Digital wireless-communications systems employ complex modulation formats to transmit voice, data,and multimedia services over limited-bandwidth channels.Wireless networks using GSM and WCDMA offer high performance with good reliability,but depend on excellent linearity from the power amplifiers in their final transmitter stages.These solid-state amplifiers must boost signals with complex modulation without undue distortion and signal leakage outside of designated frequency bands.Excessive distortion into adjacent bands (outside of the amplifier’s specified frequency band),known as adjacent-channel leakage (ACL) or adjacent-channel power (ACP), can raise the bit errors in a digital communication network and degrade or disrupt service.Fortunately,the Agilent N5182A MXG Vector Signal Generator is a powerful tool for accurately measuring the ACL or ACP performance of a wireless transmit amplifier designed to minimize wireless transmitter distortion.

The Agilent MXG vector signal generator (Fig. 1) produces the type of complex modulated signals found in modern digital wireless-communications systems, but without adding distortion of its own. The instrument is intuitive to use, allowing operators to modify key signal parameters "on the fly" locally or remotely.

Ideally, the power amplifier in a wireless base transceiver station (BTS) would boost output signals without distortion (Fig. 2). But every amplifier suffers some small amounts of distortion, and the complex signals found in digital wireless-communications systems can often push the most carefully designed power amplifier into nonlinear-behavior. The amplifier's output signals consist of imperfect inphase/quadrature (I/Q) modulated waveform spectra, out-of-channel carrier noise, and its own added intermodulation distortion. Backing off the amplifier's output power can reduce the distortion, but comprise radio link performance in terms of decreased signal-to-noise ratio (SNR). Pushing an amplifier beyond its recommended output levels is also not helpful, often driving the amplifier into gain compression (Fig. 3). This causes high levels of out-of-band signals that will interfere with other transmitted signals, causing high biterror rates (BERs) in digital receivers. Testing for adjacent-channel interference signal levels in power amplifiers is a key to ensuring a wireless network is providing optimum performance.

In traditional narrowband wireless systems, adjacent-channel leakage is tested by driving an amplifier with multiple inchannel tones and measuring the out-of-channel intermodulation distortion that the amplifier generates. In newer, wideband systems, the integrated, out-of-band signal power within a given bandwidth is measured using test signals that emulate actual modulated communications signals, then comparing this to the integrated in-channel signal power. For WCDMA systems (Fig. 4), adjacent-channel leakage ratio (ACLR) is defined as the ratio of the integrated signal power in the adjacent channel to the integrated signal power in the main channel, P(f_1intbw)/P(f_chbw) or P(f_2intbw)/P(f_chbw). In other wireless systems and with other waveform formats, ACLR is also referred to as adjacent-channel power ratio (ACPR). In GSM systems, adjacent-channel leakage is referred to as the output RF spectrum (ORFS).

Waveforms with constant envelope modulation, such as used in GSM, allow an amplifier to operate with good efficiency and close to compression. But signals with high peak-to-average ratios (or crest factors), such as WCDMA and cdma2000, can drive an amplifier into nonlinear operation and distortion. The ACLR performance of a WCDMA base station is generally -45 dBc, driving power-amplifier manufacturers to specify amplifier ACLR of -50 dBc or better.

The spectral output of a power amplifier designed for a wireless base station is comprised of the input signal (P_in) multiplied by the amplifier's gain (G) plus the amplifier's added noise and nonlinear distortion. To accurately measure the amplifier's noise, the noise and distortion of the input signal be minimized.

If the ACL characteristics of the input signal are known, its contributions to the amplifier's output ACL can also be known, by the product P_in × G. The total power of the amplifier's spectral output in the adjacent channels can be found by:

P_out (dBm) = 10log [G P_in + ( output noise + output distortion)]

P_out (dBm) = 10log (P₁ + P₂)

where:

P₁ = G P₂ and

P₂ = the output noise + distortion.

For multichannel systems such as cdma2000 and WCDMA, the amplifier output distortion is noiselike due to the large number of adjacent-channel distortions and the random nature of their phases. Since they are noise-like, if P₁ is known and Pout is known, then it is easy to determine whether or not the input signal's adjacent-channel leakage is contributing to the amplifier's ACL or ACP measurement. If the input signal leakage power contribution (P₁) is the same power as the amplifier's noise and distortion power (P₂), the total measured output power of the amplifier will be 3 dB higher than either one of them, or

P_total (dBm) = 10log(2P₁) = 3 dB + 10log(P₁)

To achieve accurate measurements of amplifier ACLR, it is necessary to use an input test signal with ACL 10 to 15 dB lower than the ACLR of the device being measured. The Agilent MXG Vector Signal Generator features a wide bandwidth from 250 kHz to 3 or 6 GHz and outstanding ACLR performance of -64 dBc needed for accurate adjacent-channel measurements of 3GPP WCDMA TM1 64DPCH1 carrier signals. For particularly demanding applications, the instrument can be equipped with Option UNV to improve ACL or ACP by an additional 7 to 9 dB depending on the modulation format.

The Agilent MXG uses a waveform header that allows an operator to program custom waveform crest factor and scaling settings, which optimize the I/Q modulator drive level and runtime scaling. Once set and saved, the parameters are automatically used when a waveform is recalled for playback. The crest factor information is in the form of the computed root mean square (RMS) value for the modulator's generated I/Q waveform and is used to automatically adjust I/Q modulator drive level for minimum adjacent-channel distortion. In automatic (AUTO) operation, the Agilent MXG will calculate an RMS value for the waveform and set the I/Q modulator drive level accordingly. For signals that have a high crest factor but a low RMS value, the computed RMS value in the header can be replaced with a more appropriate value or the I/Q modulator drive level can be adjusted manually.