IS-95

Interim Sandard 95 (IS-95), nyaéta standar seluler digital munggaran anu dumasar kanaCDMAanuditaratas kuQualcomm. Ngaran dagang pikeun IS-95 nyaétacdmaOne. IS-95 ogé dipikawanoh sabagé TIA-EIA-95.
cdmaOne nyaéta hijiStandar Telekomunikasi Mobil2Gnu ngagunakeunCDMA, nu maké cara aksés ganda (multiple access) dinaradio digital, pikeun ngirim sora, data jeung data signaling (saperti nomer telepon) antarahandsetjeungcell site.
CDMAor "code division multiple access" is adigital radiosystem that transmits stréams ofbits(PN Sequences).  CDMA permits several radios to share the same frequencies.  UnlikeTDMA"time division multiple access", a competing system used in2GGSM, 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 usedfrequency-division multiplexing.
In North America, the technology competed withDigital AMPS(IS-136, aTDMAtechnology).  It is now being supplanted byIS-2000(CDMA2000), a later CDMA-based standard. It is used in theUSA,South Korea,Canada,Mexico,India,Israel,Australia,New Zealand,Sri Lanka,Venezuela,BrazilandChina. In Q1 2007, around 15 % of the global subscribers used CDMA, while about 85 % usedGSMor3GSM.
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 éarlier set of protocol revisions, namely P_REV's one through five.
P_REV=1 was developed under anANSIstandards process with documentation referenceJ-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 termIS-95properly refers to P_REV=1, developed under theTelecommunications Industry Association(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 termedInterim 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 termedTechnical Services Bulletin 74 (TSB-74). TSB-74 was the next incremental improvement over IS-95A in the TIA standards process.
P_REV=4 is termedInterim Standard 95B (IS-95B) Phase I, and P_REV=5 is termedInterim 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). PREV=4 was by far the most popular variant of IS-95, with P_REV=5 only seeing minimal uptake in South Koréa.
P_REV=6 and beyond fall under theCDMA2000umbrella. 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.
The IS-95 standards describe anair 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) layer, L2 refers to theMedia Access Control(MAC) and Link-Access Control (LAC) sublayers, and L3 to the call-processing state machine.
IS-95 defines the transmission of signals in both theforward(network-to-mobile) andreverse(mobile-to-network) directions.
In the forward direction, radio signals are transmitted by base stations (BTS's). Every BTS is synchronized with aGPSreceiver so transmissions are tightly controlled in time. All forward transmissions areQPSKwith a chip rate of 1,228,800 per second. éach signal is spréad with aWalsh codeof length 64 and apseudo-random noisecode (PN code) of length215{\displaystyle {2}^{15}}, yielding a PN roll-over period of803{\displaystyle {\frac {80}{3}}}ms.
For the reverse direction, radio signals are transmitted by the mobile. Reverse link transmissions areOQPSKin 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 spréad withWalsh codesand thepseudo-random noisecode, which is also known as a Short Code.
Every BTS dedicates a significant amount of output power to apilot channel, which is an unmodulated PN sequence (in other words, spréad with Walsh code 0). éach 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 strongautocorrelationfunction, the forward pilot allows mobiles to determine system timing and distinguish different BTS's forhandoff.
When a mobile is "searching", it is attempting to find pilot signals on the network by tuning to particular radio frequencies, and performing an autocorrelation sweep across all possible PN phases. A strong autocorrelation 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 spréad with the Walsh and PN sequences and transmitted.
BTS's transmit async channelspréad with Walsh code 32. The sync channel frame is803{\displaystyle {\frac {80}{3}}}ms long, and its frame boundary is aligned to the pilot. The sync channel continually transmits a single message, theSync 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".
BTS's transmit at léast one, and as many as seven,paging channels starting with Walsh code 1. The paging channel frame time is 20 ms, and is time aligned to the IS-95 system (ie. 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 overhéad 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 overhéad information, itregisterswith the network, then optionally entersslotted-mode. Both of these processes are described in more detail below.
The Walsh space not dedicated to broadcast channels on the BTS sector is available fortraffic channels. 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 supportedrate set 1, providing a rate of 1200, 2400, 4800, or 9600 bit/s. P_REV=3 and beyond also providedrate set 2, yielding rates of 1800, 3600, 7200, or 14400 bit/s.
For voice calls, the traffic channel carries frames ofvocoderdata. A number of different vocoders are defined under IS-95, the éarlier 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 modérn 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 éach possible rate, and using the quality metrics of the Viterbi decoder, 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-95Radio 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 PREV=5, it was possible for a user to use up to seven supplemental "code" (traffic) channels simultanéously to incréase the throughput of a data call. Very few mobiles or networks ever provided this féature, which could in théory offer 115200 bit/s to a user.
Block InterléaverAfter convolution coding and repetition, symbols are sent to a 20 ms block interléaver, which is a 24 by 16 array.
IS-95 and its use of CDMA techniques, like any other communications system, have their throughput limited according toShannon's theorem. 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) appéar as noise, and signals carried on other Walsh codes (that are properly time aligned) are essentially removed in the de-spréading 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 inherent 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 therake receiverto improve SNR as well as performsoft handoff.
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.