Code division multiple access
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Template:Table Mobile phone standards Code division multiple access (CDMA) is a form of multiplexing (not a modulation scheme) and a method of multiple access that does not divide up the channel by time (as in TDMA), or frequency (as in FDMA), but instead encodes data with a special code associated with each channel and uses the constructive interference properties of the special codes to perform the multiplexing. CDMA also refers to digital cellular telephony systems that make use of this multiple access scheme, such as those pioneered by Qualcomm, or W-CDMA.
CDMA is a military technology first used during World War II by English allies to foil German attempts at jamming transmissions. The allies decided to transmit over several frequencies, instead of one, making it difficult for the Germans to pick up the complete signal.
CDMA has since been used in many communications systems, including the Global Positioning System (GPS) and in the OmniTRACS satellite system for transportation logistics. The latter system was designed and built by Qualcomm, and became the seed which helped Qualcomm engineers to invent Soft Handoff and fast power control, the necessary technologies that made CDMA practical and efficient for terrestrial cellular communications.
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History of CDMA
Please see: direct-sequence spread spectrum (DSSS).
Usage in mobile telephony
A number of different terms are used to refer to CDMA implementations. The original standard spearheaded by QUALCOMM was known as IS-95, the IS referring to an Interim Standard of the Telecommunications Industry Association (TIA). IS-95 is often referred to as 2G or second generation cellular. The QUALCOMM brand name cdmaOne may also be used to refer to the 2G CDMA standard.
After a couple of revisions, IS-95 was superseded by the IS-2000 standard. This standard was introduced to meet some of the criteria laid out in the IMT-2000 specification for 3G, or third generation, cellular. It is also referred to as 1xRTT which simply means "1 times Radio Transmission Technology" and indicates that IS-2000 uses the same 1.25-MHz shared channel as the original IS-95 standard. A related scheme called 3xRTT uses three 1.25-MHz carriers for a 3.75-MHz bandwidth that would allow higher data burst rates for an individual user, but the 3xRTT scheme has not been commercially deployed. More recently, QUALCOMM has led the creation of a new CDMA-based technology called 1xEV-DO, or IS-856, which provides the higher packet data transmission rates required by IMT-2000 and desired by wireless network operators.
The QUALCOMM CDMA system includes highly accurate time signals (usually referenced to a GPS receiver in the cell base station), so cell phone CDMA-based clocks are an increasingly popular type of radio clock for use in computer networks. The main advantage of using CDMA cell phone signals for reference clock purposes is that they work better inside buildings, thus often eliminating the need to mount a GPS antenna on the outside of a building.
Also frequently confused with CDMA is W-CDMA. The CDMA technique is used as the principle of the W-CDMA air interface, and the W-CDMA air interface is used in the global 3G standard UMTS and the Japanese 3G standard FOMA, by NTT DoCoMo and Vodafone; however, the CDMA family of standards (including cdmaOne and CDMA2000) are not compatible with the W-CDMA family of standards.
Another important application of CDMA—predating and entirely distinct from CDMA cellular—is the Global Positioning System, GPS.
Coverage
As CDMA is newer than GSM, it may not be available in some parts of the world. However, as the signal can be transmitted over greater distances, it may give reception in more remote or rural areas where a GSM phone does not pick up a signal.
See also Market situation section of GSM
Technical details
Mathematical foundation
CDMA exploits at its core mathematical properties of orthogonality. Suppose we represent data signals as vectors. For example, the binary string "1011" would be represented by the vector (1, 0, 1, 1). We may wish to give a vector a name, we may do so by using boldface letters, e.g. a. We also use an operation on vectors, known as the dot product, to "multiply" vectors, by summing the product of the components. For example, the dot product of (1, 0, 1, 1) and (1, -1, -1, 0) would be (1)(1)+(0)(-1)+(1)(-1)+(1)(0)=1+-1=0. Where the dot product of vectors a and b is 0, we say that the two vectors are orthogonal.
The dot product has a number of properties, and one will aid us in understanding why CDMA works. For vectors a, b, c:
- <math>\mathbf{a}\cdot(\mathbf{b}+\mathbf{c})=\mathbf{a}\cdot\mathbf{b}+\mathbf{a}\cdot\mathbf{c},\quad\mathrm{and}</math>
- <math>\mathbf{a}\cdot k\mathbf{b}=k(\mathbf{a}\cdot\mathbf{b}).</math>
The square root of a.a is a real number, and is important. We write
- <math>||\mathbf{a}||=\sqrt{\mathbf{a}\cdot\mathbf{a}}.</math>
Suppose vectors a and b are orthogonal. Then:
- <math>\mathbf{a}\cdot(\mathbf{a}+\mathbf{b})=||\mathbf{a}||^2\quad\mathrm{since}\quad\mathbf{a}\cdot\mathbf{a}+\mathbf{a}\cdot\mathbf{b}= ||a||^2+0,</math>
- <math>\mathbf{a}\cdot(-\mathbf{a}+\mathbf{b})=-||\mathbf{a}||^2\quad\mathrm{since}\quad-\mathbf{a}\cdot\mathbf{a}+\mathbf{a}\cdot\mathbf{b}= -||a||^2+0,</math>
- <math>\mathbf{b}\cdot(\mathbf{a}+\mathbf{b})=||\mathbf{b}||^2\quad\mathrm{since}\quad\mathbf{b}\cdot\mathbf{a}+\mathbf{b}\cdot\mathbf{b}= 0+||b||^2,</math>
- <math>\mathbf{b}\cdot(\mathbf{a}-\mathbf{b})=-||\mathbf{b}||^2\quad\mathrm{since}\quad\mathbf{b}\cdot\mathbf{a}-\mathbf{b}\cdot\mathbf{b}=0 -||b||^2.</math>
Implementation
Image:Cdma orthogonal signals.png
Suppose now we have a set of vectors that are mutually orthogonal to each other. Usually these vectors are specially constructed for ease of decoding -- they are columns or rows from Walsh matrices that are constructed from Walsh functions -- but strictly mathematically the only restriction on these vectors is that they are orthogonal. An example of orthogonal functions is shown in the picture on the right. Now, associate with one sender a vector from this set, say v, which is called the chip code. Associate a zero digit with the vector -v, and a one digit with the vector v. For example, if v=(1,-1), then the binary vector (1, 0, 1, 1) would correspond to (1,-1,-1,1,1,-1,1,-1). For the purposes of this article, we call this constructed vector the transmitted vector.
Each sender has a different, unique vector chosen from that set, but the construction of the transmitted vector is identical.
Now, the physical properties of interference say that if two signals at a point are in phase, they will "add up" to give twice the amplitude of each signal, but if they are out of phase, they will "subtract" and give a signal that is the difference of the amplitudes. Digitally, this behaviour can be modelled simply by the addition of the transmission vectors, component by component. So, if we have two senders, both sending simultaneously, one with the chip code (1, -1) and data vector (1, 0, 1, 1), and another with the chip code (1, 1), and data vector (0,0,1,1), the raw signal received would be the sum of the transmission vectors (1,-1,-1,1,1,-1,1,-1)+(-1,-1,-1,-1,1,1,1,1)=(0,-2,-2,0,2,0,2,0).
Suppose a receiver gets such a signal, and wants to detect what the transmitter with chip code (1, -1) is sending. The receiver will make use of the property described in the above foundation section, and take the dot product to the received vector in parts. Take the first two components of the received vector, that is, (0, -2). Now, (0, -2).(1, -1) = (0)(1)+(-2)(-1) = 2. Since this is positive, we can deduce that a one digit was sent. Taking the next two components, (-2, 0), (-2, 0).(1,-1)=(-2)(1)+(0)(-1)=-2. Since this is negative, we can deduce that a zero digit was sent. Continuing in this fashion, we can successfully decode what the transmitter with chip code (1, -1) was sending: (1, 0, 1, 1).
Likewise, applying the same process with chip code (1, 1): (1, 1).(0,-2) = -2 gives digit 0, (1, 1).(-2,0)=(1)(-2)+(1)(0)=-2 gives digit 0, and so on, to give us the data vector sent by the transmitter with chip code (1, 1): (0, 0, 1, 1).
Now, there are certain issues where this mathematical process can be disrupted. Suppose that one sender transmits at a higher signal strength than another. Then the critical orthogonality property can be disrupted, and thus the system can fail. Thus controlling power strength is an important issue with CDMA transmitters. A TDMA or FDMA receiver can in theory completely reject arbitrarily strong signals on other time slots or frequency channels. This is not true for CDMA; rejection of unwanted signals is only partial. If any or all of the unwanted signals are much stronger than the desired signal, they will overwhelm it. This leads to a general requirement in any CDMA system to approximately match the various signal power levels as seen at the receiver. In CDMA cellular, the base station uses a fast closed-loop power control scheme to tightly control each mobile's transmit power.
Suppose that noise present in a channel takes a zero bit to some other value. Then this will also disrupt the orthogonality property, and thus adding an extra level of forward error correction (FEC) coding is also vital.
So far, we have assumed that CDMA timing is absolutely exact, that is, transmitters exactly transmit at points in multiples of the length of the chip sequence. Of course, in reality, this is impractical to achieve, so all forms of CDMA use spread spectrum process gain to allow receivers to partially discriminate against unwanted signals. Signals with the desired chip code and timing are received, while signals with different chip codes (or the same spreading code but a different timing offset) appear as wideband noise reduced by the process gain.
CDMA's main advantage over TDMA and FDMA is that the number of available CDMA codes is essentially infinite. This makes CDMA ideally suited to large numbers of transmitters each generating a relatively small amount of traffic at irregular intervals, as it avoids the overhead of continually allocating and deallocating a limited number of orthogonal time slots or frequency channels to individual transmitters. CDMA transmitters simply send when they have something to say, and go off the air when they don't.
Soft Handoff
Soft handoff (or soft handover) is an innovation in mobility which was only possible with CDMA technology. It refers to the technique of adding a second base station transceiver to a connection to improve the link budget for users on the edge of a cell. As a result, signal quality and handoff robustness is improved for edge users in a CDMA system.
In TDMA and analog systems, each cell transmits on its own frequency, different from those of its neighbouring cells. If a mobile device reaches the edge of the cell currently serving its call, it is told to break its radio link and quickly tune to the frequency of one of the neighbouring cells where the call has been moved by the network due to the mobile's movement. If the mobile is unable to tune to the new frequency in time the call is dropped.
In CDMA, a set of neighbouring cells all use the same frequency for transmission and distinguish cells (or base stations) by means of a number called the "PN offset", a time offset from the beginning of the well-known pseudo-random noise sequence that is used to spread the signal from the base station. Because all of the cells are on the same frequency, listening to different base stations is now an exercise in digital signal processing based on offsets from the PN sequence, not RF transmission and reception based on separate frequencies.
As the CDMA phone roams through the network, it detects the PN offsets of the neighbouring cells and reports the strength of each signal back to the reference cell of the call (usually the strongest cell). If the signal from a neighbouring cell is strong enough, the mobile will be directed to "add a leg" to its call and start transmitting and receiving to and from the new cell in addition to the cell (or cells) already hosting the call. Likewise, if a cell's signal becomes too weak the mobile is directed to drop that leg. In this way, the mobile can move from cell to cell and add and drop legs as necessary in order to keep the call up without ever dropping the link.
In practice it is quite difficult to implement a CDMA reverse link since RF signals from two geographically distant antennas must be received simultaneously, transported to a central location, added together before decoding to maximize signal quality on the reverse link. Similarly, the forward link signal must be split and sent to two different transmitters for simultaneous, synchronized transmission so that it arrives at the receiver synchronized, to additively enhance signal quality.
When there are frequency boundaries between different carriers or sub-networks, a CDMA phone behaves in the same way as TDMA or analog and performs a hard handoff in which it breaks the existing connection and tries to pick up on the new frequency where it left off.
CDMA features
- Narrowband message signal multiplied by wideband spreading signal or pseudonoise code
- Each user has his own pseudonoise (PN) code
- Soft capacity limit: system performance degrades for all users as number of users increases
- Cell frequency reuse: no frequency planning needed
- Soft handoff increases capacity
- Near-far problem
- Interference limited: power control is required
- Wide bandwidth induces diversity: rake receiver is used
See also
External links
- {{cite web
| author= Cuschieri, Henry (Sr. Director) | title=The Third Generation Partnership Project 2 (3GPP2) | publisher=The 3GPP2 Secretariat | year=2006 | work= | url=http://www.3gpp2.org | accessdate=2006-04-09
}}
- {{cite web
| author=CHATTERJEE, Asok (Chairman) | title=The Third Generation Partnership Project (3GPP) | publisher=Project Coordination Group | year=2006 | work= | url= http://www.3gpp.org | accessdate=2006-04-09
}}
- {{cite web
| author=LaForge, Perry (Executive Director) | title= CDMA Development Group (CDG) | publisher= | year=2006 | work= | url= http://www.cdg.org | accessdate=2006-04-09
}}
- {{cite web
| author= | title=Telecom Resources - CDMA | publisher= | year=undated | work=Telecom Resources | url= http://www.freewebs.com/telecomm/cdma.html | accessdate=2006-04-09
}}
- {{cite web
| author=Poole, Ian | title=Cellular Telecommunications / Cell Phone Technology | publisher=Radio-Electronics.Com | year=2006 | work= | url=http://www.radio-electronics.com/info/cellulartelecomms | accessdate=2006-04-09
}}
- {{cite web
| author=Lee, Kira (Public Relations Officer) | title= Qualcomm CDMA Technologies (QCT)] | publisher= | year=2006 | work= | url= http://www.cdmatech.com | accessdate=2006-04-09
}}
- {{cite web
| author= | title= CDMA in Africa | publisher=GoldOasis Web Solution Enterprise | year=2006 | work=Mobile Africa | url= http://www.mobileafrica.net/cdma.php | accessdate=2006-04-09
}}
- {{cite web
| author=Lohninger, Hans | title=Direct Sequence CDMA Simulation | publisher= | year=2005-12-17 | work=Learning by Simulations | url= http://www.vias.org/simulations/simusoft_dscdma.html | accessdate=2006-04-09
}}
- {{cite web
| author=Den Beste, Steven | year=2002 | title=History of Development at Qualcomm | work= | url=http://denbeste.nu/cd_log_entries/2002/10/GSM3G.shtml | accessdate=2006-04-09 }}
External links
Further reading
- Andrew J. Viterbi. (1995) CDMA : Principles of Spread Spectrum Communication (1st edition) Prentice Hall PTR ISBN 0201633744fr:Code Division Multiple Access
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