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CODE DIVISION MULTIPLE ACCESS
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.
CDMA is a "spread spectrum" technique in which each phone in a cell uses a distinct code known to the base station to communicate with the base station. All frequencies in one cell can be used in other cells.
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".
In March 1992, the TIA established the TR-45.5 subcommittee with the charter of developing a spread-spectrum digital cellular standard. In July of 1993, the TIA gave its approval of the CDMA IS-95 standard.
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.
Current Cellular Standard
Different types of cellular systems employ various methods of multiple access. The traditional analog cellular systems, such as those based on the Advanced Mobile Phone Service (AMPS) and Total Access Communications System (TACS) standards, use Frequency Division Multiple Access (FDMA). FDMA channels are defined by a range of radio frequencies, usually expressed in a number of kilohertz (kHz), out of the radio spectrum.
For example, AMPS systems use 30 kHz "slices" of spectrum for each channel. Narrowband AMPS (NAMPS) requires only 10 kHz per channel. TACS channels are 25 kHz wide. With FDMA, only one subscriber at a time is assigned to a channel. No other conversations can access this channel until the subscriber's call is finished, or until that original call is handed off to a different channel by the system.
A common multiple access method employed in new digital cellular systems is Time Division Multiple Access (TDMA). TDMA digital standards include North American Digital Cellular (known by its standard number IS-54), Global System for Mobile Communications (GSM), and Personal Digital Cellular (PDC).
TDMA systems commonly start with a slice of spectrum, referred to as one "carrier". Each carrier is then divided into time slots. Only one subscriber at a time is assigned to each time slot, or channel. No other conversations can access this channel until the subscriber's call is finished, or until that original call is handed off to a different channel by the system.
For example, IS-54 systems, designed to coexist with AMPS systems, divide 30 kHz of spectrum into three channels. PDC divides 25 kHz slices of spectrum into three channels. GSM systems create 8 time-division channels in 200 kHz wide carriers.
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.
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.
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.
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).
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.
Traditional uses of spread spectrum are in military operations. Because of the wide bandwidth of a spread spectrum signal, it is very difficult to jam, difficult to interfere with, and difficult to identify. This is in contrast to technologies using a narrower bandwidth of frequencies. Since a wideband spread spectrum signal is very hard to detect, it appears as nothing more than a slight rise in the "noise floor" or interference level. With other technologies, the power of the signal is concentrated in a narrower band, which makes it easier to detect.
Increased privacy is inherent in CDMA technology. CDMA phone calls will be secure from the casual eavesdropper since, unlike an analog conversation, a simple radio receiver will not be able to pick individual digital conversations out of the overall RF radiation in a frequency band.
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.
Operators will have the opportunity to balance these parameters to best serve a particular area. The best balance point may change from cell site to cell site. Sites in dense downtown areas may trade off coverage for increased capacity. Conversely, at the outer edges of a system, capacity could be sacrificed for coverage area.
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.
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
CDMA for Cellular
When implemented in a cellular telephone system, CDMA technology offers numerous benefits to the cellular operator and their subscribers. These can be summarized as follows:
CDMA for Personal Communications Services (PCS)
Personal Communications Services (PCS) is anticipated to be the "omnipresent" wireless communication system of the future - a vehicle through which people can communicate to whomever they want, whenever they want, wherever they want.
CDMA is a strong solution for PCS because it offers higher capacity and increased range of coverage. By requiring fewer cell sites than traditional analog and other digital systems, CDMA is viewed as the most cost effective technology for PCS.
From enhanced spectral efficiency to improved call quality and feature flexibility, CDMA offers a host of key benefits to wireless PCS operators. Specifically, benefits which are especially relevant to PCS operators include:
Motorola's SC™ hardware and software platform makes CDMA even more attractive for PCS, considering its ability to grow with the needs of the market. For example, MCC cards, which contain the traffic channels on the system, are added to networks as customers' market penetration increases and more subscribers are supported. In addition, the digital strategies Motorola employs in its designs will ensure that in the future, the SC™ products have excellent potential to decrease in size and in cost.
CDMA for Wireless Local Loop (WiLL™)
In many developing countries, there is tremendous demand for new business and residential telephone service. More and more operators are looking to wireless technologies to rapidly provide thousands of new subscribers with high quality telephone service at a reasonable price. Motorola has introduced a set of Wireless Local Loop (WiLL™) systems which can meet the telephony requirements of many communities.
The unique features and benefits of CDMA make it an excellent technology choice for fixed wireless telephone systems.
In a fixed telephony environment, CDMA is estimated to provide up to 15-20 times the capacity of an AMPS cellular system, resulting in the highest capacity cellular-based offering for wireless local loop applications. The key to this increased WiLL™ capacity is that, in a fixed environment, CDMA power control is able to very accurately track required power, thereby resulting in reduced overall transmission power and increased capacity. Subscribers in a fixed environment require less RF power to achieve quality communications, therefore more subscribers can be placed on a CDMA channel.
In addition, CDMA optimizes use of the radio spectrum, which is an increasingly scarce resource worldwide. CDMA's single cell frequency re-use capability and non-contiguous bandwidth requirement, along with its extended coverage range, simplifies RF planning and implementation. This allows an operator to invest in fewer cell sites with faster deployment, ultimately giving the service provider increased and quicker access to revenues.
The CDMA digital interface also allows for a number of complex and unique features such as variable rate vocoders; robust error correction; frequency, space and time diversity; and multipath immunity. These features all contribute to improved system and call quality.