CODE DIVISION MULTIPLE ACCESS (CDMA)
Code Division Multiple Access (CDMA) is an American digital standard that was developed by a company named Qualcomm, based o\in California. It describes a communication channel access principle that employs spread. Spectrum technology and a special coding scheme (where each transmitter is assigned a code.
CDMA was originally deployed as a battlefield communication system because it is very hard if not completely impossible to intercept CDMA transmissions. Its digital modulation scheme is titled interim Standard 95 (15 – 95).
CDMA employs what is known as a wideband spread spectrum technology to carry digital voice and data transmissions. The CDMA conversation is assigned its own separate code.
CDMA ARCHITECTURE AND OPERATIONS
Each CDMA base station can use the same 1.25MHz carries at the same time. The only change b/w each block of 1.25MHz spectrum is the pseudorandom walsh noise code. There are a maximum of 64 allowable pseudorandom walsh noise codes per 1.25MHz carrier in the CDMA modulation scheme.
ATTRIBUTES OF CDMA
There can be nine 1.25MHz CDMA carriers per cell. Some PCS carriers are successfully using eleven 1.25MHz carriers per cell. Today, cellular carries are using two to four carriers per cell. It is also said that, each of these 1.25MHz carriers (channels) can handled 22 to 40 voice calls. CDMA employs an N = 1 ‘reuse’ format because identical sets of 1.25MHz bandwidth (spectrum) can be assigned at every base station in a CDMA system.
WIDEBAND CDMA
There is currently a newer version of code division multiple access under development known as wideband CDMA (W-CDMA). Instead of utilizing a 1.25MHz (or greater) carrier.
This new technology is supposed to significantly step up the time frame by which CDMA systems will be able to offer voice, data and at least half rate motion video from CDMA handsets. It falls under the heading of third-generation (3G) wireless technology.
The first generation is cellular. The second generation is all digital (PCS) system and, the third generation of wireless technology are those that the industry is trying to have standardized through national and international standards bodies.
COVERAGE
CDMA is mostly used in American and some parts of Asia. It is currently making progress in other parts of the world, but the coverage is still limited compared to the GSM technology. Its support is currently non-existent in Europe because the European Union mandates the sole use of GSM. Even though if they do not have this capability of international Roaming several countries do use CDMA networks.
CHAPTER TWO
GENERATING THE CDMA SIGNAL
Generating a CDMA signal is a five steps process.
GENERATING THE CDMA SIGNAL
In the CDMA technique the signal transmissions among the multiple users completely over-lap in both time and frequency. The separation between the users is made by assigning each user a unique code.
Generally, CDMA converts analog voice signal to a digital signal, encodes the digital signals, and separates voice and control data streams called channels.
• The first step is analog to digital Cnversion or A/D. The incoming voice signal is an analog signal meaning that it is changing constantly, taking on all possible values of amplitude range. The CDMA uses a digital signal for its further manipulation. In that step the analog voice signal is quantized to form a digital signal consists of a few levels.
• The second step is voice Coding or Vocoding. Voice encoding is the process of compressing the audio into as small as 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 or Inter-leaving. This step purpose is to reduce the errors when receiving the signal. Inter-leaving is a method of reducing the effects of burst error and recovering lost bits. The symbols are interleaved such that originally neighbouring symbols will be transmitted far away from each other. The various encoding methods are redundancy to the signals to help the recovery of information at the receiver in case of errors.
• The fourth step is channelizing: - A unique identification code is given to each user and the signals of all users are transmitted together, sharing the same frequency and tone. The CDMA receiver decodes the signal by multiplying it by a decoding sequence of the desired user.
The two common codes types are walsh code and PN (pseudo random noise) code:
- Walsh code: - this is used for forward CDMA channel (e.g. cell to mobile direction of communication). They 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.
- PN code: - This is used to reverse CDMA channel (e.g. mobile to cell direction of communication).
• The fifth step is digital to radio frequency (RF) conversion. The stream of bits should be somehow delivered from one end to the other. This is a method of carrying the information through the air. Digital data signals are combines into one signal and converted to a RF signal for the transmission process.
STRUCTURE OF A TYPICAL WIRELESS SYSTEM
A. BASE STATION (BTS) - Base transceiver stations provides the radio connection between mobile users and the switch. Sometimes it is called a ‘cell’ or ‘BTS’. One wireless system in a large metropolitan area may require hundred of base stations to deliver unbroken coverage and provide sufficient capacity to handle all possible users.
A BTS is general has the following unit:
- TXT (Transceiver) – which does transmission and reception of signals.
- PA (Power Amplifier) – Amplifiers the signal from DRX for transmission
through antenna.
- Combiner – for reduction of number of antenna used.
- Duplexer – for separating sending and receiving signals to/from antenna.
- Alarm Extension – collect working status alarm of various units in BTS
and extends them to operations and maintenance monitoring stations
- Control Function – Does the control of BTS. Manages the various unit of
BTS and spot configuration, status changes and software upgrades.
- Base receiver unit (BBxx) frequency hopping signal DSP.
B HOME LOCATION REGISTER (HLR)
• The HLR (Home Location Register) is the official database of all customers on a wireless system.
• It can be part of the switch or held in a server at a central location where multiple switches can interrogate it.
• It holds information like the:
(i) Current account status/validity
(ii) Phone’s technical parameters
(iii) Whether the phone is presently turned on, and if so, the identity of switch which is presently serving the phone
(iv) Secret keys for authentication to avoid fraudulent use/cloning.
C THE SWITCH
• Each call involves joining a ccf leading to one customer and a ccf leading to another person (usually on the radio side of the system and out in the Public Switched Telephone Network (PSTN). Switch makes the actual physical connections. They also store building records routing calls, implementing all calling features and interpreting dialed phone numbers.
D THE BASE STATION CONTROLLER (BSC) –
The base station controller (BSC) interface the switch and the base stations.
• It compresses speech signals for more efficient transmission over the scarce radio spectrum.
• It also controls the base stations and implements the handoff of calls from one base station to another as users drive across the system.
• Provides the intelligence behind the BTS.
• It handles allocation of radio channels.
• Receives measurements from the mobile phone.
• Its main function is to act as a concentration where many different low capacity connection to BTS becomes reduced to a small number of connections towards MSC.
• Control handover from BTS to BTS.
• Provides all required data to the operation support sub-system.
• Provides performance measuring centres.
E VOICE MAIL SYSTEM (VSM) –
When a subscriber receives an incoming call but its phone is turned off or already on a call (without a ‘call waiting feature) a voice mail system can store a message.
• The subscriber is alerted the next time they turn on their phone
• Commercial voiced mail systems offer scalable capacity up into many hundreds of hours of random – access private storage.
CHAPTER THREE
This portions of the paper will be dealing with both GSE and CDMA. The calls process in general. Both GSM and CDMA uses handoff or handover. There is soft handoff and hard handoff. Soft handoff has more advantage than hard handoff because it requires less power, which reduces interference and increase capacity. CDMA uses make-before-break while GSM uses break-before make.
SOFT HANDOFF OPERATION/SEQUENCE OF EVENTS
• After a mobile cell is initiated, the mobile stations continues to scan the nearest cells to determine the strength of the original cell to the new cell.
• When this happens, the mobile station knows that the cell has entered a new cell’s coverage area and that a handoff can be initiated.
• The mobile station than transmits a control message to the MTSO which indicate that the mobile is receiving a stronger signal from the new cell site.
• The MTSO then initiate the handoff by establishing a link to the mobile station through the new cell while maintaining the old link.
• While the mobile station is located in the transaction region between the two cell sites, the cell is supported by the communication through both cells, which is the final handover.
Not only does soft handoff greatly minimize the probability of a dropped call, but it also makes the handoff virtually undectable to the user. Soft handoff are directed by the mobile telephone as such soft handoff is also known as mobile-directed handoff.
GSM (Global System for Mobile Communication) – It is the most popular standard for mobile phones in the world today. The GSM association estimates that 82% of the global mobile market uses the standard. Its ubiquity makes international roaming very common between mobile phone operators, enabling subscribers to use their phones in many parts of the world. The ubiquity of GSM standard has been an advantage to both consumers (who benefit from switching phones) and also to network operators (who can choose equipment from any of the many vendors implementing GSM.
Its other advantage is that the standard includes one worldwide emergency telephone number 112. This makes it easier for international travelers to connect to emergency services without knowing the local emergency number.
STRUCTURE OF A GSM NETWORK (KEY ELEMENTS)
The network behind the GSM system seen by the customer is large and complicated in order to provide all of the services which are required. It is divided into a number of sections and these are each covered in separate articles.
• The base station sub-system (the base stations and their controllers).
• The network and switching sub-system (the part of the network most similar to a fixed network). This is sometimes also must called the Core network.
• The GPRS Core Network (the optional part which allows packet based internet connections).
• All of the element in the system combine to produce many GSM services such as Voice Calls and SMS.
CDMA THEORY
It is said that CDMA cell coverage depend upon the way the system is designed. The 3 primary system characteristics which are coverage, quality and capacity must be balanced in order to perform well. To get or obtain the cleanest, strongest signal, the multi-path signals are addictive to the direct signals. CDMA digital modulation scheme is titled interim standard 95 (15-95). Its major attributes are high system capacity, cost effectiveness and increment in quality of services.
ADVANTAGE OF CDMA TECHNOLOGY
- Flexible network planning: - simplified system planning through the use of same frequent in every sector of every cell site.
- Greater coverage: - power control make sure that all mobile signal arrive at the BTS at the same power level to enable proper coding. It maintain system capacity for effective operation.
A – by product of power control is to reduce power costs at the BTS as well as increase battery life and longer life span of BTS power Amplifiers. The aim of the dynamics power control is to limit transmitted power on both the link while maintaining link quality under all conditions.
- High capacity – Increase capacity of 8 – 10 times that of an Amps analog system and 5 – 5 times that of a GSM system. The amount of information that can be delivered from the source to the destination is greater than in other techniques. The fact been that the users are distinguished by Codes and not by frequencies or time slots.
- Cost: - Allowing for the possibility of fewer cell sites/BTS (which theorically is said to be the ratio of 1:4 compared to GSM) thereby save money for operators. It also enables larger profits for the providers.
- High call quality:- Good voice quality. CDMA can often, provide wireless clarity: (i) Soft handoffs requires less power, therefore reduces interference and errors. (ii) Increased talk time for portables. Probability of conjunction is less with CDMA. (iii) Bandwidth on demand: Has large bandwidth, thereby reduces fading.
- Customer satisfaction: - Due to CDMA’s unique coding customers enjoys the following:
• Privacy – it is very hard if not totally impossible to wire tapping, that is the reason why it is used in military sectors or organisations.
• CDMA’s soft handoffs, which allow make-before-break when traveling between cells, yields to fewer dropped calls.
• High speed packet switching structure and perfect power control and voice activation makes the MS power low.
- Compatibility: - dual modes that enables working in analog system as well.
CHAPTER FOUR
Kasapa chart organisation for the planning department where I undergo my attachment.
C T O
PROJECT ON SITE SEARCH
The objective of this trip was to accompany ZTE to identify and made initial contact with prospective property owners for the sites brought up by the former towards Kasapa network expansion drive. These sites are being required to enable Kasapa build more CDMA BTS and repeater sites in two regions among others. The exercise lasted for nine days from July 31 to August 8, 2008.
TEAM:
The team was made up of Sam Amoako the Manager Planning and myself Calister Mpamugo.
Resources Needed for the Site Search
- Site search form
- GPS to dictate the actual point locations and co-ordinate for the longitude, latitude and altitude as well
- Maps
- Stationery
- Cameras
- Tape Measure
- Compass – To mention but a few.
FACTORS INVOLVED IN SITE SEARCH
- Land – where the BTS or repeater towers will be built
- Knowing the altitude of the land is necessary
- Knowing the availability of direct line of sight between the site and the radio room
- The topography of the site
- The physical obstacles that might be crossing the path of the transmission
- Access to the site
- Information on the nearest cell site must be known.
CONCLUSION/SUMMARY
I may conclude that comparing GSE and CDMA technologies can be done depending on the functionality required, because both technologies has its own merits and demerits.
Code Division Multiple Access (CDMA) is an American digital standard that was developed by a company named Qualcomm, based o\in California. It describes a communication channel access principle that employs spread. Spectrum technology and a special coding scheme (where each transmitter is assigned a code.
CDMA was originally deployed as a battlefield communication system because it is very hard if not completely impossible to intercept CDMA transmissions. Its digital modulation scheme is titled interim Standard 95 (15 – 95).
CDMA employs what is known as a wideband spread spectrum technology to carry digital voice and data transmissions. The CDMA conversation is assigned its own separate code.
CDMA ARCHITECTURE AND OPERATIONS
Each CDMA base station can use the same 1.25MHz carries at the same time. The only change b/w each block of 1.25MHz spectrum is the pseudorandom walsh noise code. There are a maximum of 64 allowable pseudorandom walsh noise codes per 1.25MHz carrier in the CDMA modulation scheme.
ATTRIBUTES OF CDMA
There can be nine 1.25MHz CDMA carriers per cell. Some PCS carriers are successfully using eleven 1.25MHz carriers per cell. Today, cellular carries are using two to four carriers per cell. It is also said that, each of these 1.25MHz carriers (channels) can handled 22 to 40 voice calls. CDMA employs an N = 1 ‘reuse’ format because identical sets of 1.25MHz bandwidth (spectrum) can be assigned at every base station in a CDMA system.
WIDEBAND CDMA
There is currently a newer version of code division multiple access under development known as wideband CDMA (W-CDMA). Instead of utilizing a 1.25MHz (or greater) carrier.
This new technology is supposed to significantly step up the time frame by which CDMA systems will be able to offer voice, data and at least half rate motion video from CDMA handsets. It falls under the heading of third-generation (3G) wireless technology.
The first generation is cellular. The second generation is all digital (PCS) system and, the third generation of wireless technology are those that the industry is trying to have standardized through national and international standards bodies.
COVERAGE
CDMA is mostly used in American and some parts of Asia. It is currently making progress in other parts of the world, but the coverage is still limited compared to the GSM technology. Its support is currently non-existent in Europe because the European Union mandates the sole use of GSM. Even though if they do not have this capability of international Roaming several countries do use CDMA networks.
CHAPTER TWO
GENERATING THE CDMA SIGNAL
Generating a CDMA signal is a five steps process.
GENERATING THE CDMA SIGNAL
In the CDMA technique the signal transmissions among the multiple users completely over-lap in both time and frequency. The separation between the users is made by assigning each user a unique code.
Generally, CDMA converts analog voice signal to a digital signal, encodes the digital signals, and separates voice and control data streams called channels.
• The first step is analog to digital Cnversion or A/D. The incoming voice signal is an analog signal meaning that it is changing constantly, taking on all possible values of amplitude range. The CDMA uses a digital signal for its further manipulation. In that step the analog voice signal is quantized to form a digital signal consists of a few levels.
• The second step is voice Coding or Vocoding. Voice encoding is the process of compressing the audio into as small as 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 or Inter-leaving. This step purpose is to reduce the errors when receiving the signal. Inter-leaving is a method of reducing the effects of burst error and recovering lost bits. The symbols are interleaved such that originally neighbouring symbols will be transmitted far away from each other. The various encoding methods are redundancy to the signals to help the recovery of information at the receiver in case of errors.
• The fourth step is channelizing: - A unique identification code is given to each user and the signals of all users are transmitted together, sharing the same frequency and tone. The CDMA receiver decodes the signal by multiplying it by a decoding sequence of the desired user.
The two common codes types are walsh code and PN (pseudo random noise) code:
- Walsh code: - this is used for forward CDMA channel (e.g. cell to mobile direction of communication). They 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.
- PN code: - This is used to reverse CDMA channel (e.g. mobile to cell direction of communication).
• The fifth step is digital to radio frequency (RF) conversion. The stream of bits should be somehow delivered from one end to the other. This is a method of carrying the information through the air. Digital data signals are combines into one signal and converted to a RF signal for the transmission process.
STRUCTURE OF A TYPICAL WIRELESS SYSTEM
A. BASE STATION (BTS) - Base transceiver stations provides the radio connection between mobile users and the switch. Sometimes it is called a ‘cell’ or ‘BTS’. One wireless system in a large metropolitan area may require hundred of base stations to deliver unbroken coverage and provide sufficient capacity to handle all possible users.
A BTS is general has the following unit:
- TXT (Transceiver) – which does transmission and reception of signals.
- PA (Power Amplifier) – Amplifiers the signal from DRX for transmission
through antenna.
- Combiner – for reduction of number of antenna used.
- Duplexer – for separating sending and receiving signals to/from antenna.
- Alarm Extension – collect working status alarm of various units in BTS
and extends them to operations and maintenance monitoring stations
- Control Function – Does the control of BTS. Manages the various unit of
BTS and spot configuration, status changes and software upgrades.
- Base receiver unit (BBxx) frequency hopping signal DSP.
B HOME LOCATION REGISTER (HLR)
• The HLR (Home Location Register) is the official database of all customers on a wireless system.
• It can be part of the switch or held in a server at a central location where multiple switches can interrogate it.
• It holds information like the:
(i) Current account status/validity
(ii) Phone’s technical parameters
(iii) Whether the phone is presently turned on, and if so, the identity of switch which is presently serving the phone
(iv) Secret keys for authentication to avoid fraudulent use/cloning.
C THE SWITCH
• Each call involves joining a ccf leading to one customer and a ccf leading to another person (usually on the radio side of the system and out in the Public Switched Telephone Network (PSTN). Switch makes the actual physical connections. They also store building records routing calls, implementing all calling features and interpreting dialed phone numbers.
D THE BASE STATION CONTROLLER (BSC) –
The base station controller (BSC) interface the switch and the base stations.
• It compresses speech signals for more efficient transmission over the scarce radio spectrum.
• It also controls the base stations and implements the handoff of calls from one base station to another as users drive across the system.
• Provides the intelligence behind the BTS.
• It handles allocation of radio channels.
• Receives measurements from the mobile phone.
• Its main function is to act as a concentration where many different low capacity connection to BTS becomes reduced to a small number of connections towards MSC.
• Control handover from BTS to BTS.
• Provides all required data to the operation support sub-system.
• Provides performance measuring centres.
E VOICE MAIL SYSTEM (VSM) –
When a subscriber receives an incoming call but its phone is turned off or already on a call (without a ‘call waiting feature) a voice mail system can store a message.
• The subscriber is alerted the next time they turn on their phone
• Commercial voiced mail systems offer scalable capacity up into many hundreds of hours of random – access private storage.
CHAPTER THREE
This portions of the paper will be dealing with both GSE and CDMA. The calls process in general. Both GSM and CDMA uses handoff or handover. There is soft handoff and hard handoff. Soft handoff has more advantage than hard handoff because it requires less power, which reduces interference and increase capacity. CDMA uses make-before-break while GSM uses break-before make.
SOFT HANDOFF OPERATION/SEQUENCE OF EVENTS
• After a mobile cell is initiated, the mobile stations continues to scan the nearest cells to determine the strength of the original cell to the new cell.
• When this happens, the mobile station knows that the cell has entered a new cell’s coverage area and that a handoff can be initiated.
• The mobile station than transmits a control message to the MTSO which indicate that the mobile is receiving a stronger signal from the new cell site.
• The MTSO then initiate the handoff by establishing a link to the mobile station through the new cell while maintaining the old link.
• While the mobile station is located in the transaction region between the two cell sites, the cell is supported by the communication through both cells, which is the final handover.
Not only does soft handoff greatly minimize the probability of a dropped call, but it also makes the handoff virtually undectable to the user. Soft handoff are directed by the mobile telephone as such soft handoff is also known as mobile-directed handoff.
GSM (Global System for Mobile Communication) – It is the most popular standard for mobile phones in the world today. The GSM association estimates that 82% of the global mobile market uses the standard. Its ubiquity makes international roaming very common between mobile phone operators, enabling subscribers to use their phones in many parts of the world. The ubiquity of GSM standard has been an advantage to both consumers (who benefit from switching phones) and also to network operators (who can choose equipment from any of the many vendors implementing GSM.
Its other advantage is that the standard includes one worldwide emergency telephone number 112. This makes it easier for international travelers to connect to emergency services without knowing the local emergency number.
STRUCTURE OF A GSM NETWORK (KEY ELEMENTS)
The network behind the GSM system seen by the customer is large and complicated in order to provide all of the services which are required. It is divided into a number of sections and these are each covered in separate articles.
• The base station sub-system (the base stations and their controllers).
• The network and switching sub-system (the part of the network most similar to a fixed network). This is sometimes also must called the Core network.
• The GPRS Core Network (the optional part which allows packet based internet connections).
• All of the element in the system combine to produce many GSM services such as Voice Calls and SMS.
CDMA THEORY
It is said that CDMA cell coverage depend upon the way the system is designed. The 3 primary system characteristics which are coverage, quality and capacity must be balanced in order to perform well. To get or obtain the cleanest, strongest signal, the multi-path signals are addictive to the direct signals. CDMA digital modulation scheme is titled interim standard 95 (15-95). Its major attributes are high system capacity, cost effectiveness and increment in quality of services.
ADVANTAGE OF CDMA TECHNOLOGY
- Flexible network planning: - simplified system planning through the use of same frequent in every sector of every cell site.
- Greater coverage: - power control make sure that all mobile signal arrive at the BTS at the same power level to enable proper coding. It maintain system capacity for effective operation.
A – by product of power control is to reduce power costs at the BTS as well as increase battery life and longer life span of BTS power Amplifiers. The aim of the dynamics power control is to limit transmitted power on both the link while maintaining link quality under all conditions.
- High capacity – Increase capacity of 8 – 10 times that of an Amps analog system and 5 – 5 times that of a GSM system. The amount of information that can be delivered from the source to the destination is greater than in other techniques. The fact been that the users are distinguished by Codes and not by frequencies or time slots.
- Cost: - Allowing for the possibility of fewer cell sites/BTS (which theorically is said to be the ratio of 1:4 compared to GSM) thereby save money for operators. It also enables larger profits for the providers.
- High call quality:- Good voice quality. CDMA can often, provide wireless clarity: (i) Soft handoffs requires less power, therefore reduces interference and errors. (ii) Increased talk time for portables. Probability of conjunction is less with CDMA. (iii) Bandwidth on demand: Has large bandwidth, thereby reduces fading.
- Customer satisfaction: - Due to CDMA’s unique coding customers enjoys the following:
• Privacy – it is very hard if not totally impossible to wire tapping, that is the reason why it is used in military sectors or organisations.
• CDMA’s soft handoffs, which allow make-before-break when traveling between cells, yields to fewer dropped calls.
• High speed packet switching structure and perfect power control and voice activation makes the MS power low.
- Compatibility: - dual modes that enables working in analog system as well.
CHAPTER FOUR
Kasapa chart organisation for the planning department where I undergo my attachment.
C T O
PROJECT ON SITE SEARCH
The objective of this trip was to accompany ZTE to identify and made initial contact with prospective property owners for the sites brought up by the former towards Kasapa network expansion drive. These sites are being required to enable Kasapa build more CDMA BTS and repeater sites in two regions among others. The exercise lasted for nine days from July 31 to August 8, 2008.
TEAM:
The team was made up of Sam Amoako the Manager Planning and myself Calister Mpamugo.
Resources Needed for the Site Search
- Site search form
- GPS to dictate the actual point locations and co-ordinate for the longitude, latitude and altitude as well
- Maps
- Stationery
- Cameras
- Tape Measure
- Compass – To mention but a few.
FACTORS INVOLVED IN SITE SEARCH
- Land – where the BTS or repeater towers will be built
- Knowing the altitude of the land is necessary
- Knowing the availability of direct line of sight between the site and the radio room
- The topography of the site
- The physical obstacles that might be crossing the path of the transmission
- Access to the site
- Information on the nearest cell site must be known.
CONCLUSION/SUMMARY
I may conclude that comparing GSE and CDMA technologies can be done depending on the functionality required, because both technologies has its own merits and demerits.
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