Atmospheric propagation attenuation is known to be higher in the 2.1 GHz band than in the cellular band. Thus, to overcome this performance disadvantage and to maximize cell coverage and capacity, Unity Wireless System's first feedforward multi-carrier linear power amplifier (MCLPA) product has been designed with a high 60 W maximum average output power. The new Tornado MCLPA has been developed to satisfy the emerging 3G W-CDMA market for applications requiring high power and high linearity at a relatively low cost.

The Tornado amplifier uses a classical adaptive control feedforward design (see Figure 1 ), where the main power amplifier (MPA) distortion components are extracted using carrier cancellation in Loop 1 and amplified by the error power amplifier (EPA). The output distortion is then cancelled at the error cancellation coupler in Loop 2 (the error loop).

Direct Distortion Control

The distinguishing features of the Tornado design include a patent-pending direct distortion control (DDC) technology to control Loop 2. This technique is a low cost, ultra high selectivity, high linearity frequency tunable power detector. The DDC circuit works specifically with CDMA-type modulation schemes, including W-CDMA, cdma2000 and IS-95, and provides a number of useful advantages.

The DDC, shown in Figure 2 , first measures the carrier frequencies and then directly minimizes the MCLPA's intermodulation distortion (IMD) at the expected IMD frequencies. In addition to allowing the cancellation to be controlled, it is also possible to perform equalization of any asymmetric IMD components from the MPA, together with any asymmetric cancellation performance in the error loop. Therefore, the DDC approach ensures that the best possible level of IMD performance is obtained at all times and under all conditions.

The DDC is able to select the frequencies that are to be minimized, which means the error loop's instantaneous cancellation bandwidth requirements are lower than that required with a fixed frequency pilot injection system, which improves produceability. The DDC method also does not require a pilot injection system, meaning there is no possibility of leakage or spurious at the MCLPA output. Having no pilot injection also means that there are no error loop offsets due to pilot RF leakage. There is also marginal improvement in efficiency by not having to amplify and cancel a pilot signal. Some pilot recovery schemes also require a third carrier cancellation loop that is not required in this design, thus further improving produceability by eliminating the additional gain, phase and delay matching.

Analog Peak Predistortion

The analog peak predistortion method, shown in Figure 3 , works by cancelling the main path RF with the output from a limiting amplifier and provides both gain and phase correction of the MPA LDMOS soft limiting characteristics near P1dB. The effect of this correction method is that the MPA output power capability is increased by as much as 20 percent for the same level of adjacent channel leakage ratio (ACLR) or, alternatively, for the same output power, the ACLR is improved by approximately 3 dB. By compensating for the nonlinear gain and phase characteristics of the MPA in this way the ACLR is improved by reducing both the AM-to-AM and the AM-to-PM distortion components produced in the MPA.

EPA Enhanced Output Carrier Power

Careful design of Loop 1 lined up together with pilotless control means that there is minimal noise and spurious loading on the EPA, and the total MPA distortion at the EPA output is only approximately 0.5 W rms. This allows the output power capability of the EPA to be put to good use by supplying carrier power in phase with the MPA output. The technique is similar to cross cancellation, but the implementation is simple and non-adaptive because the carrier power being supplied is only around 10 percent of the total MCLPA output power. The output contribution of around 5 W is approximately equal to the level of MPA carrier power that is typically lost into the 50 Ω load of the 10 dB error coupler.

Key Performance Targets

Table 1 lists the key performance specifications of the Tornado MCLPA. To improve propagation in the 2.1 GHz frequency band and maximize cellular communications coverage, the amplifier was designed with a high 60 W (47.8 dBm) maximum average output power.

The Tornado MCLPA is specified to operate with one to four W-CDMA carriers using Test Model 1 with 64 channels and no preclipping. By measuring the complementary cumulative distribution function (CCDF) for a single carrier (see Figure 4 ) it can be seen that the peak-to-average ratio at 0.01 percent probability is as high as 10 dB.

The peak-to-average ratio or crest factor (CF) is defined as the quotient of peak envelope power (PEP) to average power (CF = PEP/Pav), where PEP is defined as the power of the carrier sinusoid taken over one carrier period when its amplitude is at maximum and Pav is proportional to the mean value of the squares of the signal voltage over a long period of time. The required maximum average output power and the CF of 10 dB together determine the peak power capability and dimensions of the MPA, which for Tornado is 900 W P1dB.

Linearity Performance

Using feedforward techniques the ACLR of the MPA output is typically improved by approximately 25 dB to 60 dBc at the MCLPA output, as shown in Figure 5 . Higher than 60 W output power levels would make satisfying the absolute power level requirements of the spectral emissions mask more problematic and further linearization would very likely have been necessary unless the peak-to-average requirements of the input signal were relaxed.

Efficiency

Efficiency has probably become the most important characteristic of an MCLPA as operators seek to reduce fixed operating costs, and governments and corporations seek to satisfy the Kyoto accord on greenhouse emissions. Just five years ago, before the advent of LDMOS and other efficiency enhancements, a bipolar feedforward MCLPA might have had an efficiency of just two to three percent. Using LDMOS transistors and using the worst-case W-CDMA carriers and Test Model 1 with 64 channels, Tornado achieves an efficiency of eight percent. However, using carriers with a more typical peak-to-average power of 8 dB, the MCLPA can be operated at 80 W average output power where the efficiency is close to 10 percent.

Further efficiency improvements, yielding up to 12 percent efficiency, may be obtained by accepting a reduction in ACLR performance of 5 dB and using the pre-feedforward architecture known as advanced cross cancellation. In this special case, the EPA has the same output power capability and characteristics as the MPA, and is used to supply half of the output carrier power. The efficiency improvements are the result of 0.5 dB lower loss after the EPA compared to the MPA. Also, since the MPA and EPA are closely matched, the MPA provides very good predistortion of the EPA, which further improves the overall efficiency of the EPA and MCLPA.

Cost

It is a fact that today's competitive economic environment calls for the minimum possible dollars per watt. This need has been addressed by using three techniques that maximize the output power for a given set of transistors. These techniques include peak power predistortion, a high power, low loss filter delay line, and the use of the EPA to provide additional output carrier power in a similar fashion to cross cancellation. Feedforward amplifiers are also more cost-effective and more efficient at higher output power levels.

The cost of the bill of materials (BOM) was minimised by omitting any non-essential components. The design was also simplified by reducing a DSP board to just a single chip 8051-based microcontroller. A common design for the EPA and driver power amplifier (DPA) was also used, and even something as small as the total number of high Q capacitors was minimised to reduce overall cost.

Conclusion

A minimalist approach in the Tornado MCLPA's unique design has yielded an amplifier that has achieved high power output, excellent linearity performance and low cost to satisfy today's tough requirements for the emerging 3G W-CDMA market. The Tornado MCLPA is available for demonstration on request, and the company's efficient and flexible design and manufacturing capabilities are available to undertake custom designs for similar sophisticated requirements.

Unity Wireless Systems Corp.,
Burnaby, BC, Canada (604) 267-2712, www.unitywireless.com, sales@unitywireless.com.

Circle No. 301