IS 95 CDMA
• The existing 12.5 MHz cellular bands are used to derive 10
different CDMA bands (1.25 MHz per band).
• The frequency reuse factor in CDMA is 1.
• The channel rate is 1.2288 Mbps (actually chips not bits)
• Multipath fading is exploited in CDMA. It provides for space
• RAKE receivers are used to combine the output of several
• Interim Standard 95 (IS-95) is the first CDMA-based
digital cellular standard by Qualcomm. The brand name
for IS-95 is cdmaOne. IS-95 is also known as TIA-EIA95.
• It is a 2G mobile telecommunications standard that uses
CDMA, a multiple access scheme for digital radio, to
send voice, d
data and signaling d
d i li
data ( h as a di l d
telephone number) between mobile telephones and cell
• CDMA or "code division multiple access" is a digital
radio system that transmits streams of bits (PN codes).
• CDMA permits several radios to share the same
• Unlike TDMA "time division multiple access", a
competing system used in 2G GSM all radios can be
active all the time, because network capacity does not
directly limit the number of active radios.
• Since larger numbers of phones can be served by
smaller numbers of cell-sites, CDMA-based standards
have a significant economic advantage over TDMAbased standards, or the oldest cellular standards that
d frequency-division multiplexing.
di i i
li l i
• The IS-95 standards describe an air interface, a set of
protocols used between mobile units and the network
• IS-95 is widely described as a three-layer stack,
• L1 corresponds to the physical (PHY) layer,
• L2 refers to the Media Access Control (MAC) and LinkAccess Control (LAC) sublayers, and
• L3 to the call-processing state machine.
• IS-95 defines the transmission of signals in both the forward
(network-to-mobile) and reverse (mobile-to-network) directions.
• In the forward direction, radio signals are transmitted by base
stations (BTS' )
• Every BTS is synchronized with a GPS receiver so transmissions
a e t g t y co t o ed
are tightly controlled in time. All forward transmissions are
o wa d t a s ss o s a e
QPSK with a chip rate of 1.2288 Mbps.
The Logical Channels
T ffi channels
IS-95 CDMA Forward Channel
• The forward link uses the same frequency spectrum as AMPS
• Each carrier 1.25MHz
• 4 types of logical channel: A pilot, a synchronization, 7 paging,
and 55 traffic channels
• Channels are separated using different spreading codes
• QPSK is the modulation scheme
• Orthogonal Walsh codes are used (64 total)
• After orthogonal codes, they are further spread by short PN
• Short PN spreading codes are M sequences generated by
LFSRs of length 15 with a period of 32768 chips.
IS-95 CDMA Reverse Channel
Fundamentally different from the forward channels
Uses OQPSK for power efficiency
QPSK demodulation is easy
869-894 MHz range
No spreading of the data using orthogonal codes
Same orthogonal codes are used for WAVEFORM encoding
Two types of logical channels: The access channels and the
reverse traffic channels
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