What is CDMA?
CDMA (Code-Division Multiple
Access) is a channel access method used by various radio communication
technologies. It is a form of multiplexing, which allows numerous signals to
occupy a single transmission channel, optimizing the use of available
bandwidth. The technology is used in ultra-high-frequency (UHF) cellular
telephone systems in the 800-MHz and 1.9-GHz bands.
CDMA employs analog-to-digital
conversion (ADC) in combination with spread spectrum technology. Audio input is
first digitized into binary elements. The frequency of the transmitted signal
is then made to vary according to a defined pattern (code), so it can be
intercepted only by a receiver whose frequency response is programmed with the
same code, so it follows exactly along with the transmitter frequency. There
are trillions of possible frequency-sequencing codes, which enhances privacy
and makes cloning difficult.
The original CDMA standard, also
known as CDMA One and still common in cellular telephones in the U.S offers a
transmission speed of only up to 14.4 Kbps in its single channel form and up to
115 Kbps in an eight-channel form. CDMA2000 and Wideband CDMA deliver data many
times faster.
Characteristics
of CDMA:
Ø spread spectrum
techniques use a transmission bandwidth that is several orders of magnitude
greater than the minimum required signal bandwidth. These systems were designed
using spread spectrum because of its security and resistance to jamming.
Ø CDMA can effectively reject narrow band interference.
Since narrow band interference affects only a small portion of the spread
spectrum signal, it can easily be removed through notch filtering without much
loss of information.
Ø CDMA devices use
a rake receiver, which exploits multipath delay components to improve the
performance of the system.
Ø In a CDMA
system, the same frequency can be used in every cell, because channelization is
done using the pseudo-random codes.
Ø Reusing the same
frequency in every cell eliminates the need for frequency planning in a CDMA system;
Ø CDMA systems use
the soft hand off, which is undetectable and provides a more reliable and
higher quality signal.
General
Specification of CDMA
Ø Rx: 869-894MHz
Tx: 824-849MHz
Ø 20
Channels spaced 1250kHz apart (798 users/channel)
Ø QPSK/(Offset)
OQPSK modulation scheme
Ø 1.2288Mbps
bit rate
Ø IS-95
standard
Ø Operates
at both 800 and 1900 MHz frequency bands
Advantages
of CDMA techniques:
Ø Efficient
practical utilization of fixed frequency spectrum.
Ø Flexible
allocation of resources.
Ø Many
users of CDMA use the same frequency, TDD or FDD may be used
Ø Multipath
fading may be substantially reduced because of large signal bandwidth
Ø No
absolute limit on the number of users, Easy addition of more users.
Ø Impossible
for hackers to decipher the code sent
Ø Better
signal quality
Ø No
sense of handoff when changing cells
Ø The CDMA channel
is nominally 1.23 MHz wide.
Ø CDMA networks
use a scheme called soft handoff, which minimizes signal breakup as a handset
passes from one cell to another.
Ø CDMA is
compatible with other cellular technologies; this allows for nationwide roaming.
Ø The combination
of digital and spread-spectrum modes supports several times as many signals per
unit bandwidth as analog modes.
Disadvantages
to using CDMA:
Ø As the number of
users increases, the overall quality of service decreases
Ø Self-jamming
Ø Near- Far-
problem arises
Uses
of CDMA:
Ø One of the early
applications for code division multiplexing is in GPS. This predates and is
distinct from its use in mobile phones.
Ø The Qualcomm
standard IS-95, marketed as cdmaOne.
Ø The Qualcomm
standard IS-2000, known as CDMA2000. This standard is used by several mobile
phone companies, including the Globalstar satellite phone network.
Ø The UMTS 3G
mobile phone standard, which uses W-CDMA.
Ø CDMA has been
used in the OmniTRACS satellite system for transportation logistics.
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