13 - Laserfiche WebLink

Home Browse Search

• TDIVLq thus triples a base station's traffic capacity and three or more times more calls than TDMA without any change in ERE antennas,or the systems. However,CDMA requires specialized number of active channels.This means that one equipment at both the base station and subscriber fully digital base station(or one N-AMPS base ends,and is currently undergoing field trials. station)can carry the same number of calls as three conventional analog(AMPS) base stations. Another digital format cellular systems are intro- ducing is Code Division Multiple Access or CDMA.CDMA differs greatly from TDMA.Each signal is uniquely coded with a key that spreads the signal over a very wide bandwidth,much wider than the 30-kHz TDMA standard.A CDMA receiver responds only to the desired transmitter's key and can pick out("decode") and demodulate the associated signal.CDMA is also called spread spectrum transmission.CDMA systems can ignore narrow-band undesired signals as well as CDMA signals occupying similar bandwidths but coded with different keys.Some engineers estimate that CDMA networks can handle 10 or more times more traffic than analog cellular systems, • WHAT ISA CHANNEL? number of channels that can occupy a given In radio communications,a channel is a band of segment of the radio spectrum. frequencies used to convey information. The span The signal from an N-AMPS narrowband analog of frequencies in a channel is its bandwidth in FM cellular telephone occupies 10 kHz of spectrum, hertz kilohertz,or megahertz.Depending on various while that from an AMPS conventional analog FM factors including the signal format, the transmitted cellular phone occupies 30 kHz.An AM broadcast signal may not occupy every frequency in a channel station occupies a channel about 10 kHz wide, at any one time—or ever. Often,a channel includes while an FM broadcast station transmitting high- guardband frequencies at the channel's lower and fidelity,full-range orchestral music in stereo occu- upper edges. The guardbands do not contain pies 200 kHz of bandwidth.Sending images and an appreciable signal power,but rather serve as associated stereo soundtrack entails the transmis- protective buffers to suppress interference to and sion of much more information each second than from other channels'signals. sending just sound alone.A conventional television A basic tenet of communications theory states that broadcast station occupies 6 MHz of spectrum—as a channel's bandwidth determines the amount of much as the aggregate bandwidth of 30 stereo FM information it can convey in a set period of time. broadcast signals,or that of the signals from 300 The bandwidth of the baseband modulating signal AMPS cellular phones. (the information to be communicated,such as in North American TDMA digital cellular systems, speech,music,images,or a stream of digital data the channel bandwidth is 30 kHz(the same as for words)also plays a key role in determining the AMPS),but time-slicing techniques permit the minimum bandwidth a channel must have.A simultaneous transmission of three separate channel that carries telephone-grade voice conversations in the same amount of spectrum. communications usually needs three or more kHz And in a COMA digital cellular transmission format of bandwidth.In FM transmission formats,the undergoing field tests,the signal occupies 1.23 channel bandwidth affects the useful effective MHz. With guardbands,the CDMA channel is 1.8 S communications range,so FM communications MHz wide. That channel,however,by virtue of systems often employ greater channel bandwidth spread-spectrum technology,simultaneously than that of the baseband signal. The trade-off,of handles the signals associated with many different course,is that greater bandwidth limits the total conversations.