cdmaOne
Interim Standard 95 (IS-95) was the first digital cellular technology that used code-division multiple access (CDMA).
It was developed by Qualcomm and later adopted as a standard by the Telecommunications Industry Association in TIA/EIA/IS-95 release published in 1995.
cdmaOne's technical history is reflective of both its birth as a Qualcomm internal project, and the world of then-unproven competing digital cellular standards under which it was developed.
The immature style and structure of both documents are highly reflective of the "standardizing" of Qualcomm's internal project.
The IS-95 standards describe an air interface,[1] a set of protocols used between mobile units and the network.
Reverse link transmissions are OQPSK in order to operate in the optimal range of the mobile's power amplifier.
Every BTS dedicates a significant amount of output power to a pilot channel, which is an unmodulated PN sequence (in other words, spread with Walsh code 0).
With its strong autocorrelation function, the forward pilot allows mobiles to determine system timing and distinguish different BTSs for handoff.
When a mobile is "searching", it is attempting to find pilot signals on the network by tuning to particular radio frequencies and performing a cross-correlation across all possible PN phases.
The Sync Channel Message contains information about the network, including the PN offset used by the BTS sector.
A set of messages communicate detailed network overhead to the mobiles, circulating this information while the paging channel is free.
The paging channel also carries higher-priority messages dedicated to setting up calls to and from the mobiles.
A number of different vocoders are defined under IS-95, the earlier of which were limited to rate set 1, and were responsible for some user complaints of poor voice quality.
More sophisticated vocoders, taking advantage of modern DSPs and rate set 2, remedied the voice quality situation and are still in wide use in 2005.
Under IS-95B P_REV=5, it was possible for a user to use up to seven supplemental "code" (traffic) channels simultaneously to increase the throughput of a data call.
IS-95 has a fixed bandwidth, but fares well in the digital world because it takes active steps to improve SNR.
With CDMA, signals that are not correlated with the channel of interest (such as other PN offsets from adjacent cellular base stations) appear as noise, and signals carried on other Walsh codes (that are properly time aligned) are essentially removed in the de-spreading process.
The variable-rate nature of traffic channels provide lower-rate frames to be transmitted at lower power causing less noise for other signals still to be correctly received.
These factors provide an inherently lower noise level than other cellular technologies allowing the IS-95 network to squeeze more users into the same radio spectrum.
A frame format is defined in the MAC for the traffic channel that allows the regular voice (vocoder) or data (RLP) bits to be multiplexed with signaling message fragments.
