Network planning in GSM systems is moving toward re-engineering. This trend points to the fact that developed countries are facing problems due to overloaded networks and hot spots. Before attempting to implement any re-engineered solution, however, it's essential to consider the reduction of costs in that solution's implementation. For this reason and numerous others, tower-mounted amplifiers (TMAs) are proliferating in today's wireless systems.

In both Global System for Mobile Communications (GSM) and Universal Mobile Telecommunications System (UMTS), TMAs are used to provide a balanced system design. They allow mobile operators to place an equal amount of receiving and transmitting sites.¹ TMAs also enable base stations to receive mobile signals more clearly and in a wider coverage area than they could otherwise achieve.² Mobile network operators can therefore achieve the greatest possible coverage with less base stations, which limits their costs.

Clearly, TMAs are hailed as a benefit-filled solution. Yet the presence of interference must be taken into account. This article focuses on the nonlinear distortion in TMAs as well as that distortion's impact on the overall performance of GSM systems.

When any radio engineer decides to use a TMA, his or her goal is to improve the overall sensitivity of the system. Sensitivity gives the engineer an indication of a receiver's robustness. To do so, it captures a weak signal that directly affects the range of the system.

Sensitivity also reveals the receiver's level of noise immunity. According to Equation 1, sensitivity can be understood as the minimum input power needed to get a suitable signal-to-noise ratio (SNR) at the output of the receiver. For this reason, sensitivity is based on the following: the receiver noise figure, the minimum required signal-to-noise ratio for detection, and the thermal noise of the system.³

S_i,min = NF + n₀ + SNR (dBM)

S_i,min = sensitivity

NF = noise figure of the receiver

SNR = required output signal-to-noise ratio (usually related to the acceptable bit error rate)

n₀ = thermal noise power of the receiver, n₀ = KTB, where K is the Boltzman constant, T stands for temperature, and B is the bandwidth of the system

In Equation 1, the temperature is defined by the environment in which the TMA will be installed. As a result, B is the GSM bandwidth, 200 KHz. K is a constant. SNR is imposed by the modulation technique. Hence, the only free parameter is NF. In a sense, that noise figure provides an idea of the SNR degradation that will occur when the signal traverses the receiver. One possible mathematical definition is expressed in Equation 2:

In a cascade of noisy blocks, the overall equivalent NF is given by Equation 3. By looking closely at Equation 3, one can gather some important information. For instance, the noise figure of the first block will impose the minimum noise figure of the system. (Remember that by definition, the noise figure is always either positive and higher or equal to 1.) In addition, an important conclusion can be made about the gain of the first components: The higher the gain, the higher the desensitization of the next blocks.

The sensitivity of tower-mounted amplifiers can be determined in different ways. FIGURE 1, for example, presents a basic receiving implementation that will be used to see the different effects of each subsystem. As can be seen in Equation 4, cable losses are the dominant factor in the system's noise figure. As a result, these losses are the major limiting factor for a receiver's sensitivity. To compensate for this problem, the idea is to desensitize the cable noise figure. To achieve that goal, manufacturers began providing TMAs.

A schematic TMA system is presented in FIGURE 2. The TMA subsystem is placed near the antenna. That subsystem is somewhat similar to the well-known low-noise block (LNB), which was used for several years in television satellite receivers. The main difference between them is that now, no frequency translation is needed.

To evaluate the effects of introducing a TMA, make some simple calculations. A GSM scenario typically has:

n₀ = −>121 dBm

S_i,min = −105 dBm

SNR = 9 dB (typical value)

Using Equation 1, NF is calculated to be 7 dB (F = 10^(7/10) = 5).

Say the system was designed to have a noise figure of 7 dB. In that instance, a cable will severely degrade the overall system. Taking into account a cable of 3-dB losses, the result will be Equation 5:

The resulting sensitivity will be: S = −102 dBm.

Now, imagine that a typical tower-mounted amplifier is added with NF = 1.7 dB and G = 12 dB. By applying Equation 3, the result will be Equation 6. From Equation 1, it's now possible to calculate the new sensitivity:

S_i = 3 − 121 + 9 = −109 dBm

This new sensitivity value allows better signal reception. Such improved reception can be translated into the maximum coverage area. Using the Friis equation and the Hata-Okumura propagation model, the distances depicted in FIGURE 3 are attained for an urban environment (assuming an emitter with 30 dBm of transmitted power).⁴ Simply by adding a TMA, coverage was improved by 63%.

Clearly, tower-mounted amplifiers provide important sensitivity-improvement benefits. If one does not account for the presence of interference, however, adding a tower-mounted amplifier can become a very bad design decision.³ For example, look at the nonlinear distortion that is generated at the TMA itself.

Because a TMA is an active device, it will generate some form of distortion.⁶ Such distortion is mainly due to the finite amount of energy that can be used from the power supply. For this reason, any amplifier will somehow saturate at a certain amount of input power.

If a designer takes the previous approach, in which the sensitivity of a TMA was studied, no problem will appear. In that scenario, he or she is only dealing with small signal-excursion input signals. When referring to a high-power interferer, however, the scenario changes. The power of that interferer is simply not known.