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Access technology - Information Technology - Lecture Notes, Study notes of Information Technology

COMSATS Institute of Information Technology, Abbottabad (CIITA)Information Technology

Access technology, Advanced Mobile Phone Service, Sequence Of Presentation, Cellular Access Technologies, Frequency Division Multiple Access, Time Division Multiple Access are key learning points of this lecture handout.

Typology: Study notes

2011/2012

Uploaded on 11/10/2012

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TIME
TEXT
SLIDE NO
Introduction
1. Access technology, although a current wireless phrase, is,
an open and formless term. Transmission, the process of
transmitting, of conveying intelligence from one point to another,
is a long settled, traditional way to express how signals are sent
along.
2. To access means to use, make available, or take control.
Multiple access means the cell site can handle many calls at once.
In a communication system like the analog based Advanced Mobile
Phone Service (AMPS), we access that system by using frequency
division multiple access or FDMA.
3. One of the most basic problems in any radio based
telecommunication system is how to accommodate as many
different users as possible within a given spectrum allocation. The
possible solution involves allocating the spectrum in terms of
frequency, time, space and code.
Sequence Of Presentation
4. The sequence of the presentation would be:
a. Introduction
b. Aim
c. Air Interface
d. FDMA
e. TDMA
f. CDMA
g. CDMA – Spread spectrum
h. Benefits of CDMA
j. Conclusion
Cellular Access Technologies
5. The basic problem in any radio based telecommunication
system is how to accommodate as many different users as possible
within a given spectrum allocation. The possible solution involves
allocating the spectrum in terms of frequency, time, space and
code. There are three common technologies used by cell-phone
networks for transmitting information:
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Introduction

  1. Access technology, although a current wireless phrase, is, an open and formless term. Transmission, the process of transmitting, of conveying intelligence from one point to another, is a long settled, traditional way to express how signals are sent along.
  2. To access means to use, make available, or take control. Multiple access means the cell site can handle many calls at once. In a communication system like the analog based Advanced Mobile Phone Service (AMPS), we access that system by using frequency division multiple access or FDMA.
  3. One of the most basic problems in any radio based telecommunication system is how to accommodate as many different users as possible within a given spectrum allocation. The possible solution involves allocating the spectrum in terms of frequency, time, space and code. Sequence Of Presentation
  4. The sequence of the presentation would be: a. Introduction b. Aim c. Air Interface d. FDMA e. TDMA f. CDMA g. CDMA – Spread spectrum h. Benefits of CDMA j. Conclusion Cellular Access Technologies
  5. The basic problem in any radio based telecommunication system is how to accommodate as many different users as possible within a given spectrum allocation. The possible solution involves allocating the spectrum in terms of frequency, time, space and code. There are three common technologies used by cell-phone networks for transmitting information:

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  1. Frequency Division Multiple Access (FDMA) a. The original solution was to divide the available spectrum into blocks and channels, which are then assigned exclusively to a particular user. Frequency division means calls are placed or divided by frequency, that is, one call goes on one frequency, say, 100 MHz, and another call goes on another, say, 200 MHz. You can also put digital signals on many frequencies, of course, and that would still be FDMA. But AMPS traffic is analog. FDMA separates the spectrum into distinct voice channels by splitting it into uniform chunks of bandwidth. To better understand FDMA, think of radio stations: Each station sends its signal at a different frequency within the available band. FDMA is used mainly for analog transmission. While it is certainly capable of carrying digital information, FDMA is not considered to be an efficient method for digital transmission. b. AMPS SPECTRUM (1) In October 1983, the first public cellular service in USA, known as Advanced Mobile Phone Service, was launched. AMPS is based upon analog technologies and uses AMPS where two frequencies band of 25 MHz – used for the uplink and downlink respectively – are divided into 333 duplex channels, 30 kHz wide. However, AMPS has a limited call capacity (only one user per channel) and the only service offered is voice. (2) With AMPS, the frequencies 825 - 835 MHz were allocated to operator A for uplink or reverse channels while from 835 - 845 MHz were allocated to operator B. Similarly, frequencies from 870 - 880 MHz to operator A and from 880 - 890 MHz to operator B were allocated for downlink or forward channels. Initially, the numbers of channels available with each operator were 333, but then frequency allocation

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Mobile Phone System (D-AMPS) where mobile phones are capable of accessing both analog AMPS and digital NADC networks as required. c. GSM-TDMA and PDC. The European GSM system was launched in 1992, and replaced a collection of incompatible analog cellular systems. GSM uses TDMA where 8 users are supported in channels, 200 kHz wide. GSM has also been employed in many countries outside Europe. The Personal Digital Cellular (PDC) was launched in Japan in 1993, and replaced a collection of incompatible analog systems. PDC uses TDMA where 3 users are supported in channels, 25 kHz wide. d. TDMA’s Advantages (1) Chief benefit of TDMA to carriers or cellular operators comes from increasing call capacity -- a channel can carry three conversations instead of just one. NAMPS had the same fading problems as AMPS, lacked the error correction that digital systems provided and wasn't sophisticated enough to handle encryption or advanced services. Things such as calling number identification, extension phone service and messaging. In addition, you can't monitor a TDMA conversation as easily as an analog call. So, there are other reasons than call capacity to move to a different technology. (2) Three conversations get handled on a single frequency. Call capacity increases. It is a virtue of multiplexing. A digital signal does not automatically mean less bandwidth, in fact, it means more. Multiplexing means transmitting multiple conversations on the same frequency, at once. In this case, small parts of three conversations get sent almost simultaneously. This was not the same with the old analog NAMPS, which split the frequency band into three discrete sub-frequencies of 10 kHz apiece. TDMA uses the whole frequency to transmit while NAMPS did not.

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(3) A multiplexed digital signal is what is key.

Each frequency gets divided into six repeating time slots or frames. Two slots in each frame get assigned for each call. An empty slot serves as a guard space. This may sound esoteric but it is not. Demultiplexing those conversations is no more difficult than adding the right circuit board to a personal computer. TDMA is a little different than TDM but it does have a long history in satellite working.

  1. Code Division Multiple Access (CDMA) a. In CDMA, all the users have access to all the available frequencies at all times. The CDMA system differentiates between different users by assigning a unique code to each of their transmissions. The cellular system that uses it, IS-95, tags each and every part of multiple conversations with a specific digital code. That code lets the operating system reassemble the jumbled calls at the base station. Again, CDMA is the transmission method and IS-95 is the operating system. CDMA takes an entirely different approach from TDMA. CDMA, after digitizing data, spreads it out over the entire available bandwidth. Multiple calls are overlaid on each other on the channel, with each assigned a unique sequence code. CDMA is a form of spread spectrum, which simply means that data is sent in small pieces over a number of the discrete frequencies available for use at any time in the specified range. b. CDMA – Spread Spectrum (1) CDMA is a spread spectrum technology that has been used for military communication since the 1940’s. The basic idea behind spread spectrum is to spread the signal power over a very wide range of frequencies in order to give it a noise like appearance. This makes the signal difficult to intercept and jam. In 1980’s commercial communications systems e.g. wireless LANs used CDMA for the first time. (2) All of the users transmit in the same wide- band chunk of spectrum. Each user's signal is

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bandwidth-limited systems, in which a high power level is used to overcome noise and interference. (2) Similarly, power-limited systems need careful control of transmitted power to avoid disturbing users in the same frequency band and in other cells. Bandwidth-limited systems need careful control of bandwidth to avoid disturbing other users. (3) In a CDMA system, the requirement to accurately control the transmitted power of all the users in the cell is known as near-far problem. To solve the problem, power control is used. The base station dynamically controls the transmit power level of all the mobile users in a cell, to ensure their signals are all received at an equal strength at the base station. d. CDMA has many variants as well, for example, broadband CDMA system called B-CDMA. In the coming years, wideband may dominate and find more usage in UMTS as W-CDMA. But narrowband CDMA right now is dominant in the United States, used with the operating system IS-95. CDMA Benefits

  1. One multiple access method is not better than others in all situations. However, CDMA does have a number of features that makes it attractive for cellular mobile communications systems. The CDG states that CDMA systems have seven advantages over other cellular radio transmission techniques. (GSM and IS- 136 operators will contest this list.) CDG says benefits are: a. Simplified system planning through the use of the same frequency in every sector of every cell. b. FDMA and TDMA have a fixed or hard limit on the number of users that can be accommodated within a cell. Whereas, in CDMA a soft limit gradually increases the number of users but it leads to a gradual rise in the background noise. c. Soft handoff refers to a call being handled by two BS

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while MS is moving between two cells. The communication is not interrupted in the process of handoff, since all frequencies are readily available in all cells. d. Multi-path fading is caused by different delays among the alternate paths taken by the radio signal between a BS and a MS. Wideband signals are less prone to multi-path fading. CDMA systems also uses a special kind of receiver known as Rake Receiver to reduce the effects of fading. It contains multiple receivers, each locking to one of the multi-path signals. By adding appropriate delays to early incoming signals, it is possible to recombine them all so that they reinforce rather than interfere with each other. e. Enhanced privacy due to the use of spread spectrum techniques. f. CDMA MSs operate at a much lower output power levels, resulting in more talk time. g. Bandwidth on demand CONCLUSION

  1. Over the years, in stages hard to follow, often with 2G and 3G techniques co-existing, TDMA based GSM and AT&T's IS- 136 cellular service will be replaced with a wideband CDMA system, the much hoped for Universal Mobile Telephone System. Strangely, IS- 136 will first be replaced by GSM before going to UMTS. Technologies like EDGE and GPRS will extend the life of these present TDMA systems but eventually new infrastructure and new spectrum will allow CDMA/UMTS development.
  2. Systems built on time division multiplexing will gradually be replaced with other access technologies. CDMA is the future of digital cellular radio. Time division systems are now being regarded as legacy technologies, older methods that must be accommodated in the future, but ones which are not the future itself. Right now all digital cellular radio systems are second generation, prioritizing on voice traffic, circuit switching, and slow data transfer speeds. 3G, while still delivering voice, will emphasize data, packet switching, and high-speed access.