| Code-division Multiple Access |
Website Links For Code |
Information AboutCode-division Multiple Access |
| CATEGORIES ABOUT CODE DIVISION MULTIPLE ACCESS | |
| cdma | |
| channel access methods | |
| multiplexing | |
| radio resource management | |
|
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. 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 After a couple of revisions, IS-95 was superseded by the IS-2000 standard.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 Vector s. 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''': : : The square root of a.a is a real number, and is important. We write | ||
| :<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> |
|
|