CDMA

What is CDMA?

One of the most important concepts to any cellular telephone system is that of "multiple access", meaning that multiple, simultaneous users can be supported. In other words, a large number of users share a common pool of radio channels and any user can gain access to any channel (each user is not always assigned to the same channel). A channel can be thought of as merely a portion of the limited radio resource which is temporary allocated for a specific purpose, such as someone's phone call. A multiple access method is a definition of how the radio spectrum is divided into channels and how channels are allocated to the many users of the system.

The CDMA Cellular Standard

With CDMA, unique digital codes, rather than separate RF frequencies or channels, are used to differentiate subscribers. The codes are shared by both the mobile station (cellular phone) and the base station, and are called "pseudo-Random Code Sequences." All users share the same range of radio spectrum.

For cellular telephony, CDMA is a digital multiple access technique specified by the Telecommunications Industry Association (TIA) as "IS-95".

IS-95 systems divide the radio spectrum into carriers which are 1,250 kHz (1.25 MHz) wide. One of the unique aspects of CDMA is that while there are certainly limits to the number of phone calls that can be handled by a carrier, this is not a fixed number. Rather, the capacity of the system will be dependent on a number of different factors. This will be discussed in later sections.

Multiple Access Comparison

It is easier to understand CDMA if it is compared with other multiple access technologies. The following sections describe the fundamental differences between a Frequency Division Multiple Access Analog technology (FDMA), a Time Division Multiple Access Digital technology (TDMA) and a Code Division Multiple Access Digital technology (CDMA).

FDMA - Frequency Division Multiple Access

FDMA is used for standard analog cellular. Each user is assigned a discrete slice of the RF spectrum. FDMA permits only one user per channel since it allows the user to use the channel 100% of the time. Therefore, only the frequency "dimension" is used to define channels.

Figure 1: FDMA

TDMA - Time Division Multiple Access

The key point to make about TDMA is that users are still assigned a discrete slice of RF spectrum, but multiple users now share that RF carrier on a time slot basis. Each of the users alternate their use of the RF channel. Frequency division is still employed, but these carriers are now further sub-divided into some number of time slots per carrier.

A user is assigned a particular time slot in a carrier and can only send or receive information at those times. This is true whether or not the other time slots are being used. Information flow is not continuous for any user, but rather is sent and received in "bursts." The bursts are re-assembled at the receiving end, and appear to provide continuous sound because the process is very fast.

Figure 2: TDMA

CDMA - Code Division Multiple Access

IS-95 uses a multiple access spectrum spreading technique called Direct Sequence (DS) CDMA.

Each user is assigned a binary, Direct Sequence code during a call. The DS code is a signal generated by linear modulation with wideband Pseudorandom Noise (PN) sequences. As a result, DS CDMA uses much wider signals than those used in other technologies. Wideband signals reduce interference and allow one-cell frequency reuse.

There is no time division, and all users use the entire carrier, all of the time.

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Figure 3: DS-CDMA

CDMA Technology

Though CDMA's application in cellular telephony is relatively new, it is not a new technology. CDMA has been used in many military applications, such as anti-jamming (because of the spread signal, it is difficult to jam or interfere with a CDMA signal), ranging (measuring the distance of the transmission to know when it will be received), and secure communications (the spread spectrum signal is very hard to detect).

Spread Spectrum

CDMA is a "spread spectrum" technology, which means that it spreads the information contained in a particular signal of interest over a much greater bandwidth than the original signal.

The standard data rate of a CDMA call is 9600 bits per second (9.6 kilobits per second). This initial data is "spread," including the application of digital codes to the data bits, up to the transmitted rate of about 1.23 megabits per second. The data bits of each call are then transmitted in combination with the data bits of all of the calls in the cell. At the receiving end, the digital codes are separated out, leaving only the original information which was to be communicated. At that point, each call is once again a unique data stream with a rate of 9600 bits per second.

Synchronization

In the final stages of the encoding of the radio link from the base station to the mobile, CDMA adds a special "pseudo-random code" to the signal that repeats itself after a finite amount of time. Base stations in the system distinguish themselves from each other by transmitting different portions of the code at a given time. In other words, the base stations transmit time offset versions of the same pseudo-random code. In order to assure that the time offsets used remain unique from each other, CDMA stations must remain synchronized to a common time reference.

The primary source of the very precise synchronization signals required by CDMA systems is the Global Positioning System (GPS). GPS is a radio navigation system based on a constellation of orbiting satellites. Since the GPS system covers the entire surface of the earth, it provides a readily available method for determining position and time to as many receivers as are required.

"The Balancing Act"

CDMA cell coverage is dependent upon the way the system is designed. In fact, three primary system characteristics - Coverage, Quality and Capacity - must be balanced off of each other to arrive at the desired level of system performance.

In a CDMA system these three characteristics are tightly inter-related. Even higher capacity might be achieved through some degree of degradation in coverage and/or quality. Since these parameters are all intertwined, operators can not have the best of all worlds: three times wider coverage, 40 times capacity, and "CD" quality sound. For example, the 13 kbps vocoder provides better sound quality, but reduces system capacity as compared to an 8 kbps vocoder.

CDMA Benefits

When implemented in a cellular telephone system, CDMA technology offers numerous benefits to the cellular operators and their subscribers. The following is an overview of the benefits of CDMA. Each benefit will be described in detail in the following subsections.

1. Capacity increases of 8 to 10 times that of an AMPS analog system and 4 to 5 times that of a GSM system
2. Improved call quality, with better and more consistent sound as compared to AMPS systems
3. Simplified system planning through the use of the same frequency in every sector of every cell
4. Enhanced privacy
5. Improved coverage characteristics, allowing for the possibility of fewer cell sites
6. Increased talk time for portables
7. Bandwidth on demand

Benefit 1: CDMA Capacity Increases

Capacity gains in cellular systems can be attained in one of two ways:

·         By getting more channels per MHz of spectrum.

·         By getting more channel reuse per unit of geographic area.

NAMPS is an example of a system technology which achieves greater capacity through method #1 (more channels per MHz of spectrum). Instead of one channel in 30 kHz as in AMPS, NAMPS gets three channels in 30 kHz, thereby providing three times the capacity of AMPS.

GSM is an example of a system which uses method #2 (more channel reuse per unit of geographic area). GSM allows for a 9dB C/I (carrier to interference ratio) instead of the traditional 17dB C/I used in TACS (the analog FDMA technology in the 900 MHz band). This allows GSM to place cell sites closer together and translates to about two times the capacity of TACS. Plans for a half-rate GSM system are under consideration. A half-rate GSM system, using methods #1 and #2, would result in approximately a 4 to 5 times capacity gain over analog TACS.

CDMA offers a greater system capacity than that offered by traditional analog cellular systems by using method #2. It allows reuse of the same frequency in every sector of every cell. Depending upon the starting assumptions and specific system designs, carriers should be able to achieve an 8 to 10 times capacity gain over AMPS.

It is important to note that CDMA capacity computations are based upon system wide averages. Actual capacity will vary from cell to cell and sector to sector, depending on terrain, interference levels, propagation characteristics, and a number of other factors.

CDMA and Cell Reuse

One of the key design principals of cellular telecommunications is the use of the same frequencies, over and over, in a particular geographic region. In many types of cellular systems, however, it is not possible to use every frequency in every cell site because of the interference, which would result.

With CDMA, signals can be received in the presence of high levels of interference, yet still result in the same, or better, call quality. All users on a carrier share the same RF spectrum. The same CDMA RF carrier frequency is used in every cell site, and in every sector of a sector cell site. This equates to an N=1/S frequency reuse pattern, where S is the number of sectors per cell. This N=1/S reuse of frequencies is what gives CDMA its greater capacity over AMPS and other technologies.

Eb/No and Interference Threshold

Eb/No provides a measure of the performance of a CDMA link between the mobile and the cell. It represents the signal to noise ratio for a single bit on the reverse link. It is the ratio in dB between the energy of each information bit and the noise spectral density. The noise is a combination of background interference and the interference created by other users on the system.

A decrease in the Eb/No ratio indicates that the relative level of interference, as compared to the level of the voice information, is increasing. This will lower the voice quality of the conversation. While all digital cellular systems use error correction coding, systems based on narrowband digital modulation generally use less sophisticated schemes which use up less bandwidth. In order to keep voice quality high, therefore, the operators of narrowband systems require a higher Eb/No. This leads to a need to limit the number of users on the system, lowering capacity.

CDMA, on the other hand, uses advanced forward error correction coding as well as a digital demodulator, lowering CDMA's required Eb/No ratio. Using a lower Eb/No to reach voice quality standards, CDMA achieves more capacity and uses less transmitter power than narrowband systems.

CDMA describes Eb/No noise interference in terms of the Frame Erasure Rate (FER). Using an interference threshold, the CDMA system erases frames of information that contain too many errors. The FER, then, describes the number of frames that were erased due to poor quality. Therefore, as the Eb/No level increases, the FER decreases, and system voice quality is improved.

Conversely, the higher the acceptable FER, the higher the overall cell site capacity. These two parameters, frame erasure rate and voice quality, must be balanced against each other.

Examples of Capacity Improvements

In order to understand the capacity increases that are often stated for CDMA, two examples are given below. The important thing to remember is that the capacity increase varies, depending upon the type of system one starts with. If the system is a 7-cell reuse AMPS system migrating to CDMA, the capacity increase will be more significant than that of a 4-cell reuse NAMPS system migrating to CDMA. This is simply because the NAMPS system already provides higher capacity than the AMPS system. Motorola's parameters for CDMA system design are also somewhat conservative relative to others in the industry.

Example 1: Basic Capacity Calculations - 3 Sector AMPS to 3 Sector CDMA

The calculation of CDMA's capacity gain over a given AMPS system depends on two factors:

1. the number of AMPS channels that fill the spectrum slated for a CDMA carrier

2. the number of channels supported by the CDMA carrier

One CDMA carrier requires 1.25 MHz of bandwidth. Since 3-sector AMPS has a seven cell reuse pattern, this example will spread the 1.25 MHz across 7 cell sites. Each cell site would then lose 180 kHz of spectrum (1.25 ÷ 7 Å 0.180). Thus, a total of six AMPS channels must be removed from each cell site (180 kHz ÷ 30 kHz/AMPS channel = 6). Hence, 42 AMPS channels must be removed in order to support one CDMA carrier.

Figure 4: CDMA Capacity Gains Over AMPS

Unlike AMPS, CDMA can use the same 1.25 MHz in all three sectors in each of the seven cells. Based on extensive field experience and simulations, Motorola system designs support 18 effective traffic channels per sector in a 3-sector system. This provides 54 effective channels per cell. Given the seven cells, then, CDMA supports 378 channels. Hence, in this example, CDMA achieves capacity gains of nine times that of AMPS (378 ÷ 42 = 9).

Example 2: First CDMA Carrier Allocation

It is important to note that the capacity increase brought about by the addition of the first CDMA carrier frequency differs from the previous capacity computations. It is necessary to take into account the additional AMPS channels which must be taken out of service due to guard band requirements between CDMA and AMPS channels. The first CDMA channel will actually require 1.8 MHz of spectrum, inclusive of guard bands of 0.27 MHz on each side. (Carrier + 2*guard band = 1.23 MHz + 2*0.27 MHz = 1.77 MHz .)

Because of the guard bands, 60 AMPS channels, instead of 42, must be removed from service (1.8 MHz ÷ .03 MHz = 60). When these 60 channels are replaced by the 378 CDMA channels, a 6.3 times capacity increase results. Subsequent CDMA channels will be inserted between the existing guard bands and will require only 1.25 MHz each. So the second and third carriers would bring a 9 times capacity increase.

Other Influences on Capacity

Voice Activity Detection

Voice activity detection is another variable which helps to increase the capacity of a CDMA system. IS-95 CDMA takes advantage of voice activity gain through its use of variable rate vocoders.

In a typical phone conversation a person is actively talking only about 35% of the time. The other 65% is spent listening to the other party, or is quiet time when neither party is speaking. The principle behind the variable rate vocoder is to have it run at high speed, providing the best speech quality, only when voice activity is detected.

When no voice activity is detected, the vocoder will drop its encoding rate, because there is no reason to have high speed encoding of silence. The encoded rate can drop to 4, 2, or even 1 kbps. Thus the variable rate vocoder uses up channel capacity only as needed. Since the level of "interference" created by all of the users directly determines system capacity, and voice activity detection reduces the noise level in the system, capacity can be maximized.

CDMA Power Control

Another very important parameter that is key to providing enhanced capacity with CDMA is power control. The primary design goal of a CDMA system is for all users to be received by the base station at the same power level, and to make that power level as low as possible while still maintaining a high quality call. Any more power than needed adds unnecessarily to the overall noise level on the CDMA channel, and cuts down capacity. Therefore, the more precise the power control, the greater the capacity. Power control is also employed in analog and TDMA systems, but it is not as precise as it is in CDMA.

In CDMA, the base station communicates to the mobile station, instructing the mobile to adjust its power up or down. The mobile station transmits only enough power to maintain a link, so the average transmitted power is much lower than that required for an analog system.

In a CDMA system, the cell site continually measures the received signal from the mobile, compares it to the desired power level, and then makes a decision to raise or lower a specific mobile's transmit power as frequently as once every 1.25 milliseconds (800 times per second).

CDMA adjusts mobile power levels up and down in 84 steps of 1 dB each. This method ensures that no matter how close or far a mobile is from the cell site, each one is received at the same power level.

Benefit 2: Improved Call Quality

Cellular telephone systems using CDMA are able to provide higher quality sound and fewer dropped calls than systems based on other technologies. A number of features inherent in the system produce this high quality.

• Advanced error detection and error correction schemes greatly increase the likelihood that frames are interpreted correctly.
• Sophisticated vocoders offer high speed coding and reduce background noise.
• CDMA takes advantage of various types of diversity to improve speech quality:
• frequency diversity (protection against frequency selective fading)
• spatial diversity (two receive antennas)
• path diversity (rake receiver improves reception of a signal experiencing multipath "interference," and actually enhances sound quality)
• time diversity (interleaving and coding)
• Soft Handoffs contribute to high voice quality by providing a "make before break" connection. "Softer" Handoffs between sectors of the same cell provide similar benefits.
• Precise power control assures that all mobiles are very close to the optimum power level to provide the highest voice quality possible.
• The voice quality for CDMA has been rated very high in mean opinion score (MOS) tests which compare it to other technologies. 

Advanced Error Detection and Error Correction

The IS-95 CDMA air interface standard specifies powerful error detection and correction algorithms. Corrupted voice data can be detected and either corrected or manipulated to minimize the impact of data errors on speech quality.

Sophisticated Vocoders

PCM is the vocoding standard (64 Kbps) used in landline systems. It is simple, which was necessary in the 1960s, but not very efficient. It has the sound quality wireless would like to match. Wired communications still uses PCM, since bandwidth has become rather inexpensive via fiber optic cable and/or microwave links.

The CDMA vocoder also increases call quality by suppressing background noise. Any noise that is constant in nature, such as road noise, is eliminated. Constant background sound is viewed by the vocoder as noise which does not convey any intelligent information, and is removed as much as possible. This greatly enhances voice clarity in noisy environments, such as the inside of cars, or in noisy public places.

In an effort to improve voice quality without sacrificing capacity or transmit range, CDMA vendors are implementing an advanced 8 kbps vocoder known as the EVRC (Enhanced Variable Rate Coder).

Multiple Levels of Diversity

CDMA takes advantage of a number of types of diversity, all of which lead to improved speech quality. The four types are frequency diversity, spatial diversity, path diversity and time diversity.

Frequency Diversity

With radio, fades or "holes" in frequency will occur. Fades occur in a multi-path environment when two or more signals combine and cancel each other out. Narrow band transmissions are especially prone to this phenomenon. For wide band signals such as CDMA, this is much less of a problem. The wide band signal is, of course, also subjected to frequency selective fading, but the majority of the signal is unaffected and the overall effect is minimal.

Figure 5: CDMA Quality Benefits from Frequency Diversity

As an example, consider what happens when there is a 12 dB deep, 400 kHz wide, frequency selective fade. For a wide band CDMA signal which spans 1.25 MHz, this fade affects only about 1/3 of the entire signal's bandwidth. Since the energy of a phone call is spread across the entire signal, the effect of the fade is looked at as an average, and represents an overall drop in signal of approximately 2 dB.

If this same 400 kHz, 12 dB fade falls on top of a narrow band 200 kHz signal, as in GSM, the results are quite different. The entire 200 kHz signal is then affected by this fade. The result will be an overall drop in signal of the full 12 dB. This is a much more serious hit to the signal, and could lead to severe degradation in voice quality, or even a dropped call.

Spatial Diversity

Spatial Diversity refers to the use of two receive antennas separated by some physical distance. The principle of spatial diversity recognizes that when a mobile is moving about, it creates a pattern of signal peaks and nulls. When one of these nulls falls on one antenna it will cause the received signal strength to drop. However, if a second antenna is placed some physical distance away, it will be outside of the signal null area and thus receive the signal at an acceptable signal level.

Path Diversity

With radio communications, there is usually more than one RF path from the transmitter to the receiver. Therefore, multiple versions of the same signal are usually present at the receiver. However, these signals, which have arrived along different paths, are all time shifted with respect to each other because of the differences in the distance each signal has traveled. This "multipath" effect is created when a transmitted signal is reflected off of objects in the environment (buildings, mountains, planes, trucks, etc.). These reflections, combined with the transmitted signal, create a moving pattern of signal peaks and nulls.

When a narrow band receiver moves through these nulls there is a sudden drop in signal strength. This fading will cause either lower, more noisy speech quality or, if the fading is severe enough, the loss of signal and a dropped call.

Figure 6: CDMA Quality Benefits from Path Diversity

Although multipath is usually detrimental to an analog or TDMA signal, it is actually an advantage to CDMA, since the CDMA rake receiver can use multipath to improve a signal. The CDMA receiver has a number of receive "fingers" which are capable of receiving the various multipath signals. The receiver locks onto the three strongest received multipath signals, time shifts them, and then sums them together to produce a signal that is better than any of the individual signal components. Adding the multipath signals together enhances the signal rather than degrading it.

Time Diversity

CDMA systems use a number of forward error correcting codes, followed by interleaving.

Error correction schemes are most effective when bit errors in the data stream are spread more evenly over time. By separating the pieces of data over time, a sudden disruption in the CDMA data will not cause a corresponding disruption in the voice signal. When the frames are pieced back together by the decoder, any disrupted voice data will have been in small pieces over a relatively longer stretch of the actual speech, reducing or eliminating the impact on the voice quality of the call.

Interleaving, which is common to most digital communication systems, ensures that contiguous pieces of data are not transmitted consecutively. Even if you lose one small piece of a word, chances are great that the rest of the word will get through clearly.

Soft Handoff

With traditional hard handoffs, which are used in all other types of cellular systems, the mobile drops a channel before picking up the next channel. When a call is in a soft handoff condition, a mobile user is monitored by two or more cell sites and the transcoder circuitry compares the quality of the frames from the two receive cell sites on a frame-by-frame basis. The system can take advantage of the moment-by-moment changes in signal strength at each of the two cells to pick out the best signal.

This ensures that the best possible frame is used in the CDMA decoding process. The transcoder can literally toggle back and forth between the cell sites involved in a soft handoff on a frame-by-frame basis, if that is what is required to select the best frame possible.

Figure 7: CDMA Soft Handoff Improves Frame Quality

Soft handoffs also contribute to high call quality by providing a "make before break" connection. This eliminates the short disruption of speech one hears with non-CDMA technologies when the RF connection breaks from one cell to establish the call at the destination cell during a handoff. Narrow band technologies "compete" for the signal, and when Cell B "wins" over Cell A, the user is dropped by cell A (hard handoff). In CDMA the cells "team up" to obtain the best possible combined information stream. Eventually, Cell A will no longer receive a strong enough signal from the mobile, and the transcoder will only be obtaining frames from Cell B. The handoff will have been completed, undetected by the user. CDMA handoffs do not create the "hole" in speech that is heard in other technologies.

Figure 8: CDMA Soft Handoff Utilizes Two or More Cells

Some cellular systems suffer from the "ping pong effect" of a call getting repetitively switched back and forth between two cells when the subscriber unit is near a cell border. At worst, such a situation increases the chance of a call getting dropped during one of the handoffs, and at a minimum, causes noisier handoffs. CDMA soft handoff avoids this problem entirely.

And finally, because a CDMA call can be in a soft handoff condition with up to three cells at the same time, the chances of a loss of RF connection (a dropped call) is greatly reduced.

CDMA also provides for "softer" handoffs. A "softer" handoff occurs when a subscriber is simultaneously communicating with more than one sector of the same cell.

Precise Power Control

CDMA power control not only increases capacity (as described earlier) but also increases speech quality by minimizing and overcoming interference. CDMA's power control algorithms are all designed to reduce the overall signal strength level to the bare minimum required to maintain a quality call.

Benefit 3: Simplified System Planning

All users on a CDMA carrier share the same RF spectrum. This N=1/S reuse of frequencies (where S = number of sectors per cell) is one factor which gives CDMA its greater capacity over AMPS and other technologies, but it also makes certain aspects of system planning more straightforward. Engineers will no longer have to perform the detailed frequency planning which is necessary in analog and TDMA systems.

Because frequency planning is unnecessary, that element of engineering work formerly required as part of an initial system design is eliminated. Even more significantly, frequency re-tunes for expansion of a system are also eliminated. If a customer wants to add cell sites or channels, it will no longer require an entirely new frequency plan to do so.

Note that when CDMA is added as an overlay on an existing analog system, frequency planning would be required to clear spectrum for the CDMA carriers.

Benefit 4: Enhanced Privacy

Increased privacy over other cellular systems, both analog and digital, is inherent in CDMA technology. It is extremely difficult for someone to jam the CDMA signal. In addition, since the digitized frames of information are spread across a wide slice of spectrum, it is unlikely that a casual eavesdropper will be able to listen in on a conversation. Furthermore, there are future plans to offer digital encryption to provide even greater levels of security and privacy.

Note: Other types of systems can also include encryption, offering this highest level of privacy.

Benefit 5: Improved Coverage

At the startup of a new system, there are fewer subscribers, so fewer cells are required to handle the traffic. However, there is still the need to provide wide initial geographic coverage.

A CDMA cell site has a greater range than a typical analog or digital cell site. Therefore fewer CDMA cell sites are required to cover the same area. Depending on system loading and interference, the reduction in cells could be as much as 50% when compared to GSM!

CDMA's greater range is due to the fact that CDMA uses a more sensitive receiver than other technologies.

Please note that cell reduction is true for customers STARTING with CDMA. But customers with existing analog systems may choose to co-locate CDMA cells with existing cells.

Benefit 6: Increased Portable Talk Time

Because of precise power control and other system characteristics, CDMA subscriber units normally transmit at only a fraction of the power of analog and TDMA phones. This will enable portables to have longer talk and standby time. (This direct comparison assumes, of course, similar cell sizes between the CDMA and analog or TDMA systems.)

Benefit 7: Bandwidth on Demand

A wideband CDMA channel provides a common resource that all mobiles in a system utilize based on their own specific needs, whether they are transmitting voice, data, facsimile, or other applications. At any given time, the portion of this "bandwidth pool" that is not used by a given mobile is available for use by any other mobile. This provides a tremendous amount of flexibility - a flexibility that can be exploited to provide powerful features, such as higher data rate services. In addition, because mobiles utilize the "bandwidth pool" independently, these features can easily coexist on the same CDMA channel.

CDMA Implementation

CDMA Channels

Just when one grasps an understanding of the CDMA carrier, which is 1.25 MHz wide, someone talks about "traffic channels" and confuses the issue. The fact is that with CDMA, the path by which voice or data passes is the entire carrier, as described previously.

CDMA traffic channels are different: they are dependent on the equipment platform, such as Motorola's SC™ products, on which the CDMA is implemented. Motorola designates channels in three ways: effective traffic channels, actual traffic channels and physical traffic channels.

• The number of "Effective" traffic channels includes the traffic carrying channels less the soft handoff channels. The capacity of an effective traffic channel is equivalent to the traffic carrying capacity of an analog traffic channel.
• The number of "Actual" traffic channels includes the effective traffic channels, plus channels allocated for soft handoff.
• The number of "Physical" traffic channels includes the Pilot channels, the Sync channels, the Paging channels, the Soft Handoff Overhead channels and the Effective (voice and data) traffic channels.

CDMA uses the terms "forward" and "reverse" channels just like they are used in analog systems. Base transmit equates to the forward direction, and base receive is the reverse direction. ("Forward" is what the subscriber hears and "reverse" is what the subscriber speaks.)

 

CDMA Forward Channels

Pilot Channel

The pilot channel is used by the mobile unit to obtain initial system synchronization and to provide time, frequency, and phase tracking of signals from the cell site.

Sync Channel

This channel provides cell site identification, pilot transmit power, and the cell site pilot pseudo-random (PN) phase offset information. With this information the mobile units can establish the System Time as well as the proper transmit power level to use to initiate a call.

Paging Channel

The mobile unit will begin monitoring the paging channel after it has set its timing to the System Time provided by the sync channel. Once a mobile unit has been paged and acknowledges that page, call setup and traffic channel assignment information is then passed on this channel to the mobile unit.

Forward Traffic Channel

This channel carries the actual phone call and carries the voice and mobile power control information from the base station to the mobile unit.

CDMA Reverse Channels

Access Channel

When the mobile unit is not active on a traffic channel, it will communicate to the base station over the access channel. This communication includes registration requests, responses to pages, and call originations. The access channels are paired with a corresponding paging channel.

Reverse Traffic Channel

This channel carries the other half of the actual phone call and carries the voice and mobile power control information from the mobile unit to the base station.

CDMA Modulation

Both the Forward and Reverse Traffic Channels use a similar control structure consisting of 20 millisecond frames. For the system, frames can be sent at either 14400, 9600, 7200, 4800, 3600, 2400, 1800, or 1200 bps.

For example, with a Traffic Channel operating at 9600 bps, the rate can vary from frame to frame, and can be 9600, 4800, 2400, or 1200 bps. The receiver detects the rate of the frame and processes it at the correct rate. This technique allows the channel rate to dynamically adapt to the speech or data activity. For speech, when a talker pauses, the transmission rate is reduced to a low rate. When the talker speaks, the system instantaneously shifts to using a higher transmission rate. This technique decreases the interference to other CDMA signals and thus allows an increase in system capacity.

CDMA starts with a basic data rate of 9600 bits per second. This is then spread to a transmitted bit rate, or chip rate (the transmitted bits are called chips), of 1.2288 MHz. The spreading process applies digital codes to the data bits, which increases the data rate while adding redundancy to the system.

The chips are transmitted using a form of QPSK (quadrature phase shift keying) modulation which has been filtered to limit the bandwidth of the signal. This is added to the signal of all the other users in that cell. When the signal is received, the coding is removed from the desired signal, returning it to a rate of 9600 bps. When the decoding is applied to the other users' codes, there is no despreading; the signals maintain the 1.2288 MHz bandwidth. The ratio of transmitted bits or chips to data bits is the coding gain. The coding gain for the IS-95 CDMA system is 128, or 21 dB.

CDMA Advanced Features

Advanced Features Overview

As was mentioned earlier, the CDMA Development Group continues to define features and capabilities for CDMA cellular systems in order to provide better service to subscribers and new revenue generating services for the operators. The current set of advanced features which have been defined by the CDG and TIA and are being incorporated into systems include:

• Multiple, High Quality Vocoders
• CDMA Short Message Services (SMS)
• Over the Air Activation
• Sleep Mode
• CDMA Data and Fax

Multiple, High Quality Vocoders

One of the unique strengths of the CDMA standard is the support for multiple vocoders simultaneously within a system. By comparison, IS-54 TDMA can not easily support multiple vocoders operating simultaneously. As improved vocoders become available they can be incorporated without major changes to the CDMA infrastructure equipment.

Every vocoder design must achieve a balance among the factors of voice quality, device complexity and bandwidth efficiency. Device complexity includes requirements for processor speed, memory size and allowable delay.

The new 13 kbps Code Excited Linear Predictive (CELP) vocoder and the 8 kbps Enhanced Variable Rate Vocoder (EVRC) vocoder which are planned for CDMA promise better voice quality over the current CDMA 8 kbps vocoder. (Though not reflected in the name, the 13 kbps vocoder uses variable rate transmission, just as the original 8 kbps CELP vocoder does.)

The 13 kbps vocoder currently being standardized by the CDG, has shown MOS scores consistent with toll quality voice. The improved voice quality is not free, however, since capacity and cell site range are affected by the higher data rate vocoder.

Although the use of 13 kbps vocoders has a negative impact on system capacity, the capacity penalty can be minimized, and even virtually eliminated, by the "intelligent" control of 13 kbps vocoder usage. For example, stepping back to 8 kbps vocoders in congested cells on a cell by cell, call by call basis.

Coverage is a central concern for many new operators who need to quickly provide wide geographic coverage (a large system "footprint"). The 13 kbps vocoder decreases the cell range below that of NAMPS systems, though it is still greater than GSM's cell radius.

The 8 kbps EVRC vocoder offers most of the quality improvement of the 13 kbps vocoder (when compared to the existing 8 kbps vocoder), without the penalty in cell site capacity and range.

To take advantage of CDMA's dynamic selection of multiple vocoders within a system, Motorola is working with several CDMA customers to define multiple service tiers tied to vocoder utilization. For example, the top tier will be a high quality voice service based upon the 13 kbps vocoder. The bottom tier will be a standard voice quality service provided by the 8 kbps vocoder. A middle service tier will provide high quality voice service when it is available and provide standard 8 kbps voice quality during the peak usage periods.

Mean Opinion Scores

Important Note: It is extremely important to recognize that Mean Opinion Scores (MOS) are relative measures of voice quality. They can effectively be used to rate one vocoder better or worse than another, provided both were evaluated in the same test, by the same people, under the same conditions. The same vocoder may get very different scores in different tests with different listeners! In addition, a rating of 3.4 means nothing without a suitable reference point, which is why LD-CELP or ADPCM is usually included in listening tests.

The first table below shows the MOS from a test made at the AT&T listening labs. The tests were paid for by AT&T, Motorola and Qualcomm. As can be seen, the 13 kbps CELP vocoder does well in most conditions.

The following scores were achieved with a clear channel:

16 kbps LD-CELP 3.72 ("wireline quality" reference)

8 kbps CDMA (IS-96) 3.46

13 kbps CELP 4.01

The 13 kbps CELP vocoder was also tested at higher frame erasure rates:

13 kbps CELP with 1% FER 3.94

13 kbps CELP with 2% FER 3.89

The table below shows the performance of the new 8 kbps EVRC in a test conducted by the TIA during July of 1995. (As emphasized in the note above, it would be incorrect to compare the quantitative scores from this test with those from other tests.) The scores from this test show that the 8 kbps EVRC compares favorably to the 16k LD-CELP in most conditions.

The following scores were achieved with a clear channel:

16 kbps LD-CELP 3.58 ("wireline quality" reference)

8 kbps CDMA (IS-96A) 3.33

8 kbps EVRC 3.62

The 8 kbps EVRC was also tested at higher frame erasure rates:

8 kbps EVRC with 1% FER 3.60

8 kbps EVRC with 2% FER 3.46

CDMA Short Message Services (SMS)

The Short Message Service (SMS) allows for the exchange of alpha-numeric messages via the CDMA infrastructure. Messages can originate from either the network or from a mobile station. The mobile may be an appropriately equipped voice telephone, a data terminal or a specialized short message entry system.

In addition, CDMA SMS will support the following capabilities:

• 256 Character Mobile Terminated Short Messaging (outbound to mobile). By comparison, NAMPS SMS supports 14 characters.
• Urgent Message Indicator: This feature allows any digital page, short message, or voice mail indication sent to a subscriber to be marked as urgent when it is recalled by the subscriber.
• Date/Time Stamp: This feature provides any digital page, short message, or voice mail indication sent to a subscriber with a date/time stamp when it is recalled by the subscriber.
• Message Acknowledgment: This feature returns an acknowledgment from the receiving mobile following the receipt of a short message (page, message, etc.) by the mobile.
• Off-Hook Services: This feature allows any digital page, short message, or voice mail indication to be sent to a cellular user on an active call. Typically, these features are only available when the phone is not in use.
• Broadcast: A common broadcast message is sent to all mobile stations within a selected zone (one or more sectors). Broadcast messages will be overwritten by higher priority short messages. Value-added services like advertisements, service bulletins, etc. can be supported via this feature. Note that the subscriber unit can be configured to ignore broadcast messages.
• Mobile Originated Short Messaging: The mobile unit has the option of originating a short text message for delivery by the network to the identified mobile or land destination address.

CDMA Short Message Service implementation will support feature transparency with NAMPS Short Message Service. NAMPS SMS provides digital paging, short message receipt at the mobile station (14 characters), and voice mail indication.

Over-the-Air Activation

"Over-the-Air Activation" is a capability which is key to the future business plans of many wireless operators. This feature, with specifications developed by the CDG, allows a potential cellular service subscriber to activate (become authorized for) new cellular service without the intervention of a third party, such as an authorized dealer.

One of the primary objectives of Over-the-Air Activation is the ability to provide a secure authentication key to a mobile station to facilitate the authentication process. Authentication is the process by which information is exchanged between a mobile station and the network for the purpose of confirming and validating the identity of the mobile station. A successful outcome to the authentication process occurs only when it is demonstrated that the mobile station and the network possess identical sets of Shared Secret Data.

The Over-the-Air Activation feature consists of "over-the-air" programming of Number Assignment Modules (NAMs), which are used to authorize cellular telecommunications service with a specific service provider. The feature incorporates an Authentication Key Exchange Agreement algorithm. This algorithm allows the network to exchange Authentication Key parameters with a mobile station. These parameters are used to generate the Authentication Key which is then used to generate the Shared Secret Data. The Authentication Key Exchange Agreement algorithm enhances security for the subscriber and reduces the potential for fraudulent use of cellular telecommunications service.

Sleep Mode

Sleep Mode is a feature which allows the user to place the mobile station into an operating mode in which it periodically wakes up to be notified of any pending SMS messages. The purpose of this feature is to substantially extend battery life beyond the current state-of-the-art. Estimates by subscriber unit manufacturers are on the order of several days of "stand-by-time" with standard batteries.

Upon notification of the pending SMS messages, the subscriber can choose to retrieve the messages from the service provider. During the time when this feature is activated, the mobile station user can still originate calls.

CDMA Data and Fax

CDMA circuit switched services provide asynchronous data and facsimile transmission using an Interworking Unit (IWU). The IWU provides the functions needed for mobile equipment to communicate with fixed end equipment in a public network. This architecture adapts the air interface and land lines by providing re-transmission protocols unique to the CDMA air interface, called the Radio Link Protocol (RLP) and rate adaptation to the land line modems.

The various data services are initiated as service options during call setup, or anytime during a call. The service option negotiation process specifies whether the service option will be used for the primary and secondary traffic. Therefore, the user can switch between voice, data, and fax service simply by initiating and terminating the appropriate service options.

Circuit Mode Asynchronous Data/Fax Rates

The CDMA system will support synchronous and asynchronous data services that emulate a traditional modem connection to the PSTN. For the CDMA system, the modem will not be located in the subscriber unit, but in the network. This allows for direct digital communications over the radio channel. This preserves the advantages of true digital transmission, eliminating the need to convert from digital to analog, then back to digital.

Using this approach, the system design will enable a subscriber unit to transparently communicate with any landline modem. The landline user modem can support any existing V-Series modulation techniques. In the current revision of the air interface, data rates of up to 9.6 kbps are supported for the 8 kbps vocoder and up to 14.4 kbps for the 13 kbps vocoder. The following table lists the primary signaling standards and transmission rates that CDMA will support. This list is not meant to be exhaustive; other modem types can be supported as options.

Table - Supported Primary Modem Standards.

Modem Standard	Rate (if Applicable)
V.21	300 bps
V.22/V.22bis	1200/2400 bps
V.32/V32bis	9600/14400 bps
Bell 103/212A	300/1200 bps
V.42/V.42bis	These error correction and compression standards will also be transparently supported by the core PCS system.

Any of the transmission rates in the CDMA system can be used for data. To support efficient transmission, flow control is an integral part of the CDMA system. Thus, the air interface transmission rate need not match the landline rate. For example, the air interface rate can be higher than the landline rate to reduce delay. Similarly, the air interface rate can be lower than the landline rate to support more users having simultaneous traffic demands.

Support For Higher Data Rates

CDMA systems will eventually support higher data rates, and thus can provide more throughput for emerging modem standards. A major design objective is to meet the 64 kbps capacity of the standard ISDN B-channel to provide such services as compressed video transmission. This rate could be supported by an eventual system rate of 76.8 kbps.

"Bandwidth on demand" refers to CDMA's flexibility to implement new and innovative features that may eventually support higher data rates and simultaneous voice and data transmission based upon how the spectrum and carriers are applied. Theoretically, a wide band carrier such as CDMA can support very high data rates which could be used to send voice, data, video, facsimile and other services.

Simultaneous Voice and Data

The CDMA system will support the simultaneous transmission of voice and data. The two digital streams will be multiplexed on a frame by frame basis with voice being given priority over data to maintain voice quality. As higher data rate channels are introduced later, data throughput will increase. Several modes of operation including turning on and off voice service during a data call or adding data to a voice call in progress will be supported.

Packet Data Services

CDMA is defined as a lower level protocol. The CDMA system will support higher level protocols commonly used for data communications, such as TCP/IP.

Simultaneous voice and data will be available in the case of packet data applications.

Simultaneous Subscriber Unit Ringing

This feature operates much like a land line extension phone service. Cellular carriers can provide high-minute users with CDMA phones and allow the subscriber to compare analog and CDMA performance head to head. If the customer prefers CDMA, they can keep (purchase?) the CDMA phone. This reduces the risk of losing subscribers who do not prefer the sound quality of digital (as was the case in many TDMA systems).

Subscriber Access Control

Subscriber Access Control allows an operator to selectively control mobile access to the CDMA system on a per cell/sector basis. The control is implemented through the use of Access Overload Classes (ACCOLC). Each mobile is assigned an ACCOLC class at the time of subscription. When a mobile attempts to access the CDMA system in a cell/sector where the feature is active, the mobile will compare its ACCOLC with the set of allowable access classes currently broadcast by the cell/sector. All mobiles denied access may optionally be redirected by the Base Station System to the analog system within the same coverage area.

Potential uses of the Subscriber Access Control feature include during field testing, during initial system deployment and during system maintenance windows. For example, a controlled environment can be provided for testing purposes by denying CDMA service to some or all commercial mobiles, while allowing service to test mobiles. The CDMA service denial will result in the re-direction of the denied mobiles to the analog system. Upon completion of pre-commercial testing, an operator may wish to gradually phase in CDMA traffic. Again, the Subscriber Access Control feature can be used to control the loading of an initial system, thus allowing for more reliable and controlled monitoring of system performance. Finally, during system maintenance periods, Subscriber Access Control can be used to re-direct dual mode mobiles in a specific service area, while not interrupting service to a subscriber.