Literature Review Online Billing System

Introduction

Mobile communications are rapidly becoming more and more necessary for everyday activities. With so many more users to accommodate, more efficient use of bandwidth is a priority among cellular phone system operators. Equally important is the security and reliability of these calls.

One solution that has been offered is a Code division multiple access system. CDMA is one method for implementing a multiple-access communication system. MULTIPLE ACCESS is a technique where many subscribers or local stations can share the use of a communication channel at the same time or nearly so despite the fact originate from widely different locations. A channel can be thought of as merely a portion of the limited radio resource, which is temporarily allocated for a specific purpose, such as someone’s phone call.

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A multiple access method is a definition of how the radio spectrum is divided into channels and how the channels are allocated to the many users of the system. Since there are multiple users transmitting over the same channel, a method must be established so that individual users will not disrupt one another. There are essentially three ways to do this. Code Division Multiple Access is a new technology used in wireless communication devices. This technology made its commercial debut in the early nineties.

A significant advantage of the CDMA is the fact that, unlike other modulation schemes, it does not have to allocate part of the frequency for each user. It allocates the whole frequency spectrum to each user, distinguishing each signal with a unique pseudo-random sequence. CDMA stands for "Code Division Multiple Access. " It is a form of spread-spectrum, an advanced digital wireless transmission technique. Instead of using frequencies or time slots, as do traditional technologies, it uses mathematical codes to transmit and distinguish between multiple wireless conversations.

Its bandwidth is much wider than that required for simple point-to-point communications at the same data rate because it uses noise-like carrier waves to spread the information contained in a signal of interest over a much greater bandwidth. However, because the conversations taking place are distinguished by digital codes, many users can share the same bandwidth simultaneously. We are moving into a new era of communications and information technology. Personal Competitiveness in business relies more and more on increasing personal productivity and responsiveness.

Today everybody is on the move, and mobile is the only way to keep in contact with that person. 1 But nowadays peoples want multimedia facilities from their mobile handset. But it requires a high data rate, high efficiency, and many more technical things, which are available in the third generation. (CDMA) so CDMA Technology makes existing mobile handsets more efficient and attractive. CDMA (3G) mobile devices and services will transform wireless communications into online, real-time connectivity. 3G wireless technology will allow an individual to have immediate access to location-specific services that offer information on demand.

The first generation of mobile phones consisted of analog models that emerged in the early 1980s. The second generation of digital mobile phones appeared about ten years later, along with the first digital mobile networks. During the second generation, the mobile telecommunications industry experienced exponential growth both in terms of subscribers as well as new types of value-added services. Mobile phones are rapidly becoming the preferred means of personal communication, creating the world's largest consumer electronics industry.

The rapid and efficient deployment of new wireless data and Internet services has emerged as a critical priority for communications equipment manufacturers. Network components that enable wireless data services are fundamental to the next-generation network infrastructure. Wireless data services are expected to see the same explosive growth in demand that Internet service and wireless voice services have seen in recent years. 1. What is CDMA? Code Division Multiple Access (CDMA) is a digital cellular spread-spectrum modulation technique that implements distributed voice and data networks.

CDMA works by converting speech into digital information, which is then transmitted as a radio signal over a wireless network. CDMA does not assign a specific frequency to each user but the full available spectrum. Each signal is encoded differently using a unique code. This way, CDMA enables a large number of users to share the same frequency band at the same time without interference. The receiving device is instructed to use the code to extract the data from the received signal.

History

The first generation of cellular systems, which include the AMPS (Advanced Mobile Phone Systems), was introduced in the early 1980s. These systems use analog frequency modulation (FM) and have a frequency division multiple access (FDMA) based media access control (MAC) architecture. Within a few years, market demands and capacity requirements began to grow to hit the practical limitations. These limitations motivated the development of second-generation cellular systems, which improved compatibility and accommodated higher capacity than first-generation systems.

These systems use digital modulation and processing techniques. TDMA (Time Division Multiple Access), GSM (Global System for Mobile Communication), and (narrowband) CDMA belong to the second-generation systems. CDMA was introduced in 1994 by Qualcomm, Inc. Using direct sequence code division multiple access, it claimed to provide 10 times more capacity than analog systems. Far more than TDMA or GSM. Today, CDMA is the basis of the third-generation market in the United States and other places in the world.

Background

A cellular system is called so because it divides the service area into small transmission areas called cells.

Each cell contains a base station (BTS), which consists of a transceiver and a receiver in order to connect to mobile phones in the cell. Each cell is assigned a group of radio channels (frequencies).

The Cellular Challenge

The world's first cellular networks were introduced in the early 1980s, using analog radio transmission technologies such as AMPS (Advanced Mobile Phone System). Within a few years, cellular systems began to hit a capacity ceiling as millions of new subscribers signed up for service, demanding more and more airtime. Dropped calls and busy network signals became common in many areas.

To accommodate more traffic within a limited amount of radio spectrum, the industry developed a new set of digital wireless technologies called TDMA (Time Division Multiple Access) and GSM (Global System for Mobile). TDMA and GSM used a time-sharing protocol to provide three to four times more capacity than analog systems. But just as TDMA was being standardized, an even better solution was found in CDMA.

Commercial Development

The founders of QUALCOMM realized that CDMA technology could be used in commercial cellular communications to make even better use of the radio spectrum than other technologies.

They developed the key advances that made CDMA suitable for cellular, then demonstrated a working prototype and began to license the technology to telecom equipment manufacturers. The first CDMA networks were commercially launched in 1995 and provided roughly 10 times more capacity than analog networks - far more than TDMA or GSM. Since then, CDMA has become the fastest-growing of all wireless technologies, with over 100 million subscribers worldwide. In addition to supporting more traffic, CDMA brings many other benefits to carriers and consumers, including better voice quality, broader coverage, and stronger security. CHAPTER 3 3. 0

Discussion

Multiple Access Systems A Multiple access system is a technology that specifies the way multiple users can share the same transmission medium. There are three main types of multiple access systems, each of which has its own way of sharing the bandwidth.

• Frequency Division Multiple Access (FDMA)
• Time Division Multiple Access (TDMA)
• Code Division Multiple Access (CDMA)

FDMA and TDMA are narrowband technologies, and CDMA is wideband. FDMA and TDMA In the FDMA technology, signals from various users are assigned different frequencies.

When a frequency channel is assigned to a user, no other user of the same cell or in the neighboring cell can use it at the same time. In TDMA technology, the information from each user is conveyed in time intervals called time slots. A few users using a different time slot might share the same frequency. When all the available time slots in a given frequency are used, a new user connecting to the system must be assigned a time slot on a different frequency. In a way, TDMA is very similar to a computer with only one processor that seems to run multiple processes simultaneously.

Only one person is actually using the frequency channel at any given moment and then has to give up the channel to allow other users to use it. Code division multiple access (CDMA) systems are spread spectrum systems in which the users are able to transmit simultaneously in the same frequency channel and use the entire system's spectrum.

Frequency division multiple access

In this technique, the available bandwidth is split up into non-overlapping frequency bands, and these disjoint subbands of frequency are allocated to the different users on a continuous time basis.

In order to reduce interference between users allocated adjacent channel bands, channel bands are used to act as buffer zones, as illustrated in figure(1). These guard bands are necessary because of the impossibility of achieving ideal filtering for separating the different users. It could be compared to AM or FM broadcasting radio, where each station has a frequency assigned.

Time division multiple access

In this technique, each user is allocated the full spectral occupancy of The channel, but only for a short duration of time called a time slot.

Buffers zones are in the form of guard times inserted between the assigned time slots. This is done to reduce interference between users by allowing for time uncertainty that arises due to system imperfections, especially in synchronization schemes.

Code division multiple access

The above drawbacks are overcome in this third technique in which the users are spread across both frequency and time in the same Channel. This is a hybrid combination of FDMA and TDMA. For example, frequency hopping may be employed to ensure during each successive time slot, the frequency bands assigned to the users are recorded in a random manner.

During time slot 1, user 1 occupies frequency band 1, user 2 occupies frequency band 2, user 3 occupies band 3, and so on. During time slot 2, user 1 hops to frequency band 3, user 2 hops to band 1, user 3 hops to band 2, and so on. An important advantage of CDMA over FDMA and TDMA is that it can provide secure communication. 6 3. 5 Generating the CDMA signal In the CDMA technique, the signal transmissions among multiple users completely overlap in both time and frequency. The separation between the users is made by assigning each user a unique code.

Generally, CDMA converts an analog voice signal to a digital signal, encodes the digital signals, and separates voice and control data into data streams called channels. Generating a CDMA signal is a five steps process: The first step is analog to digital conversion or A/D. The incoming voice signal is an analog signal meaning that it is constantly changing, taking on all possible values of amplitude range. The CDMA uses a digital signal for its further manipulations. That digital signal is characterized by discrete states. In that step, the analog voice signal is quantized to form a digital signal consisting of a few levels.

The second step is voice coding or Vocoding. Voice encoding is the process of compressing the audio into as small a stream of bits as possible. The vocoder takes advantage of the pauses in speech to accomplish maximum compression. The Vocoder’s rate must be variable to fit the rate of the user’s speech activity. The third step is encoding and interleaving. This step's purpose is to reduce the errors when receiving the signal. Interleaving is a method of reducing the effects of burst errors and recovering lost bits. The symbols are interleaved such that originally neighboring symbols will be transmitted far away from each other.

In addition to that, the various encoding methods add redundancy to the signals to help the recovery of information at the receiver in case of errors. The fourth step is channelizing. The signal of each user is further encoded to create a separation between different users. A unique identification code is given to each user, and the signals of all users are transmitted together, sharing the same frequency and time. The CDMA receiver decodes the signal by multiplying it by a decoding sequence of the desired user. Two common code types are the Walsh code and PN (pseudo-random noise) code.

The Walsh code is used for forwarding the CDMA channel (e.g., cell to the mobile direction of communication). Walsh codes are orthogonal, meaning that the code of each user can be decoded at the receiver only by using the same Walsh code used to transmit the signal. The PN code is used for reverse CDMA channels (e.g., the mobile-to-cell direction of communication). A series of digital signals 0? s and 1? s go into an antipodal mapping device to produce a bit stream of negative and positive 1? s. Each user signal is then multiplied by the PN code series. At this moment, the signals occupy a wide frequency spectrum. The PN sequence rate is much higher than that of the original signal. It is generated in a deterministic manner and is repetitive. However, there are about 4. Four trillion combinations of this code, and for practical purposes, we may assume that this sequence is truly random.

Advantages

Voice is the major source of traffic and revenue for wireless operators, but packet data will emerge in the coming years as an important source of incremental revenue. CDMA2000 delivers the highest voice capacity and packet data throughput using the least amount of spectrum for the lowest cost.

• Higher data throughput
• Increased battery life

Benefits include:

• Quick paging channel operation
• Improved reverse link performance
• New common channel structure and operation
• Reverse link gated transmission

Transmit diversity:

Transmit diversity consists of de-multiplexing and modulating data into two orthogonal signals, each of them transmitted from a different antenna at the same frequency. The two orthogonal signals are generated using either Orthogonal Transmit Diversity (OTD) or Space-Time Spreading (STS).

The receiver reconstructs the original signal using the diversity signals, thus taking advantage of the additional space and/or frequency diversity.

Disadvantages

1. Multi-user interference or multiple access interference(MAI)
2. Multi-path fading
3. Near- for the problem.

Conclusion

CDMA is a radically new concept in wireless communication. It has gained widespread international acceptance by cellular radio system operators as an upgrade that will dramatically increase both their system's capacity and service quality.

Moreover, spread spectrum technology is both more secure, less probable to intercept and jam, highly private, and offers higher transmission quality than TDMA because of its increased resistance to multipath distortion. The principal type of CDMA systems is direct sequence CDMA, frequency hopping CDMA, and multicarrier CDMA. The major problem in CDMA is multiple Access interference (MAI) which arises due to the deviation of the spreading codes from perfect orthogonally.

The capacity of CDMA is interference limited. The obvious way to increase the capacity of the CDMA is to reduce the level of interference. This is achieved by reducing cross-correlation, power control, and antenna arrays.

Reference:

1. Simon Hawkins –Communication Systems-John Willy & Sons, Fourth edition
2. CDMA TECHNIQUES FOR THIRD GENERATION MOBILE SYSTEM by Francis Swarts, PETER Van Rooyan, Ian Opperman & Michael P. Loyyer
3. http://www. telecomresearch.com

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