Wireless Communication

Chapter: Network Concepts and Standards
Section: Cellular Systems

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.
  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
 

Attributes of CDMA in Cellular Systems

There are many attributes of CDMA which are of great benefit to the cellular system
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.

The IS-95 System

Similar to most other digital vehicular cellular systems, IS95 uses speech coding at about 9.6 kbit/s.

Forward link: base to mobile

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.

Reverse Link: mobile to base

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.

link to Quicktime file

IS95 Cellular Reuse

In IS95, all base stations use the same channel (C = 1). The interference between cells

Power Control

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JPL's Wireless Communication Reference Website 1993, 1995.

Source Credit: Jack Glas (T.U. Delft) contributed to this page.