FH-CDMA
FH-CDMA : Frequency hopping is one of two basic?modulation?techniques used in?spread spectrum?signal?transmission. It is the repeated switching of frequencies during radio transmission, often to minimize the effectiveness of "electronic warfare" - that is, the unauthorized interception or jamming of telecommunications. It also is known as frequency- hopping code division multiple access (FH-CDMA). Spread spectrum modulation techniques have become more common in recent years. Spread spectrum enables a signal to be transmitted across a?frequency band?that is much wider than the minimum?bandwidth?required by the information signal. The transmitter "spreads" the energy, originally concentrated in narrowband, across a number of frequency band channels on a wider electromagnetic spectrum. Benefits include improved privacy, decreased narrowband interference, and increased signal capacity. In an FH-CDMA system, a transmitter "hops" between available frequencies according to a specified?algorithm, which can be either random or preplanned. The transmitter operates in synchronization with a receiver, which remains tuned to the same center frequency as the transmitter. A short burst of data is transmitted on a narrowband. Then, the transmitter tunes to another frequency and transmits again. The receiver thus is capable of hopping its frequency over a given bandwidth several times a second, transmitting on one frequency for a certain period of time, then hopping to another frequency and transmitting again. Frequency hopping requires a much wider bandwidth than is needed to transmit the same information using only one carrier frequency. The spread spectrum approach that is an alternative to FH-CDMA is direct sequence code division multiple access (DS-CDMA), which chops the data into small pieces and spreads them across the frequency domain. FH-CDMA devices use less power and are generally cheaper, but the performance of DS-CDMA systems is usually better and more reliable. The biggest advantage of frequency hopping lies in the coexistence of several access points in the same area, something not possible with direct sequence
No records Found