Attenuation Solution for TDMA-to-GSM BTS Upgrades
When implementing the evolution of TDMA wireless networks to GSM/GPRS and then W-CDMA, service providers “overlay” GSM/GPRS by integrating a new base transceiver station (BTS) with the existing TDMA BTS. Carrier signals from the new BTS are fed through the TDMA transmit chain to the existing antennas, and receiver resources are shared as well. Signal levels from the GSM BTS amplifiers must be attenuated substantially before they enter the amplifiers of the existing TDMA BTS, and this attenuation should be variable to ensure that the TDMA amplifiers are not overdriven. The AY-H45S combination fixed/variable attenuator makes the attenuation considerably easier and more reliable than mixing and matching various discrete attenuators when optimizing performance of the system.
The GSM/GPRS/W-CDMA route to 3G was chosen by TDMA-based service providers because both technologies have some common signaling and protocol features. GSM increases spectral efficiency (and thus capacity) of the network, and, as the world’s most widely used access method, is supported by a wide variety of handsets and other equipment. The addition of GPRS provides the IP-based core network that allows mobile positioning and wireless data services to be provided along with a smooth transition to EDGE (which can handle three times more data subscribers than GPRS), and then to the third-generation multimedia capabilities of W-CDMA. This evolution is not implemented all at once, since there will continue to be many TDMA subscribers for years to come. In addition, the FCC requires that networks dedicate some capacity to first-generation analog (FM) signals.
The primary initial challenge for service providers making transition is how to most efficiently add a new access method to an existing network. Putting up new cell sites is extraordinarily expensive and not even a possibility in most places. Building a completely independent GSM/GPRS system at existing sites is also expensive, while adding new antennas is often impossible because of tower loading and other restrictions. As a result, service providers have chosen the overlay approach, which is far less expensive than any other alternative, and requires no approval from local government agencies. It also can be applied if there is no possibility of adding antennas to an existing tower because the existing antennas can be shared. The approach makes the transition from technology of one generation to another seamless without service disruption.
The worst-case approach, in which no new antennas can be accommodated, is best described by example. In this case, a Lucent Series II BTS currently handling FM and TDMA traffic is supplemented by an Ericsson GSM BTS within the equipment enclosure. The key to successful deployment of this technique lies in accurately calculating the FM, TDMA and GSM/GPRS power budget based on the specific characteristics of each technology. These characteristics vary between FM (which inherently provides clipping and limits distortion) and TDMA/GSM signals (that have peak-to-average signal levels as high as 2:1). This step is necessary to ensure that the multi-carrier amplifiers in the existing system are never overdriven, which would cause high levels of intermodulation distortion (IMD) to occur.
Once the acceptable combination of each type of signal is determined, the new channels from the GSM base station are fed to an AY-H45S attenuator. It consists of a 50 dB (±2 dB) high power fixed attenuator integrated into the same package with a variable attenuator with continuous resolution from 0 to 15 dB. The value of the fixed attenuator varies with the application, that is, 40 dB (some other value may be desired in some cases). The two-attenuator device can conservatively dissipate 50 W and features a frequency range of 800 to 900 MHz, VSWR less than 1.3 and maximum input power of 50 W CW.
The fixed portion of the attenuator reduces the output (typically +43 dBm) to –5 dBm acceptable for injection into the TDMA amplifiers, and the variable attenuator is used to provide “fine” adjustment. The entrance point of the signals is an unused port on the 9:1 low level power combiner on the Series II BTS (see Figure 1). This combiner directly follows a 4:1 unit that has already combined the signals of the TDMA radios. The additional GSM carriers are then fed from the 9:1 combiner to a 3:1 combiner, preamplifier, and finally the multi-carrier linear power amplifier. This amplifier can deliver an output of 240 W, but is reduced to 160 W at the antenna by factors such as filtering and cable losses.
The role played by the AY-H45S attenuator is simple, but it can make the overlay process easier in several ways. First, it provides a standard, fixed level of attenuation that immediately reduces the signal level from the GSM BTS amplifiers down to a low level. This allows the signals to be fed to the TDMA BTS without the need for high power combiners and other high power components that would be required if the signals were combined after the linear amplifier stage.
The “fine tuning” capability provided by the integrated variable attenuator enables amplifier performance to be optimized. The precise signal input level that provides the best combination of IMD performance and output can then be obtained by connecting a spectrum analyzer to the monitor port on the 9:1 combiner and adjusting the variable attenuator. The AY-H45S eliminates the need for an engineer or technician to have an assortment of attenuators with fixed values on hand in order to reach the desired value, or to use separate fixed and variable attenuators. Each of these alternatives adds cabling and interconnects, potentially reduces reliability, and is inconvenient.
While a Lucent Series II BTS is used here as an example, the values of the attenuators, power-handling ability and frequency range can be customized to suit the requirements of other overlay scenarios, or for that matter any application in which it is convenient to combine a high fixed level of attenuation with the “fine-tuning” ability of a variable attenuator in a single package.
Aeroflex-KDI Integrated Products,