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Since 1989, the wireless industry began to explore by adapting digital technology replacing the existing analog network as a means of improving capacity due to the increase of wireless service demands. Digital has a number of advantages over analog transmission: Economizes on bandwidth Allows easy integration with personal communication systems (PCS) devices Maintains superior quality of voice transmission over long distances Difficult to decode Can use lower average transmitter power Enables smaller and less expensive individual receivers and transmitters Offer voice privacy At that period, Time Division Multiple Access (TDMA) was chosen to replace Motorola's Frequency Division Multiple Access (FDMA) technology which known today as narrowband analog mobile phone service. FDMA allocates a single channel to one user at a time. FDMA is wasteful of bandwidth: the channel is assigned to a single conversation whether or not somebody is speaking. Moreover, it cannot handle alternate forms of data, only voice transmissions. Qualcomm then introduced the Code Division Multiple Access (CDMA) technology to compete against TDMA technology to provide digital service.
Now the two major competing systems (TDMA and CDMA), have been a topic for debate throughout the wireless community over which technology has the superior quality. WHAT IS TDMA? TDMA is digital transmission technology that allows a number of users to access a single radio-frequency (RF) channel without interference by allocating unique time slots to each user with thin a channel. The digital conversations from a single transmitter occupy different time slots in several bands at the same time In TDMA, the access technique used exercises three users sharing a 30 -kHz carrier frequency.
TDMA is also the technology used in Europe where Global System for Mobile Communications (GSM) is the digital standard used to provide wireless access and in Japan where Personal Digital Cellular (PDC) is used as the standard. TDMA was chosen for these standards due to the reason that it enables some essential features for system operation in an advanced cellular or PCS environment. A single channel can carry all four conversations if each conversation is divided in short fragments, is assigned a time slot, and is transmitted in corresponding time bursts. TDMA also offers the ability to carry data rates of 64 kbps to 120 Mbps. This enables service providers to offer short message services as well as bandwidth-intensive applications such as multimedia and videoconferencing. Unlike other spread spectrum technologies, TDMA ensures the avoidance of interference from other users transmitting at the same time on the same frequency since users it separates the users in time.
Also, TDMA installations offer substantial savings in base-station equipment, space, and maintenance, an important factor as cell sizes grow ever smaller. One of the disadvantages of TDMA is that each user has a predefined time slot. Users roaming from one cell to another are not allotted a time slot. Therefore, if the user moves to another time slot, and all slots are occupied, the conversation may be disconnected. Another problem with TDMA is that it is prone to multipath distortion. A signal from a base-station to the mobile phone can take many directions.
Thus, the signal can bounce of several buildings which can cause interference. WHAT IS CDMA? CDMA stands for Code Division Multiple Access. This means that the signal is coded, not only providing better security but also allowing multiple, simultaneous users. 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). With CDMA, unique digital codes, rather that separate RF frequencies or channels are used to differentiate subscribers within on frequency.
The codes are shared by both the mobile phone and the base station. The codes are called pseudo-random code sequences. CDMA uses radio spectrum that is 1. 25 MHz wide. Increased privacy is a common trait of CDMA technology.
CDMA calls will be secure from eavesdroppers. Unlike analog conversation, a simple radio receiver will not be able to pick up individual digital conversations out of the overall RF frequency band. CDMA also provides soft handoff. In a cellular system, a users call switches from one cell site to another as the user travels. With CDMA, users are on the same frequency for the whole network, thus the mobile phone must only negotiate the pseudo-random code instead of switching to a whole new frequency which is known as a hard handoff.
Therefore, the soft handoff offers transparent cell site switching, instead of having distorted audio due to hard handoffs. A problem with CDMA is channel pollution. This occurs when signals from too many base stations are present and non are more dominant than the others. This then causes rapid degradation of audio quality. COMMENTARY The principle problem facing the wireless communication community is how to allow a large number of users access allotment of frequencies. In todays wireless communication systems, there are two principle methods being used to multiplex users on the same frequencies, one by time, and the other by code.
CDMA no doubt has the highest spectral efficiency of all the digital wireless technology. It can accommodate more users per MHz of bandwidth than any other technology. CDMA supporters claim bandwidth efficiency of up to 13 times that of TDMA and between 20 to 40 times that of analog transmission. CDMA has other benefits over other systems such as: Better signal quality Privacy of coded digital communications Soft handoffs when changing cell sites Easy addition of more users It is quite obvious that CDMA technology is the wave of the future. But the main concern is the ability of the carriers to implement the system. TDMA systems have been operable for quite some time.
Thus the network is by now well established. Fast time to market is essential to carriers due to the phenomenal pace of todays wireless communications market, and many providers choose to incest in TDMA systems that have already been developed and proven. Furthermore, the cost of installing new CDMA stations is much more expensive. A CDMA station would cost $ 300, 000 in comparison to $ 80, 000 per TDMA base station.
Other implementation complication includes backward compatibility. Consumers must possess dual mode phones in order to be compatible with other systems. But as CDMA coverage grows slowly, this will not be a problem. With present developments, it is apparent that CDMA will eventually displace TDMA as the primary wireless technology. Since TDMA is a proven technology, it will be a viable alternative for markets that require lower capacity demands.
Because CDMA is ideally suited for the high traffic demand of populated urban areas, CDMA systems will dominate these markets. REFERENCE LIST web web web web web web
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