional 800 MHz antennas, for example, most manufacturers top out at 12 dBd.

Antenna diversity almost always is used in cellular networks, but it’s rarely used in public-safety radio networks.

Conversely, voting receivers are commonly used in public-safety networks, but rarely used in cellular networks. Note that a voting receiver system actually is a form of antenna diversity, with the antennas separated by miles rather than feet.

So how do we estimate uplink performance when one or more of these techniques are used? To answer this question, let’s consider a second example where voting receivers are used to help correct the link imbalance. In this case, the network consists of three simulcast repeater sites with directional transmit antennas, omnidirectional receive antennas and voting receivers (see Figure 1 on page 70).

We can no longer apply antenna reciprocity directly because reciprocity only applies if the same antenna is used for transmit and receive. Thus, we cannot simply scale downlink drive-test measurements to estimate uplink performance. Instead, we need drive-test measurements from the omnidirectional antennas.

But how do we collect measurements from omnidirectional receive antennas? We could revert back to the direct uplink measurement approach with all of its complexity and cost or we could consider an alternative: Transmit a test signal from an omnidirectional antenna.

If the system does not employ tower-top amplifiers, one option is to temporarily install a duplexer on the receive antenna line and connect a test transmitter to the duplexer. The duplexer prevents the test transmitter from interfering with the site receivers while introducing some insertion loss.

If tower-top amplifiers are used, an alternative approach is to temporarily install an omnidirectional test antenna on the tower and connect the test transmitter to it. With test transmitters connected to each omnidirectional antenna, we collect downlink measurements in the conventional way and scale them for uplink performance as described above.

Jay Jacobsmeyer is president of Pericle Communications Co., a consulting engineering firm located in Colorado Springs, Colo. He holds bachelor’s and master’s degrees in Electrical Engineering from Virginia Tech and Cornell University, respectively, and has more than 20 years experience as a radio frequency engineer.

References:

1. W.C. Jakes, ed., Microwave Mobile Communications, IEEE Press Reissue, 1994.

2. J.M. Jacobsmeyer and G.W. Weimer, “Guidelines for Conducting Drive Test Surveys for 800 MHz Rebanding,” October 1, 2005. Available at www.pericle.com.

Equation 1

L_max (dB) = ERP (dBm) + 2.2 + G_r (dBd) + 2.2 – P_th(dBm)

where ERP is the effective radiated power,
the factor 2.2 dB is the gain of a half-wavelength dipole,
G_r is the gain of the receive antenna, and
P_th is the sensitivity of the receiver.

Equation 2

▸ = L_max (downlink) – L_max (uplink)