Chapter17.ppt (Size: 843.5 KB / Downloads: 78)
Cell: Geographical division unit of wireless networks.
• Access Point: Gateway to the network for mobile phones in a cell to contacts.
• Levels of cells in hierarchical cellular network:
– picocell: Each covers up to 100 meters, useful for wireless/cordless applications and devices (e.g. PDAs) in an office or home.
– microcell: Each covers up to 1,000 meters in cities or local areas, e.g. radio access pay-phones on the streets.
– cell: Each has up to 10,000 meters coverage, good for national or continental networks.
– macrocell: World-wide coverage, e.g. satellite phones.
Analog Wireless Networks
1G cellular phones used analog technology and FDMA.
– AMPS (Advanced Mobile Phone System) in North America, operating at 800-900 MHz frequency band.
a) Each of the two-way communication is allocated 25 MHz with (MS Transmit) in the band of 824 to 849 MHz and (BS Transmit) in the band of 869 to 894 MHz.
b) Each of the 25 MHz band is then divided up for two Operator bands, A and B, giving each 12.5 MHz.
c) FDMA further divides each of the 12.5 MHz operator bands into 416 channels – each channel having a bandwidth of 30 KHz.
– TACS (Total Access Communication System) and NMT (Nordic Mobile Telephony) were similar standards in Europe and Asia.
A layout for FDMA cellular system
Each cell in the seven-cell cluster is assigned a unique set of frequency channels, the interference from neighboring cells is negligible.
• The same set of frequency channels (denoted as f1 to f7) will be reused once in each cluster. The so called reuse factor is K = 7.
• In an AMPS system, for example, the maximum number of channels (including control channels) available in each cell is reduced to 416/7 ≈ 59.
TDMA and GSM
TDMA creates multiple channels in multiple time slots while allowing them to share the same carrier frequency.
• GSM was established by CEPT, a standard for a mobile communication network throughout Europe:
– GSM 900: operate in the 900 MHz frequency range.
– GSM 1800: the original GSM standard modified to operate at the 1.8 GHz frequency range.
• In North America:
– GSM 1900: GSM network uses frequencies at the range of 1.9 GHz.
– TIA/EIA IS-54B and the IS-136 standards – the most predominant use of TDMA technology.
– IS-136, superseding IS-54B, operates in the frequencies of 800 MHz and 1.9 GHz (the PCS frequency range).
Spread Spectrum and CDMA
Spread spectrum: A technology in which the bandwidth of a signal is spread before transmission.
– Distinct advantages of being secure and robust against intentional interference (jamming).
– Applicable to digital as well as analog signals because both can be modulated and “spread”.
– It is the digital applications in particular CDMA that made the technology popular in various wireless data networks.
– Two ways of implementing spread spectrum: frequency hopping and direct sequence.