RE: CDMA (Download Full Seminar Report)
Tejas V. Shah
46_CDMA.doc (Size: 658 KB / Downloads: 84)
Generally a fixed amount of frequency spectrum is allocated to a cellular system by the national regulator (e.g. in the United states,the Federal communication commission). Multiple-access techniques are then deployed so that many users can share the available spectrum in an efficient manner. Multiple access systems specify how signals from different sources can be combined efficiently for transmission over a given radio frequency band and then separated at the destination without mutual interference.The three basic multiple access methods currently in use in cellular systems are:
• Frequency division multiple access (FDMA)
• Time division multiple access (TDMA)
• Code division multiple access (CDMA)
In case of FDMA ,users share the available spectrum in the frequency domain,and user is allocated a part of the frequency band called the traffic channel.
In TDMA techniques that are utilized in many digital cellular systems,the available spectrum is partitioned into narrow frequency bands or frequency channels (as in FDMA),which in turn are divided into a number of time slots.An individual user is assigned a time slot that permits access to the frequency channel for the duration of the time slot.
The CDMA systems utilizes the spread spectrum technique,whereby a spreading code(called a pseudo-random noise or PN code) is used to allow multiple users to share a block of frequency spectrum.In CDMA cellular systems (e.g. IS-95 in the United States) that use direct sequence spread (DSS) spectrum techniques,the(digital) information from an individual user is modulated by means of the unique PN code (spreading sequence) assigned to each user.All the PN –code-modulated signals from different users are then transmitted over the entire CDMA frequency channel (e.g.,1.23 MHZ in case of IS-95).Since the signal in the case of CDMA utilize the entire allocated block of spectrum,no guard bands of any kind are necessary within the allocated block.
CDMA permits a more uniform distribution of energy in the emitted bandwidth Short for Code Division Multiple Access, a digital cellular technology that uses spread-spectrum techniques. Unlike competing systems, such as GSM that use TDMA, CDMA does not assign a specific frequency to each user. Instead, every channel uses the full available spectrum. Individual conversations are encoded with a pseudo-random digital sequence. The older version of the CDMA technology and now it is now known as cdmaOne as well as IS-95. The other types of CDMA technology has CDMA2000,WCDMA (Wideband CDMA). The spread spectrum may be viewed as a kind of modulation scheme in which the modulated(spread spectrum) signal bandwidth is much greater than the message(baseband) signal bandwidth. Thus, spread spectrum is a wideband scheme.
The final assessment on the potential superiority of CDMA systems over TDMA systems,in terms of capacity,cost, and speech quality,will emerge only after both systems have been in operation in dense,urban areas with full complements of subscribers and services.
A CDMA system is clearly not a collision avoidance system like FDMA and TDMA. The opposite is true and explains the differences in the behavior of CDMA systems compared to FDMA and TDMA. In general, the collisions at the channel is a disadvantage of CDMA system and can be mitigated by careful selection of the sequence and power control that is close to perfect. CDMA is restricted to a short distance charging area(SDCA). Currently, there are 2600 SDCAs within the country. A CDMA-based phone can thus ‘roam’ only within its SDCA. This is NOT a technological restriction. In India, Reliance Infocom and Tata Indicom use CDMA technology to provide WILL services. In remote rural areas, where installing cables is difficult as well as expensive, CDMA-based WILL networks can be deployed quickly. A CDMA doesn’t have a SIM card, which makes m-commerce difficult.
Daily application possible with CDMA is daily downloading, text communication such as chat,e-mail,sms,member search etc.sending photo on the air,entertainment and games.
This paper is intended to provide an introduction to CDMA use in wireless telephone systems. The focus is on explaining, in generally non-technical language, both the key aspects of CDMA technology, and the primary benefits the technology offers to wireless communication system operators and their subscribers. There is a tremendous amount of detailed technical information which is intentionally not covered in this forum.
It has been necessary, though, to assume at least a rudimentary familiarity with cellular telephone systems, including the basic characteristics of radio and the RF spectrum, as well as fundamental system design concepts such as frequency re-use.
Motorola welcomes your comments and feedback on this paper.
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".
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. This will be discussed in later sections.
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 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.