14-04-2014, 04:01 PM
Introduction to WCDMA
WCDMA[.ppt (Size: 1.54 MB / Downloads: 21)
Duplexing method
WCDMA supports both FDD and TDD modes of operation
Frequency Division Duplex (FDD)
separate 5 MHz carrier frequencies are used for uplink and downlink, respectively
Time Division Duplex (TDD)
only one 5 MHz is timeshared between uplink and downlink
Chip rate
chip rate of 3.84 Mcps leads to a carrier bandwidth (channel bandwidth) of approximately 5 MHz
chip:the length of time to transmit either a "0" or a "1" in a binary pulse code
chip rate:number of chips per second
DS-CDMA systems with a bandwidth of about 1 MHz (narrowband CDMA systems)
wide carrier bandwidth of WCDMA supports high user data rates
Multirate concept
use a variable spreading factor and multicode to support very high bit rates (up to 2 Mbps)
multicode
in multicode CDMA systems, each user can be provided with multiple spreading codes of fixed length, depending on users' rate requests
motivations for multicode CDMA
increase the information rate over a given spread bandwidth
allow for the flexibility of multiple data rates
Detection
WCDMA employs coherent detection (連續偵測) on uplink and downlink based on the use of pilot symbols (導引符號) or common pilot (共用導引)
coherent detection (coherent demodulation)
a method of recovering the original signal that requires an exactly same carrier frequency and phase (propagation delay causes carrier-phase offset) as those used in the transmitting end
the received signal is mixed, in some type of nonlinear device, with a signal from a local oscillator, to produce an intermediate frequency, from which the modulating signal is recovered (detected)