Third-generation (3G) wireless communications systems are designed to improve upon the performance and services available from their first- and second-generation (1G and 2G) predecessors. The goal of 3G systems is true seamless global mobile communications sharing full compatibility with selected access technologies such as wireless local loop (WLL), cellular, cordless, and satellite-communications

systems. One technical challenge to the advent of seamless global-terminal mobility is the difficulty in achieving a common global-frequency plan. In every world region, at least part of the necessary spectrum is already allocated for other radio services.

Following a spectrum allocation around 2 GHz (roughly 1880 to 2200 MHz, depending upon geographic region) by the World Administrative Radio Conference (WARC) in 1992, the International Telecommunication Union—Radio-communication sector (ITU-R) began to define a wish list for 3G-system requirements. A range of technologies were proposed to meet these requirements, including orthogonal frequency-division multiplex (OFDM), opportunity-driven multiple access (ODMA), time-division synchronous code-division multiple access (TDSCDMA), and wideband CDMA (WCDMA). A technical body called the Third-Generation Partnership Project (3GPP) was organized to analyze the proposed technologies, with WCDMA selected as the preferred technology for 3G systems. The 3GPP standards organization has since written a technical-requirements specification in which chapter 25.101 includes the key performance requirements for the RF hardware portion of a WCDMA mobile terminal.

The 3GPP also defined two choices of operation for a WCDMA terminal: a frequency-division-duplex (FDD) mode and a time-division-duplex (TDD) mode. In the former, physical channels are defined by the RF channel number and the channelization code. The FDD mode is suitable for fast-moving mobile use. The uplink and downlink functions are separated in the frequency domain, and the approach offers greater downlink capacity then uplink capacity. The FDD approach employs a 100-percent duty cycle on both the uplink and downlink functions. In the TDD mode, physical channels are defined by the RF channel number, the channelization code, and the time slot. This approach is suitable for indoor or slow-moving mobile use. The uplink and downlink functions have similar capacities and occupy the same channel, with discontinuous transmission (DTx) on both the uplink and downlink.

DTx is a method for optimizing the efficiency of wireless voice-communications systems by momentarily powering-down or muting a mobile or portable telephone in the absence of voice input. In a typical two-way conversation, each party speaks slightly less than one-half the time, so if a transmitter (Tx) is on during voice input only, the telephone's duty cycle can be cut to less than 50 percent. This conserves battery power, eases the workload of Tx components, and frees time for the channel—allowing the system to take advantage of available bandwidth by sharing the channel with other signals. DTX circuits operate with voice-activity detection (VAD), which in wireless Txs is sometimes called voice-operated transmission (VOX). The 3GPP specifications also include FDD terminals for 60-MHz chunks only, with 190-MHz duplex spacing: 2110 to 2170 MHz for mobile receive and 1920 to 1980 MHz for mobile transmit.

Chapter 25.101 of the 3GPP specification covers the receive and transmit electrical requirements for FDD 3G mobile terminals. Before exploring WCDMA Tx requirements, it may help to review some of the key Tx parameters and their importance in Tx design. Adjacent-channel power ratio (ACPR), for example, is a measure of the amount of interference or power in an adjacent-frequency channel. Usually defined as the ratio of the average power in the adjacent frequency channel (or offset) to the average power in the transmitted-frequency channel, ACPR describes the amount of distortion due to nonlinearities in the Tx hardware.

ACPR is critical for WCDMA Txs, because CDMA modulation produces closely spaced spectral components in a modulated carrier. Intermodulation of those components causes a spectral regrowth of "shoulders" around the center-carrier frequency. Nonlinearities in the Tx can disperse those spectral-regrowth components into adjacent channels.

Error vector magnitude (EVM) is the vector (magnitude and phase) difference at a given instant between an ideal error-free reference and the actual transmitted signal. Because it changes continuously during every symbol transition, EVM is defined as the root-mean-square (RMS) value of the error vector over time. EVM is critical for WCDMA Tx performance because it indicates modulation quality in the transmitted signal. A large value of EVM results in degraded transceiver performance by causing poor detection accuracy.

Frequency error is the difference between specified and actual carrier frequencies. A large frequency error degrades transceiver performance by causing adjacent-channel interference and poor detection accuracy. Spurious and harmonic signals are tones produced by different signal combinations in the Tx, and harmonics are distortion products produced by nonlinear behavior in the Tx. Harmonics occur at integer multiples of the transmitted signal.