BTS Architecture Evolution

Andrew Wright, Director, Product Research
Oliver Nesper, DSP Design Engineer

PCS and Cellular basestation designs have dramatically evolved since the analog first-generation systems were originally introduced. Figure 1 below illustrates the single carrier BTS architecture, where each information-bearing RF carrier was amplified and combined at RF prior to propagating to the egress antenna. The ohmic power loss that occurred in the power combining network was typically dismissed as immaterial due to the inherent 50% efficiency associated with each class C amplifier that could be utilized with the constant envelope FM radio waveform. As an alternative, expensive cavity combiners could be employed to mitigate a portion of the ohmic combining loss.



Figure 1 Comparison Between Single Carrier Multi Amplifier and MultiCarrier Single

Amplifier Basestation Architectures

The evolution of second generation cellular communication systems was spurred by the need for more system capacity and a significant increase in the clarity of the voice communication link. This caused digital modulation schemes, which offer a dramatic increase in spectral efficiency, to be utilized. Unfortunately, such schemes do not offer constant RF amplitude envelopes. This implies that highly efficient class C amplifier technologies could not be employed. This sparked a change in BTS architectures because employing linear class AB amplifiers in the same post amplification ohmic combining architecture rapidly caused basestation efficiencies to become unmanageable. This forced the evolution of a multi carrier pre-amplification combining architecture that forms a composite multi carrier signal that is fed to the amplifier assembly. Figure 4 above also illustrates this topology.

Unfortunately, the combination of multiple RF carriers with fluctuating envelopes causes the crest factor or peak-to-average statistics of the composite waveform to expand. The amplification of the composite multi carrier signal cannot now be faithfully amplified and reproduced in a distortion-free manner by a simple class AB amplifier. To overcome this difficulty linear Feed Forward amplifiers are employed to counter the distortion problems and provide sufficient linearity that spectral regrowth does not pollute adjacent channels. This requirement, however, causes the efficiency of the amplifiers assembly to be further degraded to levels that are typically less than 10%.

The next logical architectural step is to eliminate significant component costs by provisioning a digital baseband carrier combining technology that permits only a single radio up conversion card to be employed. This is illustrated in Figure 2.



Figure 2 Basic Digital Multi-Carrier Single Amplifier Basestation Architectures

This new architecture permits evolutionary technologies such as Predistortion and Waveshaping to be employed which further reduce manufacturing costs, eliminate analog design complexity and simultaneously permit significant increases in power amplifier efficiency to be achieved. This new digital approach is portrayed in Figure 3. The Waveshaping element permits multiple information sources from a plethora of modems to be combined and shifted to baseband carrier frequencies that when translated to RF will form specific RF carriers. Most importantly the combination of random sources always causes very large crest factor waveforms to be generated.

Due to system linearity requirements this significantly impacts linearity because an amplifiers average power operating point needs to backed off to accommodates the signal peaks. Backing off an amplifier significantly degrades efficiency. Waveshaping is a key process that occurs during combining and permits a significant reduction in the crest factor of a multi carrier WCDMA Waveform.



Figure 3 Waveshaped & Predistortion Digital Multi Carrier Amplifier Basestation

Architectures

Digital predistortion is an approach to amplifier linearization that permits the efficiency of the multi carrier amplifier to be dramatically increased. The principle of predistortion is intrinsically very simple, requiring a non-linear distortion function to be built in the numerical digital baseband signal processing domain that is commensurate (“equal”) but opposite to the distortion function exhibited by the amplifier. A highly linear distortion free system is achieved when the cascade of these two non-linear distortion functions equates to a linear system. The beauty of this approach is that the analog power amplifier is permitted to become a simple class AB platform. This frees BTS vendors from the burden and complexity of manufacturing feed forward amplifiers. Moreover, because the amplifier is not burdened with the need for error amplifier distortion correction circuitry, the efficiency of the system is significantly enhanced.

Once this baseband signal processing has been completed a single digital stream is fed to a digital to analog convertor and passed into a single RF up convertor. This in turn is fed to the amplifier and subsequently to the antenna. A desirable attribute of this architecture is the significant reduction in analog circuitry associated with a single radio up convertor system. The difficulties of analog circuit design and manufacturing can not be underestimated and so any approach that significantly reduces this requirement is readily adopted by BTS vendors.

About the Author

Andrew Wright is Director of Wireless and Signal Processing Product Research at PMC-Sierra. Dr Wright is a former co-founder and CTO of Datum Telegraphic Inc. and holds a Ph.D. in Microwave Engineering (meteorology). Since 1995, he has specialized in signal processing solutions for third generation wireless systems.

Oliver Nesper is a DSP Design Engineer in the Access Product Division. He has worked on the development of the PALADIN (Predistortion) and PALADIN Waveshaper products. Prior to that he was with Spectrum Signal Processing as a hardware development engineer working on the design of Soft Radio Receiver Platforms.