Translate your WCDMA specs into receiver requirements

The requirement for high-speed data services over next-generation cellular networks is inevitable as the demand for video, multimedia, games, and broadband wide-area mobility grows. With more than 55 million subscribers, 108 commercial operators in 47 countries, and 67 combined wideband CDMA (WCDMA) and EDGE networks, WCDMA technology is well-positioned to capitalize on these trends and is experiencing growing market acceptance. At the same time, the integration and design challenges of current 3G radios is reminiscent of first-generation, super-heterodyne GSM/GPRS implementations.

With the advent of new commercial solutions, handset designers not only must decide which modes and frequency bands they plan to support, but are left to differentiate among transceiver specs with limited design experience and production data. Unlike more mature 2.5G time division multiple access (TDMA) designs, WCDMA is a spread-spectrum, frequency-division duplexing (FDD) system requiring additional design considerations. It is, therefore, helpful to clarify key receiver requirements and the implications on the evolving 3G radio and production handset.

WCDMA provides an upgrade path from GSM/GPRS/EDGE technologies for increased data rates, improved capacity and lower delivery costs for application services. The WCDMA frequency spectrum is distributed among nine bands dependent upon geography (see the figure). Because there are limited production-proven multi-band WCDMA transceiver offerings, frequency-band support is an initial receiver consideration.

Japan currently has the largest subscriber base with about 50% of the total 3G users. Europe is second, with more than 10 million users in Italy and the UK alone. A single-band WCDMA transceiver should support Band I to address these lead markets, as well as support compatibility in Korea and China.

For multi-band transceivers, Bands V and VI (a subset of band V) overlap the GSM850 allocation, which is important in the US and for roaming in Japan. Band V is the logical selection choice with additional future growth from transitioning Latin America regions and potential application in China and other Asia-Pacific regions. Network operators in the US are further expected to mandate that one band is insufficient, and support for Band II, which covers the PCS1900 spectrum, is required. Band IV is also of North American interest but considered a future requirement as FCC rules and auctions are pending in the latter half of 2006. Similarly bands III and IX (a subset of III), VII, and VIII are considered 2007 and beyond expansion bands for Japan, North America, and Europe respectively.

Shown is the WCDMA frequency spectrum by the various regions.

In WCDMA, a difference from TDMA systems like GSM is the treatment of the overall signal-to-noise ratio (SNR). From a conceptual perspective, a radio signal is multiplied by a spreading signal (which is a pseudo-noise code sequence) with a higher rate than the data rate of the message. The resultant 5-MHz wide signal appears random, but if the intended recipient has the right code, this process is reversed and the original signal is extracted. For WCDMA, the user bit rate is 12.2 kbits/s and the signal is spread with a 3.84 megachips per second (Mcps) spreading code. During despreading, the signal energy is then effectively gained up while the noise remains the same. Using the equation for processing gain (Gp),^{10log(3.84 Mcps/12.2 kbits/s)}, this equates to 25 dB.

In WCDMA, a difference from TDMA systems like GSM is the treatment of the overall SNR. From a conceptual perspective, a 5-MHz wide radio signal is multiplied by a spreading signal (which is a pseudo-noise code sequence) with a higher rate than the data rate of the message. The resultant signal appears random but if the intended recipient has the right code, this process is reversed and the original signal is extracted.

Incorporating this into the familiar concept of the minimum detectable signal (MDS), the minimum transceiver noise figure (NF) can be determined and the sensitivity relationship for the receive chain written as:

where K is Boltzman's constant equal to 1.38 × 10^-23J/°K (defining the average energy per particle which can be coupled electrically per degree Kelvin); T is the temperature of the system in degrees Kelvin; B is the bandwidth of the system. F is the noise factor of the system; and

is the effective ratio of signal energy to noise required by the baseband processor to correctly demodulate the received data. Using an ambient temperature of 290°K (17°C) and an RMS noise bandwidth of 3.84 MHz, the basic noise power density (KTB) of a WCDMA receiver is found to be -108 dBm/3.84 MHz.