About Opentopia
Opentopia Directory Encyclopedia Tools

Code division multiple access

Encyclopedia : C : CO : COD : Code division multiple access

  1. redirect
Mobile phone and data
standards
0G
0.5G
1G
2G
2.5G
2.75G
3G
3.5G
3.75G
4G
Frequency bands

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, and W-CDMA by the International Telecommunication Union or ITU.

CDMA has since been used in many communications systems, including the Global Positioning System (GPS) and in the OmniTRACS satellite system for transportation logistics.

Contents

Usage in mobile telephony

A number of different terms are used to refer to CDMA implementations. The original US standard defined 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. The CDMA has been submitted for approval as a mobile air interface standard to the ITU International Telecommunication Union.

Whereas GSM is a specification of an entire network infrastructure, the CDMA interface relates only to the air interface - the radio part technology. E.g. GSM specifies an infrastructure based on internationally approved standard while CDMA allows each operator to provide the network features as it finds suited. On the air interface, the signalling suite (GSM: ISDN SS7) work has been progressing to harmonise these.

After a couple of revisions, IS-95 was superseded by the IS-2000 standard.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 US national standards (including cdmaOne and CDMA2000) are not compatible with the W-CDMA family of International Telecommunication Union (ITU) standards.

Another important application of CDMA—predating and entirely distinct from CDMA cellular—is the Global Positioning System, GPS.

Coverage and Technologies

When it comes to cell size, this depends on the power used by the handset, the terrain and frequency. Effective algorithms can reduce the noise, but will require that superfluous information is sent to validate the transfer. Hence the radio frequency and power of the handset determines the cell size. We see surfers ride high waves with lots of energy as these approaches the shallow shore - and radio waves are no different. Drop a big stone in a pond and compare the waves with that made by a small stone. Then if you can - study how much harder you have to throw the small stone to make the waves go just as far as the big stone. The long waves need less energy to travel far - so lower frequencies are used to make larger cell. The higher the frequency - the shorter will the coverage be. This is used by mobile networks to vary the size of the cell. In cities you need many small cells, and high frequencies allows sites to be placed closer, and more subscribers provided service. On the country side, a lower frequency allows the same base station to provide broader coverage.See also Market situation section of GSM

Various companies use various variants of CDMA to provide fixed line network - using Wireless local loop WLL. Since they can plan with the maximum number of subscribers in the cell, and these are all stationary, this application of CDMA can be found in most parts of the world.

CDMA is suited for data transfer, with bursty behaviour, and where delays can be accepted. It is used in WLAN - and the cell size here is 500 feet because of the high frequency (2.4GHz) and low power. The suitability for data transfer is the reason for why W-CDMA seems to be "winning technology" for the data part of 3G mobile networks.

Technical details

Code Division Multiplexing

Synchronous CDMA, or Code Division Multiplexing (CDM) 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 CDM works. For vectors a, b, c:

[\mathbf\cdot(\mathbf+\mathbf)=\mathbf\cdot\mathbf+\mathbf\cdot\mathbf,\quad\mathrm]
[\mathbf\cdot k\mathbf=k(\mathbf\cdot\mathbf).]
The square root of a.a is a real number, and is important. We write
[||\mathbf||=\sqrt\cdot\mathbf}.]
Suppose vectors a and b are orthogonal. Then:
[\mathbf\cdot(\mathbf+\mathbf)=||\mathbf||^2\quad\mathrm\quad\mathbf\cdot\mathbf+\mathbf\cdot\mathbf= ||a||^2+0,]
[\mathbf\cdot(-\mathbf+\mathbf)=-||\mathbf||^2\quad\mathrm\quad-\mathbf\cdot\mathbf+\mathbf\cdot\mathbf= -||a||^2+0,]
[\mathbf\cdot(\mathbf+\mathbf)=||\mathbf||^2\quad\mathrm\quad\mathbf\cdot\mathbf+\mathbf\cdot\mathbf= 0+||b||^2,]
[\mathbf\cdot(\mathbf-\mathbf)=-||\mathbf||^2\quad\mathrm\quad\mathbf\cdot\mathbf-\mathbf\cdot\mathbf=0 -||b||^2.]

Example

An example of 4 orthogonal digital signals.
Enlarge
An example of 4 orthogonal digital signals.

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).

Asynchronous CDMA

The previous example of orthogonal Walsh sequences describes how 2 users can be multiplexed together in a synchronous system, a technique that is commonly referred to as Code Division Multiplexing (CDM). The set of 4 Walsh sequences shown in the figure will afford up to 4 users, and in general, an NxN Walsh matrix can be used to multiplex N users. Multiplexing requires all of the users to be coordinated so that each transmits their assigned sequence v (or the complement, -v) starting at exactly the same time. Thus, this technique finds use in base-to-mobile links, where all of the transmissions originate from the same transmitter and can be perfectly coordinated.

On the other hand, the mobile-to-base links cannot be precisely coordinated, particularly due to the mobility of the handsets, and require a somewhat different approach. Since it is not mathematically possible to create signature sequences that are orthogonal for arbitrarily random starting points, unique "pseudo-random" or "pseudo-noise" (PN) sequences are used in asynchronous CDMA systems. These PN sequences are statistically uncorrelated, and the sum of a large number of PN sequences results in Multiple Access Interference (MAI) that is approximated by a Gaussian noise process (via the theorem of the "law of large numbers" in statistics). If all of the users are received with the same power level, then the variance (e.g., the noise power) of the MAI increases in direct proportion to the number of users.

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.

Since each user generates MAI, controlling the signal strength is an important issue with CDMA transmitters. A CDMA, TDMA or FDMA receiver can in theory completely reject arbitrarily strong signals using different codes, time slots or frequency channels due to the orthogonality of these systems. 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.

CDMA's main advantage over TDMA and FDMA is that it can use the spectrum more efficiently in mobile telephony applications. TDMA systems must carefully synchronize the transmission times of all the users to ensure that they are received in the correct timeslot and do not cause interference. Since this cannot be perfectly controlled in a mobile environment, each timeslot must have a guard-time, which reduces the probability that users will interfere, but decreases the spectral efficiency. Similarly, FDMA systems must use a guard-band between adjacent channels, due to the random doppler shift of the signal spectrum which occurs due to the user's mobility. The guard-bands will reduce the probability that adjacent channels will interfere, but decrease the utilization of the spectrum.

Most importantly, CDMA offers a key advantage in the flexible allocation of resources. There are a fixed number of orthogonal codes, timeslots or frequency bands that can be allocated for CDMA, TDMA and FDMA systems, which remain underutilized due to the bursty nature of telephony and packetized data transmissions. There is no strict limit to the number of users that can be supported in a CDMA system, only a practical limit governed by the desired bit error probability, since the SIR (Signal to Interference Ratio) varies inversely with the number of users. In a bursty traffic environment like mobile telephony, the advantage afforded by CDMA is that the performance (bit error rate) is allowed to fluctuate randomly, with an average value determined by the number of users times the percentage of utilization. Suppose there are 2N users that only talk half of the time, then 2N users can be accomodated with the same average bit error probability as N users that talk all of the time. The key difference here is that the bit error probability for N users talking all of the time is constant, whereas it is a random quantity (with the same mean) for 2N users talking half of the time.

In other words, CDMA is ideally suited to a mobile network where large numbers of transmitters each generate a relatively small amount of traffic at irregular intervals. CDM, TDMA and FDMA systems cannot recover the underutilized resources inherent to bursty traffic due to the fixed number of orthogonal codes, time slots or frequency channels that can be assigned to individual transmitters. For instance, if there are N time slots in a TDMA system and 2N users that talk half of the time, then half of the time there will be more than N users needing to use more than N timeslots. Furthermore, it would require significant overhead to continually allocate and deallocate the orthogonal code, time-slot or frequency channel resources. By comparison, CDMA transmitters simply send when they have something to say, and go off the air when they don't, keeping the same PN signature sequence as long as they are connected to the system.

Soft Handoff

Soft handoff (or soft handover) is an innovation in mobility inherited from GSM. It refers to the technique of adding a second (or more) base station (or in GSM as many as 5) base stations to a connection to be certain that "next base is ready as you move through the terrain. However, it can also be used to move a call from one base station that is approaching congestion - to another with better capacity. As a result, signal quality and handoff robustness is improved for edge users in GSM and 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.

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

See also

External links

==

 

From Wikipedia, the Free Encyclopedia. Original article here. Support Wikipedia by contributing or donating.
All text is available under the terms of the GNU Free Documentation License See Wikipedia Copyrights for details.

Search Titles
0123456789
ABCDEFGHIJ
KLMNOPQRST
UVWXYZ?

E-mail this article to:

Personal Message: