Abstract
Recent years have witnessed the rapid progress in handheld devices. This has resulted in a growing number of mobile phones or PDAs that have a built-in camera to record still pictures or live videos. Encouraged by the success of second generation cellular wireless networks, researchers are now pushing the 3G standard to support a seamless integration of multimedia data services. One of the main products is WCDMA (Holma & Toskala, 2001), short for wideband code division multiple access. WCDMA networks have 80 million subscribers in 46 countries at the time of this writing. WCDMA can be viewed as a successor of the 2G CDMA system. In fact, many WCDMA technologies can be traced back to the 2G CDMA system. However, WCDMA air interface is specifically designed with envision to support real time multimedia services. To name some highlights, WCDMA: • Supports both packet-switched and circuitswitched data services. Mobile best-effort data services, such as Web surfing and file downloads, are available through packet service. • Has more bandwidth allocated for downlink and uplink than the 2G systems. It uses a 5 MHz wide radio signal and a chip rate of 3.84 mcps, which is about three times higher than CDMA2000. • Support a downlink data rate of 384 kbps for wide area coverage and up to 2 Mbps for hot-spot areas, which is sufficient for most existing packet-data applications. WCDMA Release 5 (Erricson, 2004) adopts HSDPA (High-speed downlink packet access), which increases peak data rates to 14 Mbps in the downlink. To achieve high data rate, WCDMA uses several new radio interface technologies, including (1) shared channel transmission, (2) higher-order modulation, (3) fast link adaptation, (4) fast scheduling, and (5) hybrid automatic-repeat-request (HARQ). These technologies have been successfully used in the downlink HSDPA, and will be used in upcoming improved uplink radio interface in the future. The rest of this article will explain the key components of the radio interface in WCDMA.
TopRecent years have witnessed the rapid progress in hand-held devices. This has resulted in a growing number of mobile phones or PDAs that have a built-in camera to record still pictures or live videos. Encouraged by the success of second generation cellular wireless networks, researchers are now pushing the 3G standard to support a seamless integration of multimedia data services. One of the main products is WCDMA (Holma & Toskala, 2001), short for wideband code division multiple access. WCDMA networks have 80 million subscribers in 46 countries at the time of this writing.
WCDMA can be viewed as a successor of the 2G CDMA system. In fact, many WCDMA technologies can be traced back to the 2G CDMA system. However, WCDMA air interface is specifically designed with envision to support real time multimedia services. To name some highlights, WCDMA:
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Supports both packet-switched and circuit-switched data services. Mobile best-effort data services, such as Web surfing and file downloads, are available through packet service.
- •
Has more bandwidth allocated for downlink and uplink than the 2G systems. It uses a 5 MHz wide radio signal and a chip rate of 3.84 mcps, which is about three times higher than CDMA2000.
- •
Support a downlink data rate of 384 kbps for wide area coverage and up to 2 Mbps for hot-spot areas, which is sufficient for most existing packet-data applications. WCDMA Release 5 (Erricson, 2004) adopts HSDPA (High-speed downlink packet access), which increases peak data rates to 14 Mbps in the downlink.
To achieve high data rate, WCDMA uses several new radio interface technologies, including (1) shared channel transmission, (2) higher-order modulation, (3) fast link adaptation, (4) fast scheduling, and (5) hybrid automatic-repeat-request (HARQ). These technologies have been successfully used in the downlink HSDPA, and will be used in upcoming improved uplink radio interface in the future. The rest of this article will explain the key components of the radio interface in WCDMA.
TopBackground
The first CDMA cellular standard is IS-95 by the U.S. Telecommunication Industry Association (TIA). In CDMA system, mobile users use the same radio channel within a cell when talking to the base station. Data from different users is separated because each bit is direct-sequence spread by a unique access code in the time domain. This even allows adjacent cells to use the same radio frequency with acceptable transmission error rate, which results in perhaps the most important advantages CDMA-based cellular network has over TDMA and FDMA: high cell capacity. CDMA network is able to support more users and higher data rates than TDMA/FDMA based cellular networks for the same frequency bandwidth (Gilhousen, Jacobs, Padovani, Viterbi, Weaver, & Wheatley, 1991).
On the CDMA downlink, the base station simultaneously transmits the user data for all mobiles. On the uplink, mobile transmissions are in an asynchronous fashion and their transmission power is controlled by the base station. Due to the asynchronous nature in the uplink, Signal-Interference-Ratio (SIR) in uplink is much lower than downlink. Thus, the capacity of a CDMA network is typically limited by its uplink. Improving uplink performance has been one of the most active research topics in the CDMA community.
Three main competing CDMA-based 3G systems are CDMA2000 (Esteves, 2002), WCDMA (Holma & Toskala, 2001), and TD-SCDMA, all based on direct-sequence CDMA (DS-CDMA). Commonalities among these systems are: close-loop power control, high data rate in downlink, link-level adaption, TDM fashion transmission, fair queue scheduling, and so forth. Main differences reside in the frequency bandwidth of carrier, link-adaption methods, and different implementation of signaling protocol. TD-SCDMA use TDD separation between uplink and downlink.
Aside from these industry standards, hybrid systems integrating WCDMA and WLAN have attracted a great deal of attention recently. One such system is described by Wang and Liu (2005). Such systems might offer the final solution toward true multimedia experiences in wireless.
Key Terms in this Chapter
Cochannel Interference: In wireless environments, multiple devices may transmit with the same physical frequency. These signals will superimpose each other and their summation will be detected at receiver. The transmissions of other devices are thus interference/noise to the intended device.
BPSK: Refers to Binary Phase Shift Keying, a constant amplitude modulation technique to convert a binary 1 or 0 to appropriate waveforms. The two waveforms have the same amplitude and their phase is separated by 180 degrees.
WCDMA: Wideband Direct Spread CDMA technique. It is a major standard/specification for third-generation (3G) cellular network. WCDMA is designed to offer high data rate to support 3G mobile functionalities, such as Internet surfing, video download/upload, and interactive real-time mobile applications. WCDMA was selected as the air interface for UMTS, the 3G data part of GSM. Attempts were made to unify WCDMA (3GPP) and CDMA-1X (3GPP2) standards in order to provide a single framework.
OFDM: Orthogonal Frequency Division Multiplexing is a modulation technique for transmission over a frequency-selective channel. OFDM divides the channel into multiple orthogonal frequencies, and each frequency will use a subcarrier where data streams are transmitted. Because the concurrent subcarriers carry different data streams, OFDM allows high spectral efficiency.
CDMA: Refers to Code Division Multiple Access. It is a spread spectrum multiplexing transmission method which uses a redundant information encoding method to deal with cochannel interference. In a most simplified setup, an information bit will be duplicated N times, and each chip will be modulated by a scramble sequence before transmitted to the media. The receiving end will descramble the received signal using the same scramble sequence to recover the transmitted information.
Uplink: Transmission from the mobile terminals to the base station.
Downlink: Transmission from the base station to the mobile terminal.