![Samsung CDMA Phone](https://upload.wikimedia.org/wikipedia/commons/7/76/Samsung_CDMA_Phone.jpg)
Interim Standard 95 (IS-95) was the first ever
CDMA
Code-division multiple access (CDMA) is a channel access method used by various radio communication technologies. CDMA is an example of multiple access, where several transmitters can send information simultaneously over a single communication ...
-based digital cellular technology. It was developed by
Qualcomm
Qualcomm () is an American multinational corporation headquartered in San Diego, California, and incorporated in Delaware. It creates semiconductors, software, and services related to wireless technology. It owns patents critical to the 5G, 4 ...
and later adopted as a standard by the
Telecommunications Industry Association
The Telecommunications Industry Association (TIA) is accredited by the American National Standards Institute (ANSI) to develop voluntary, consensus-based industry standards for a wide variety of Information and Communication Technologies (Inform ...
in TIA/EIA/IS-95 release published in 1995. The proprietary name for IS-95 is cdmaOne.
It is a
2G mobile telecommunications standard that uses
code-division multiple access
Code-division multiple access (CDMA) is a channel access method used by various radio communication technologies. CDMA is an example of multiple access, where several transmitters can send information simultaneously over a single communication ...
(CDMA), a
multiple access
In telecommunications and computer networks, a channel access method or multiple access method allows more than two terminals connected to the same transmission medium to transmit over it and to share its capacity. Examples of shared physical med ...
scheme for
digital radio
Digital radio is the use of digital technology to transmit or receive across the radio spectrum. Digital transmission by radio waves includes digital broadcasting, and especially digital audio radio services.
Types
In digital broadcasting syst ...
, to send voice, data and signaling data (such as a dialed telephone number) between mobile
telephones
A telephone is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most efficiently the human voice, into el ...
and
cell sites. CDMA transmits streams of
bits (
PN codes). CDMA permits several radios to share the same frequencies. Unlike
TDMA "time division multiple access", a competing system used in
2G GSM
The Global System for Mobile Communications (GSM) is a standard developed by the European Telecommunications Standards Institute (ETSI) to describe the protocols for second-generation ( 2G) digital cellular networks used by mobile devices such ...
, 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 TDMA-based standards, or the oldest cellular standards that used
frequency-division multiplexing
In telecommunications, frequency-division multiplexing (FDM) is a technique by which the total bandwidth available in a communication medium is divided into a series of non-overlapping frequency bands, each of which is used to carry a separate ...
.
In North America, the technology competed with
Digital AMPS (IS-136, a
TDMA technology). It was supplanted by
IS-2000
CDMA2000 (also known as C2K or IMT Multi‑Carrier (IMT‑MC)) is a family of 3G mobile technology standards for sending voice, data, and signaling data between mobile phones and cell sites. It is developed by 3GPP2 as a backwards-compatible ...
(CDMA2000), a later CDMA-based standard.
Protocol revisions
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 term IS-95 generically applies to the earlier set of protocol revisions, namely P_REV's one through five.
P_REV=1 was developed under an
ANSI
The American National Standards Institute (ANSI ) is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organi ...
standards process with documentation reference ''J-STD-008''. J-STD-008, published in 1995, was only defined for the then-new North American PCS band (Band Class 1, 1900 MHz). The term ''IS-95'' properly refers to P_REV=1, developed under the
Telecommunications Industry Association
The Telecommunications Industry Association (TIA) is accredited by the American National Standards Institute (ANSI) to develop voluntary, consensus-based industry standards for a wide variety of Information and Communication Technologies (Inform ...
(TIA) standards process, for the North American cellular band (Band Class 0, 800 MHz) under roughly the same time frame. IS-95 offered interoperation (including handoff) with the analog cellular network. For digital operation, IS-95 and J-STD-008 have most technical details in common. The immature style and structure of both documents are highly reflective of the "standardizing" of Qualcomm's internal project.
P_REV=2 is termed ''Interim Standard 95A (IS-95A)''. IS-95A was developed for Band Class 0 only, as in incremental improvement over IS-95 in the TIA standards process.
P_REV=3 is termed ''Technical Services Bulletin 74 (TSB-74)''. TSB-74 was the next incremental improvement over IS-95A in the TIA standards process.
P_REV=4 is termed ''Interim Standard 95B (IS-95B) Phase I'', and P_REV=5 is termed ''Interim Standard 95B (IS-95B) Phase II''. The IS-95B standards track provided for a merging of the TIA and ANSI standards tracks under the TIA, and was the first document that provided for interoperation of IS-95 mobile handsets in both band classes (dual-band operation). P_REV=4 was by far the most popular variant of IS-95, with P_REV=5 only seeing minimal uptake in South Korea.
P_REV=6 and beyond fall under the
CDMA2000
CDMA2000 (also known as C2K or IMT Multi‑Carrier (IMT‑MC)) is a family of 3G mobile technology standards for sending voice, data, and signaling data between mobile phones and cell sites. It is developed by 3GPP2 as a backwards-compatible ...
umbrella. Besides technical improvements, the IS-2000 documents are much more mature in terms of layout and content. They also provide backwards-compatibility to IS-95.
Protocol details
![CdmaOneNetworkGeneralView](https://upload.wikimedia.org/wikipedia/commons/0/09/CdmaOneNetworkGeneralView.svg)
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, where L1 corresponds to the physical (
PHY
PHY is an abbreviation for the physical layer of the OSI model and refers to the circuitry required to implement physical layer functions.
PHY or Phy may also refer to:
* Phy, the drug methadone
* Phetchabun Airport (IATA code), Thailand
See ...
) layer, L2 refers to the
Media Access Control
In IEEE 802 LAN/MAN standards, the medium access control (MAC, also called media access control) sublayer is the layer that controls the hardware responsible for interaction with the wired, optical or wireless transmission medium. The MAC sublay ...
(MAC) and Link-Access Control (LAC) sublayers, and L3 to the call-processing state machine.
Physical layer
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's). Every BTS is synchronized with a
GPS
The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...
receiver so transmissions are tightly controlled in time. All forward transmissions are
QPSK
Phase-shift keying (PSK) is a digital modulation process which conveys data by changing (modulating) the phase of a constant frequency reference signal (the carrier wave). The modulation is accomplished by varying the sine and cosine inputs at a ...
with a chip rate of 1,228,800 per second. Each signal is spread with a
Walsh code
The Hadamard code is an error-correcting code named after Jacques Hadamard that is used for error detection and correction when transmitting messages over very noisy or unreliable channels. In 1971, the code was used to transmit photos of M ...
of length 64 and a
pseudo-random noise
In cryptography, pseudorandom noise (PRN) is a signal similar to noise which satisfies one or more of the standard tests for statistical randomness. Although it seems to lack any definite pattern, pseudorandom noise consists of a deterministic s ...
code (
PN code
A pseudorandom binary sequence (PRBS), pseudorandom binary code or pseudorandom bitstream is a binary sequence that, while generated with a deterministic algorithm, is difficult to predict and exhibits statistical behavior similar to a truly rando ...
) of length 2
15, yielding a PN roll-over period of
ms.
For the reverse direction, radio signals are transmitted by the mobile. Reverse link transmissions are
OQPSK in order to operate in the optimal range of the mobile's power amplifier. Like the forward link, the chip rate is 1,228,800 per second and signals are spread with
Walsh codes and the
pseudo-random noise
In cryptography, pseudorandom noise (PRN) is a signal similar to noise which satisfies one or more of the standard tests for statistical randomness. Although it seems to lack any definite pattern, pseudorandom noise consists of a deterministic s ...
code, which is also known as a Short Code.
Forward broadcast channels
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). Each BTS sector in the network is assigned a PN offset in steps of 64 chips. There is no data carried on the forward pilot. With its strong
autocorrelation
Autocorrelation, sometimes known as serial correlation in the discrete time case, is the correlation of a signal with a delayed copy of itself as a function of delay. Informally, it is the similarity between observations of a random variable ...
function, the forward pilot allows mobiles to determine system timing and distinguish different BTS's for
handoff
In cellular telecommunications, handover, or handoff, is the process of transferring an ongoing call or data session from one channel connected to the core network to another channel. In satellite communications it is the process of transfer ...
.
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. A strong correlation peak result indicates the proximity of a BTS.
Other forward channels, selected by their Walsh code, carry data from the network to the mobiles. Data consists of network signaling and user traffic. Generally, data to be transmitted is divided into frames of bits. A frame of bits is passed through a convolutional encoder, adding forward error correction redundancy, generating a frame of symbols. These symbols are then spread with the Walsh and PN sequences and transmitted.
BTSs transmit a ''sync channel'' spread with Walsh code 32. The sync channel frame is
ms long, and its frame boundary is aligned to the pilot. The sync channel continually transmits a single message, the ''Sync Channel Message'', which has a length and content dependent on the P_REV. The message is transmitted 32 bits per frame, encoded to 128 symbols, yielding a rate of 1200 bit/s. The Sync Channel Message contains information about the network, including the PN offset used by the BTS sector.
Once a mobile has found a strong pilot channel, it listens to the sync channel and decodes a Sync Channel Message to develop a highly accurate synchronization to system time. At this point the mobile knows whether it is roaming, and that it is "in service".
BTSs transmit at least one, and as many as seven, ''paging channel''s starting with Walsh code 1. The paging channel frame time is 20 ms, and is time aligned to the IS-95 system (i.e. GPS) 2-second roll-over. There are two possible rates used on the paging channel: 4800 bit/s or 9600 bit/s. Both rates are encoded to 19200 symbols per second.
The paging channel contains signaling messages transmitted from the network to all idle mobiles. 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.
When a mobile is idle, it is mostly listening to a paging channel. Once a mobile has parsed all the network overhead information, it ''registers'' with the network, then optionally enters ''slotted-mode''. Both of these processes are described in more detail below.
Forward traffic channels
The Walsh space not dedicated to broadcast channels on the BTS sector is available for ''traffic channel''s. These channels carry the individual voice and data calls supported by IS-95. Like the paging channel, traffic channels have a frame time of 20ms.
Since voice and user data are intermittent, the traffic channels support variable-rate operation. Every 20 ms frame may be transmitted at a different rate, as determined by the service in use (voice or data). P_REV=1 and P_REV=2 supported ''rate set 1'', providing a rate of 1200, 2400, 4800, or 9600 bit/s. P_REV=3 and beyond also provided ''rate set 2'', yielding rates of 1800, 3600, 7200, or 14400 bit/s.
For voice calls, the traffic channel carries frames of ''vocoder'' data. 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.
The mobile receiving a variable-rate traffic frame does not know the rate at which the frame was transmitted. Typically, the frame is decoded at each possible rate, and using the quality metrics of the
Viterbi decoder
A Viterbi decoder uses the Viterbi algorithm for decoding a bitstream that has been
encoded using a convolutional code or trellis code.
There are other algorithms for decoding a convolutionally encoded stream (for example, the Fano algorithm). Th ...
, the correct result is chosen.
Traffic channels may also carry circuit-switch data calls in IS-95. The variable-rate traffic frames are generated using the IS-95 ''Radio Link Protocol (RLP)''. RLP provides a mechanism to improve the performance of the wireless link for data. Where voice calls might tolerate the dropping of occasional 20 ms frames, a data call would have unacceptable performance without RLP.
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. Very few mobiles or networks ever provided this feature, which could in theory offer 115200 bit/s to a user.
Block Interleaver
After convolution coding and repetition, symbols are sent to a 20 ms block interleaver, which is a 24 by 16 array.
Capacity
IS-95 and its use of CDMA techniques, like any other communications system, have their throughput limited according to
Shannon's theorem
In information theory, the noisy-channel coding theorem (sometimes Shannon's theorem or Shannon's limit), establishes that for any given degree of noise contamination of a communication channel, it is possible to communicate discrete data (dig ...
. Accordingly, capacity improves with SNR and bandwidth. 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.
Active (slow) power control is also used on the forward traffic channels, where during a call, the mobile sends signaling messages to the network indicating the quality of the signal. The network will control the transmitted power of the traffic channel to keep the signal quality just good enough, thereby keeping the noise level seen by all other users to a minimum.
The receiver also uses the techniques of the
rake receiver
A rake receiver is a radio receiver designed to counter the effects of multipath fading. It does this by using several "sub-receivers" called ''fingers'', that is, several correlators each assigned to a different multipath component. Each finger ...
to improve SNR as well as perform
soft handoff Soft handover or soft handoff refers to a feature used by the CDMA and W-CDMA standards, where a cell phone is simultaneously connected to two or more cells (or cell sectors) during a call. If the sectors are from the same physical cell site (a sec ...
.
Layer 2
Once a call is established, a mobile is restricted to using the traffic channel. 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. The signaling message fragments are pieced together in the LAC, where complete signaling messages are passed on to Layer 3.
Deployment
cdmaOne was used in the following areas:
*North America
*Japan and South Korea (2G GSM is not available in these two countries)
*Hong Kong (by
Hutchison Telecom
Hutchison Asia Telecom Group or HAT, is a division of Hong Kong-based multinational conglomerate CK Hutchison Holdings. The division provides telecommunications services to several Asian countries. The division was formerly incorporated as H ...
, other operators and Hutchison Telecom itself also provides GSM)
See also
*
PN code
A pseudorandom binary sequence (PRBS), pseudorandom binary code or pseudorandom bitstream is a binary sequence that, while generated with a deterministic algorithm, is difficult to predict and exhibits statistical behavior similar to a truly rando ...
*
Comparison of mobile phone standards
This is a comparison of standards of mobile phones. A new generation of cellular standards has appeared approximately every tenth year since 1G systems were introduced in 1979 and the early to mid-1980s.
Issues
Global System for Mobile Commun ...
*
CDMA spectral efficiency
CDMA spectral efficiency refers to the system spectral efficiency in bit/s/Hz/site or Erlang/MHz/site that can be achieved in a certain CDMA based wireless communication system. CDMA techniques (also known as spread spectrum) are characterized by ...
References
External links
TR-45 Engineering CommitteeCDMA Standards Developing Group
{{Telecommunications
3rd Generation Partnership Project 2 standards
Telecommunications-related introductions in 1995
Qualcomm