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57-451: It is a 2G mobile telecommunications standard that uses CDMA, a multiple access scheme for digital radio , to send voice, data and signaling data (such as a dialed telephone number) between mobile telephones and cell sites . CDMA transmits streams of bits ( PN codes ). CDMA permits several radios to share the same frequencies. Unlike time-division multiple access (TDMA), a competing system used in 2G GSM , all radios can be active all

114-407: 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 function, the forward pilot allows mobiles to determine system timing and distinguish different BTSs for handoff . When a mobile is "searching", it

171-418: A QPSK symbol can allow the phase of the signal to jump by as much as 180° at a time. When the signal is low-pass filtered (as is typical in a transmitter), these phase-shifts result in large amplitude fluctuations, an undesirable quality in communication systems. By offsetting the timing of the odd and even bits by one bit-period, or half a symbol-period, the in-phase and quadrature components will never change at

228-509: A TDMA-based standard, as well as with the TDMA-based GSM. It was supplanted by IS-2000 (CDMA2000), a later CDMA-based standard. 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

285-429: A base signal, the carrier wave (usually a sinusoid ), in response to a data signal. In the case of PSK, the phase is changed to represent the data signal. There are two fundamental ways of utilizing the phase of a signal in this way: A convenient method to represent PSK schemes is on a constellation diagram . This shows the points in the complex plane where, in this context, the real and imaginary axes are termed

342-473: 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

399-482: A minimum. The receiver also uses the techniques of the rake receiver to improve SNR as well as perform soft 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

456-503: 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 the Media 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 the forward (network-to-mobile) and reverse (mobile-to-network) directions. In

513-532: 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

570-430: A system is termed coherent (and referred to as CPSK). CPSK requires a complicated demodulator, because it must extract the reference wave from the received signal and keep track of it, to compare each sample to. Alternatively, the phase shift of each symbol sent can be measured with respect to the phase of the previous symbol sent. Because the symbols are encoded in the difference in phase between successive samples, this

627-482: A user. After convolution coding and repetition, symbols are sent to a 20 ms block interleaver, 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 to Shannon'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

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684-537: 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 , 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

741-420: Is accomplished by varying the sine and cosine inputs at a precise time. It is widely used for wireless LANs , RFID and Bluetooth communication. Any digital modulation scheme uses a finite number of distinct signals to represent digital data. PSK uses a finite number of phases, each assigned a unique pattern of binary digits . Usually, each phase encodes an equal number of bits. Each pattern of bits forms

798-624: 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

855-412: 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

912-451: Is called differential phase-shift keying (DPSK) . DPSK can be significantly simpler to implement than ordinary PSK, as it is a 'non-coherent' scheme, i.e. there is no need for the demodulator to keep track of a reference wave. A trade-off is that it has more demodulation errors. There are three major classes of digital modulation techniques used for transmission of digitally represented data: All convey data by changing some aspect of

969-488: 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 80 3 {\displaystyle {\frac {80}{3}}} ms long, and its frame boundary

1026-491: Is functionally equivalent to 2-QAM modulation. The general form for BPSK follows the equation: This yields two phases, 0 and π. In the specific form, binary data is often conveyed with the following signals: where f is the frequency of the base band. Hence, the signal space can be represented by the single basis function where 1 is represented by E b ϕ ( t ) {\displaystyle {\sqrt {E_{b}}}\phi (t)} and 0

1083-409: Is more general than that of BPSK and also indicates the implementation of higher-order PSK. Writing the symbols in the constellation diagram in terms of the sine and cosine waves used to transmit them: This yields the four phases π/4, 3π/4, 5π/4 and 7π/4 as needed. This results in a two-dimensional signal space with unit basis functions The first basis function is used as the in-phase component of

1140-406: Is one of the most spread modulation schemes in application to LEO satellite communications. This variant of QPSK uses two identical constellations which are rotated by 45° ( π / 4 {\displaystyle \pi /4} radians, hence the name) with respect to one another. Usually, either the even or odd symbols are used to select points from one of the constellations and

1197-437: Is only one bit per symbol, this is also the symbol error rate. Sometimes this is known as quadriphase PSK , 4-PSK, or 4- QAM . (Although the root concepts of QPSK and 4-QAM are different, the resulting modulated radio waves are exactly the same.) QPSK uses four points on the constellation diagram, equispaced around a circle. With four phases, QPSK can encode two bits per symbol, shown in the diagram with Gray coding to minimize

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1254-463: Is represented by − E b ϕ ( t ) {\displaystyle -{\sqrt {E_{b}}}\phi (t)} . This assignment is arbitrary. This use of this basis function is shown at the end of the next section in a signal timing diagram. The topmost signal is a BPSK-modulated cosine wave that the BPSK modulator would produce. The bit-stream that causes this output

1311-428: Is shown above the signal (the other parts of this figure are relevant only to QPSK). After modulation, the base band signal will be moved to the high frequency band by multiplying cos ⁡ ( 2 π f c t ) {\displaystyle \cos(2\pi f_{c}t)} . The bit error rate (BER) of BPSK under additive white Gaussian noise (AWGN) can be calculated as: Since there

1368-414: Is the simplest form of phase shift keying (PSK). It uses two phases which are separated by 180° and so can also be termed 2-PSK. It does not particularly matter exactly where the constellation points are positioned, and in this figure they are shown on the real axis, at 0° and 180°. Therefore, it handles the highest noise level or distortion before the demodulator reaches an incorrect decision. That makes it

1425-448: The bit error rate (BER) – sometimes misperceived as twice the BER of BPSK. The mathematical analysis shows that QPSK can be used either to double the data rate compared with a BPSK system while maintaining the same bandwidth of the signal, or to maintain the data-rate of BPSK but halving the bandwidth needed. In this latter case, the BER of QPSK is exactly the same as

1482-422: The constellation points chosen are usually positioned with uniform angular spacing around a circle . This gives maximum phase-separation between adjacent points and thus the best immunity to corruption. They are positioned on a circle so that they can all be transmitted with the same energy. In this way, the moduli of the complex numbers they represent will be the same and thus so will the amplitudes needed for

1539-401: The symbol that is represented by the particular phase. The demodulator , which is designed specifically for the symbol-set used by the modulator, determines the phase of the received signal and maps it back to the symbol it represents, thus recovering the original data. This requires the receiver to be able to compare the phase of the received signal to a reference signal – such

1596-495: The BER of BPSK – and believing differently is a common confusion when considering or describing QPSK. The transmitted carrier can undergo numbers of phase changes. Given that radio communication channels are allocated by agencies such as the Federal Communications Commission giving a prescribed (maximum) bandwidth, the advantage of QPSK over BPSK becomes evident: QPSK transmits twice

1653-429: 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

1710-522: The LAC, where complete signaling messages are passed on to Layer 3. cdmaOne was used in the following areas: 2G">2G The requested page title contains unsupported characters : ">". Return to Main Page . OQPSK Phase-shift keying ( PSK ) is a digital modulation process which conveys data by changing (modulating) the phase of a constant frequency carrier wave . The modulation

1767-455: 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

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1824-500: 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

1881-490: The cosine and sine waves. Two common examples are "binary phase-shift keying" ( BPSK ) which uses two phases, and "quadrature phase-shift keying" ( QPSK ) which uses four phases, although any number of phases may be used. Since the data to be conveyed are usually binary, the PSK scheme is usually designed with the number of constellation points being a power of two. BPSK (also sometimes called PRK, phase reversal keying, or 2PSK)

1938-440: The data rate in a given bandwidth compared to BPSK - at the same BER. The engineering penalty that is paid is that QPSK transmitters and receivers are more complicated than the ones for BPSK. However, with modern electronics technology, the penalty in cost is very moderate. As with BPSK, there are phase ambiguity problems at the receiving end, and differentially encoded QPSK is often used in practice. The implementation of QPSK

1995-408: The description given for BPSK above. The binary data that is conveyed by this waveform is: 11000110 . Offset quadrature phase-shift keying ( OQPSK ) is a variant of phase-shift keying modulation using four different values of the phase to transmit. It is sometimes called staggered quadrature phase-shift keying ( SQPSK ). Taking four values of the phase (two bits ) at a time to construct

2052-417: The even (or odd) bits are used to modulate the in-phase component of the carrier, while the odd (or even) bits are used to modulate the quadrature-phase component of the carrier. BPSK is used on both carriers and they can be independently demodulated. As a result, the probability of bit-error for QPSK is the same as for BPSK: However, in order to achieve the same bit-error probability as BPSK, QPSK uses twice

2109-486: The forward direction, radio signals are transmitted by base stations (BTS's). Every BTS is synchronized with a GPS receiver so transmissions are tightly controlled in time. All forward transmissions are QPSK with a chip rate of 1,228,800 per second. Each signal is spread with a Walsh code of length 64 and a pseudo-random noise code ( PN code ) of length 2, yielding a PN roll-over period of 80 3 {\displaystyle {\frac {80}{3}}} ms. For

2166-426: The in-phase and quadrature axes respectively due to their 90° separation. Such a representation on perpendicular axes lends itself to straightforward implementation. The amplitude of each point along the in-phase axis is used to modulate a cosine (or sine) wave and the amplitude along the quadrature axis to modulate a sine (or cosine) wave. By convention, in-phase modulates cosine and quadrature modulates sine. In PSK,

2223-550: 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,

2280-452: The magnitude of jumps is smaller in OQPSK when compared to QPSK. The license-free shaped -offset QPSK (SOQPSK) is interoperable with Feher-patented QPSK ( FQPSK ), in the sense that an integrate-and-dump offset QPSK detector produces the same output no matter which kind of transmitter is used. These modulations carefully shape the I and Q waveforms such that they change very smoothly, and

2337-406: 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. The Walsh space not dedicated to broadcast channels on the BTS sector is available for traffic channel s. These channels carry

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2394-493: The most robust of all the PSKs. It is, however, only able to modulate at 1   bit/symbol (as seen in the figure) and so is unsuitable for high data-rate applications. In the presence of an arbitrary phase-shift introduced by the communications channel , the demodulator (see, e.g. Costas loop ) is unable to tell which constellation point is which. As a result, the data is often differentially encoded prior to modulation. BPSK

2451-406: The odd-numbered bits have been assigned to the in-phase component and the even-numbered bits to the quadrature component (taking the first bit as number 1). The total signal – the sum of the two components – is shown at the bottom. Jumps in phase can be seen as the PSK changes the phase on each component at the start of each bit-period. The topmost waveform alone matches

2508-402: The other symbols select points from the other constellation. This also reduces the phase-shifts from a maximum of 180°, but only to a maximum of 135° and so the amplitude fluctuations of π / 4 {\displaystyle \pi /4} -QPSK are between OQPSK and non-offset QPSK. One property this modulation scheme possesses is that if the modulated signal is represented in

2565-430: 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

2622-403: The phase can change by 180° at once, while in OQPSK the changes are never greater than 90°. The modulated signal is shown below for a short segment of a random binary data-stream. Note the half symbol-period offset between the two component waves. The sudden phase-shifts occur about twice as often as for OQPSK (since the signals no longer change together), but they are less severe. In other words,

2679-436: The power (since two bits are transmitted simultaneously). The symbol error rate is given by: If the signal-to-noise ratio is high (as is necessary for practical QPSK systems) the probability of symbol error may be approximated: The modulated signal is shown below for a short segment of a random binary data-stream. The two carrier waves are a cosine wave and a sine wave, as indicated by the signal-space analysis above. Here,

2736-418: 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 code, which is also known as a Short Code. Every BTS dedicates a significant amount of output power to

2793-454: 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

2850-401: The same time. In the constellation diagram shown on the right, it can be seen that this will limit the phase-shift to no more than 90° at a time. This yields much lower amplitude fluctuations than non-offset QPSK and is sometimes preferred in practice. The picture on the right shows the difference in the behavior of the phase between ordinary QPSK and OQPSK. It can be seen that in the first plot

2907-421: The signal and the second as the quadrature component of the signal. Hence, the signal constellation consists of the signal-space 4 points The factors of 1/2 indicate that the total power is split equally between the two carriers. Comparing these basis functions with that for BPSK shows clearly how QPSK can be viewed as two independent BPSK signals. Note that the signal-space points for BPSK do not need to split

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2964-481: The signal stays constant-amplitude even during signal transitions. (Rather than traveling instantly from one symbol to another, or even linearly, it travels smoothly around the constant-amplitude circle from one symbol to the next.) SOQPSK modulation can be represented as the hybrid of QPSK and MSK : SOQPSK has the same signal constellation as QPSK, however the phase of SOQPSK is always stationary. The standard description of SOQPSK-TG involves ternary symbols . SOQPSK

3021-413: The symbol (bit) energy over the two carriers in the scheme shown in the BPSK constellation diagram. QPSK systems can be implemented in a number of ways. An illustration of the major components of the transmitter and receiver structure are shown below. Although QPSK can be viewed as a quaternary modulation, it is easier to see it as two independently modulated quadrature carriers. With this interpretation,

3078-405: 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 North America, the technology competed with Digital AMPS (IS-136),

3135-399: 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

3192-580: Was developed under an ANSI 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 (TIA) standards process, for the North American cellular band (Band Class 0, 800 MHz) under roughly

3249-671: 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 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. The IS-95 standards describe an air interface ,

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