IS95: CDMA Cellular Telephony
IS 95 is a cellular phone system based on Direct Sequence
CDMA multiple access. Thus, multiple users simultaneously share the same (wideband) channel. Designers from Qualcomm claim
a 20 fold increase in capacity over analog cellular telephony.
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| Audio commentary: Peter M. Grant, Distinguished
IEEE Lecturer 1997, discusses cellular CDMA systems (MPEG audio). See also:
Full talk, MPEG plug-in on
- In DS-CDMA systems, are accommodated in the same RF bandwidth.
- Each user is identified by a different spreading code
- Energy received over multipath channels can be resolved to avoid cancellation of
waves
- Transmissions are asynchronous on the uplink, but synchronous on the downlink
- Power control is needed to mitigate near-far problems
- Capacity is limited my multiple-access interference from other users
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Attributes of CDMA in Cellular Systems
There are many attributes of CDMA which are of great benefit to the cellular
system
- Soft handoff.
Since every cell uses the same radio frequency band,
the only
difference between user channels is the spreading
code sequences. Therefore, there is no jump
from one frequency
to another frequency when a user moves between cells. The mobile terminal receives
the same signal in one cell as it does in the next, and thus there is no harsh
transition from one receiving mode to another. Two or more neighboring
base stations can receive
the signal of a particular user, because they all use the same channel.
Moreover, two base stations
can simultaneously transmit to the same user terminals. The mobile
(rake) receiver can resolve
the two signals separately and combine them (see diversity).
This feature is called
soft handoff.
- Soft capacity or graceful degradation.
In FDMA and TDMA,
N channels can be
used virtually without interference from other users in the same cell
but potential users N+1, N+2, ...,
are blocked until a channel is released. The capacity of FDMA and TDMA is
therefore fixed at N users and the link quality is determined by the frequency reuse pattern.
In theory, it does not
matter whether the
spectrum is divided into frequencies, time slots, or codes, the capacity
provided from these three multiple access schemes is the same. However,
in CDMA,
all the users in all cells
share one radio
channel and are separated by codes. Therefore, an additional user
may be added by sacrificing somewhat the
link quality,
with the effect that
voice quality is just slightly
degraded compared to that of the normal N-channel cell. Thus,
degradation of performance with an increasing number
of simultaneous users is "graceful"
in CDMA systems, versus the hard limits placed
on FDMA and TDMA systems.
- Multipath tolerance.
Spread spectrum techniques are effective in
combating the
frequency selective fading that takes place in multipath
channels. The underlying principle is that when a signal is spread over
a wide bandwidth, a frequency selective fade will corrupt only a small
portion of the signal's power spectrum, while passing the remaining spectrum
unblemished. As a result, upon despreading there is a better probability
that the signal can be recovered correctly. For an unspread
signal whose spectral density happens to be misplaced in a deep fade, an
unrecoverable signal at the receiver is virtually assured.
To optimally combine signals received over various delayed paths, a rake receiver
can be used.
- No channel equalization needed.
When the transmission rate is much higher than 10
kbps in both FDMA and TDMA, an equalizer is needed for reducing the intersymbol
interference caused by time delay spread. This is because when the bit period
becomes smaller than about ten times the time delay spread,
intersymbol interference
becomes significant.
However, in CDMA a correlatoris needed at minimum.
To achieve
good performance a rake receiver
is needed combat delay spread.
- Privacy.
An important requirement of spreading signals is that they
be "noise-like", or pseudorandom. Despreading the signal requires
knowledge of the user's code, and for a binary code with spreading
factor N there exist 2N possible random sequences.
In military systems these codes are kept secret, so
it is very difficult
for an unauthorized attacker to tap into or transmit on another user's channel. Often it is even difficult to detect the presence of a spread-spectrum signal because it is below the noise
that is present in the transmit bandwidth.
Note that in cellular systems, the codes are fully described in publicly available standards.
In digital systems, security against eavesdropping (confidentiality) is obtained through encryption.
This is a highly desirable alternative to the analog FDMA cellular phone system
in wide use today, where with an inexpensive scanner one can tune in to
the private conversations of unwary neighbors.
There are, of course, a number of disadvantages associated with CDMA; two of
the most severe are the problem of "self-interference," and the related problem
of the "near-far"
effect.
- Self-interference arises from the presence of delayed replicas of signal due to multipath. The delays cause the spreading sequences of the different users to
lose their orthogonality, as by design they are orthogonal only at zero phase offset.
Hence in despreading a given user's waveform,
nonzero contributions to that user's signal arise from the transmissions
of the other users in the network.
This is distinct from both TDMA and
FDMA, wherein for reasonable time or frequency guardbands, respectively,
orthogonality of the received signals can be preserved.
- The near-far problem arises from the fact that signals closer to the receiver of
interest are received with smaller attenuation than are signals located further
away. Therefore the strong signal from the nearby transmitter will mask the
weak signal from the remote transmitter. In TDMA and FDMA, this is not a
problem since mutual interference can be filtered. In
CDMA, however, the near-far effect combined with imperfect orthogonality
between codes (e.g. due to different time sifts), leads to substantial
interference.
Accurate and fast power control
appears essential to ensure reliable operation of multi-user
DS-CDMA systems.
The IS-95 System
Similar to most other digital vehicular cellular systems, IS95 uses speech coding at about 9.6 kbit/s.
Here all signal originate at the same transmitter. Thus it is fairly simple to reduce
mutual interference from users within the same cell, by assigning
orthogonal Walsh-Hadamard codes.
There are logical channels for pilot, paging, sync and traffic.
One of the Walsh codes is the all "one" word (1,1,1,1,...),
which would result in a narrowband signal. Thus a maximum length PN sequence is
superimposed, which is the same for all users and has the same time
phase for all users.
The long PN code provide a measure of voice privacy and improves
time synchronization. The short PN code in the forward link has a
limited resolution but makes synchronization easier.
- Chip rate 1.2288 Mchip/s = 128 times 9600 bit/sec
- Codes: combines 64 Walsh-Hadamard (for orthogonality among users)
and a maximum length code sequence (for effective spreading and multipath resistance)
- Transmit bandwidth 1.25 MHz
- Convolutional coding with rate 1/2
- pilot tone for synchronization
On the reverse link, every user uses the same set of
short sequences for modulation. The length of these sequences is
215, i.e., it is a modified 15 bit Linear Feedback Shift Register
maximum length sequence
(215+1-1).
Each access channel and each traffic channel gets a different long PN sequence. The long sequences are used to separate the signals from different users on the reverse link (CDMA).
Walsh codes are used solely to provide m-ary orthogonal modulation waveform.
The reverse link uses rate 1/3 convolutional coding.
Video
Phil Karn (Qualcomm) explains the basics of the IS95 spreading concept in a presentation at U.C. Berkeley. The spreading method
on the reverse link contains a short code and a long code. The short code is mainly
for acquisition. The long code is used to separate users.
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link to Quicktime file
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IS95 Cellular Reuse
In IS95, all base stations use the same channel (C = 1).
The interference between cells
- Highly depends on path loss law
- Would theoretically diverge to infinity for free space loss with "20 log d"
- According to Qualcomm, surrounding cells contribute
to the total interference as follows:
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1st tier: 6 cells 6% per cell,
- 2nd tier: 12 cells 0.2% per cell
- 3rd tier: 18 cells 0.03% per cell
- 4th tier: 24 cells 0.01% per cell
where percentages are relative to power from own cell
- CDMA performance is optimized if all signals are received with the same power
- Update needed every 1 msec. (cf. rate of fading)
- Performance is sensitive to imperfections of only a dB
- For flat (frequency non-selective) Rayleigh and Rician fading, perfect power control is impossible. Fades are so deep that the average gain needed to compensate
is unbounded: Expectation [attenuation-2] goes to infinity.
Source Credit: Jack Glas (T.U. Delft) contributed to this page.
