Code-division multiple access

Code-division multiple access (CDMA) is a channel access method used by various radio communication technologies.[1]

CDMA is an example of multiple access, where several transmitters can send information simultaneously over a single communication channel. This allows several users to share a band of frequencies (see bandwidth). To permit this without undue interference between the users, CDMA employs spread spectrum technology and a special coding scheme (where each transmitter is assigned a code).[1]

CDMA is used as the access method in many mobile phone standards. IS-95, also called "cdmaOne", and its 3G evolution CDMA2000, are often simply referred to as "CDMA", but UMTS, the 3G standard used by GSM carriers, also uses "wideband CDMA", or W-CDMA, as well as TD-CDMA and TD-SCDMA, as its radio technologies.

History

The technology of code-division multiple access channels has long been known. In the Soviet Union (USSR), the first work devoted to this subject was published in 1935 by Dmitry Ageev.[2] It was shown that through the use of linear methods, there are three types of signal separation: frequency, time and compensatory. The technology of CDMA was used in 1957, when the young military radio engineer Leonid Kupriyanovich in Moscow made an experimental model of a wearable automatic mobile phone, called LK-1 by him, with a base station. LK-1 has a weight of 3 kg, 20–30 km operating distance, and 20–30 hours of battery life.[3][4] The base station, as described by the author, could serve several customers. In 1958, Kupriyanovich made the new experimental "pocket" model of mobile phone. This phone weighed 0.5 kg. To serve more customers, Kupriyanovich proposed the device, which he called "correlator."[5][6] In 1958, the USSR also started the development of the "Altai" national civil mobile phone service for cars, based on the Soviet MRT-1327 standard. The phone system weighed 11 kg (24 lb). It was placed in the trunk of the vehicles of high-ranking officials and used a standard handset in the passenger compartment. The main developers of the Altai system were VNIIS (Voronezh Science Research Institute of Communications) and GSPI (State Specialized Project Institute). In 1963 this service started in Moscow, and in 1970 Altai service was used in 30 USSR cities.[7]

Uses

Au CDMA 1X WIN W31SAII gravelly silver expansion
A CDMA2000 mobile phone
  • One of the early applications for code-division multiplexing is in the Global Positioning System (GPS). This predates and is distinct from its use in mobile phones.
  • The Qualcomm standard IS-95, marketed as cdmaOne.
  • The Qualcomm standard IS-2000, known as CDMA2000, is used by several mobile phone companies, including the Globalstar network.
  • The UMTS 3G mobile phone standard, which uses W-CDMA.
  • CDMA has been used in the OmniTRACS satellite system for transportation logistics.

Steps in CDMA modulation

CDMA is a spread-spectrum multiple-access[8] technique. A spread-spectrum technique spreads the bandwidth of the data uniformly for the same transmitted power. A spreading code is a pseudo-random code that has a narrow ambiguity function, unlike other narrow pulse codes. In CDMA a locally generated code runs at a much higher rate than the data to be transmitted. Data for transmission is combined by bitwise XOR (exclusive OR) with the faster code. The figure shows how a spread-spectrum signal is generated. The data signal with pulse duration of (symbol period) is XORed with the code signal with pulse duration of (chip period). (Note: bandwidth is proportional to , where = bit time.) Therefore, the bandwidth of the data signal is and the bandwidth of the spread spectrum signal is . Since is much smaller than , the bandwidth of the spread-spectrum signal is much larger than the bandwidth of the original signal. The ratio is called the spreading factor or processing gain and determines to a certain extent the upper limit of the total number of users supported simultaneously by a base station.[9]

Generation of CDMA
Generation of a CDMA signal

Each user in a CDMA system uses a different code to modulate their signal. Choosing the codes used to modulate the signal is very important in the performance of CDMA systems.[1] The best performance occurs when there is good separation between the signal of a desired user and the signals of other users. The separation of the signals is made by correlating the received signal with the locally generated code of the desired user. If the signal matches the desired user's code, then the correlation function will be high and the system can extract that signal. If the desired user's code has nothing in common with the signal, the correlation should be as close to zero as possible (thus eliminating the signal); this is referred to as cross-correlation. If the code is correlated with the signal at any time offset other than zero, the correlation should be as close to zero as possible. This is referred to as auto-correlation and is used to reject multi-path interference.[10]

An analogy to the problem of multiple access is a room (channel) in which people wish to talk to each other simultaneously. To avoid confusion, people could take turns speaking (time division), speak at different pitches (frequency division), or speak in different languages (code division). CDMA is analogous to the last example where people speaking the same language can understand each other, but other languages are perceived as noise and rejected. Similarly, in radio CDMA, each group of users is given a shared code. Many codes occupy the same channel, but only users associated with a particular code can communicate.

In general, CDMA belongs to two basic categories: synchronous (orthogonal codes) and asynchronous (pseudorandom codes).

Code-division multiplexing (synchronous CDMA)

The digital modulation method is analogous to those used in simple radio transceivers. In the analog case, a low-frequency data signal is time-multiplied with a high-frequency pure sine-wave carrier and transmitted. This is effectively a frequency convolution (Wiener–Khinchin theorem) of the two signals, resulting in a carrier with narrow sidebands. In the digital case, the sinusoidal carrier is replaced by Walsh functions. These are binary square waves that form a complete orthonormal set. The data signal is also binary and the time multiplication is achieved with a simple XOR function. This is usually a Gilbert cell mixer in the circuitry.

Synchronous CDMA exploits mathematical properties of orthogonality between vectors representing the data strings. For example, binary string 1011 is represented by the vector (1, 0, 1, 1). Vectors can be multiplied by taking their dot product, by summing the products of their respective components (for example, if u = (a, b) and v = (c, d), then their dot product u·v = ac + bd). If the dot product is zero, the two vectors are said to be orthogonal to each other. Some properties of the dot product aid understanding of how W-CDMA works. If vectors a and b are orthogonal, then and:

Each user in synchronous CDMA uses a code orthogonal to the others' codes to modulate their signal. An example of 4 mutually orthogonal digital signals is shown in the figure below. Orthogonal codes have a cross-correlation equal to zero; in other words, they do not interfere with each other. In the case of IS-95, 64-bit Walsh codes are used to encode the signal to separate different users. Since each of the 64 Walsh codes is orthogonal to all other, the signals are channelized into 64 orthogonal signals. The following example demonstrates how each user's signal can be encoded and decoded.

Example

Cdma orthogonal signals
An example of 4 mutually orthogonal digital signals

Start with a set of vectors that are mutually orthogonal. (Although mutual orthogonality is the only condition, these vectors are usually constructed for ease of decoding, for example columns or rows from Walsh matrices.) An example of orthogonal functions is shown in the adjacent picture. These vectors will be assigned to individual users and are called the code, chip code, or chipping code. In the interest of brevity, the rest of this example uses codes v with only two bits.

Each user is associated with a different code, say v. A 1 bit is represented by transmitting a positive code v, and a 0 bit is represented by a negative code −v. For example, if v = (v0, v1) = (1, −1) and the data that the user wishes to transmit is (1, 0, 1, 1), then the transmitted symbols would be

(v, −v, v, v) = (v0, v1, −v0, −v1, v0, v1, v0, v1) = (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 v chosen from that set, but the construction method of the transmitted vector is identical.

Now, due to physical properties of interference, if two signals at a point are in phase, they add to give twice the amplitude of each signal, but if they are out of phase, they subtract and give a signal that is the difference of the amplitudes. Digitally, this behaviour can be modelled by the addition of the transmission vectors, component by component.

If sender0 has code (1, −1) and data (1, 0, 1, 1), and sender1 has code (1, 1) and data (0, 0, 1, 1), and both senders transmit simultaneously, then this table describes the coding steps:

Step Encode sender0 Encode sender1
0 code0 = (1, −1), data0 = (1, 0, 1, 1) code1 = (1, 1), data1 = (0, 0, 1, 1)
1 encode0 = 2(1, 0, 1, 1) − (1, 1, 1, 1) = (1, −1, 1, 1) encode1 = 2(0, 0, 1, 1) − (1, 1, 1, 1) = (−1, −1, 1, 1)
2 signal0 = encode0 ⊗ code0
= (1, −1, 1, 1) ⊗ (1, −1)
= (1, −1, −1, 1, 1, −1, 1, −1)
signal1 = encode1 ⊗ code1
= (−1, −1, 1, 1) ⊗ (1, 1)
= (−1, −1, −1, −1, 1, 1, 1, 1)

Because signal0 and signal1 are transmitted at the same time into the air, they add to produce the raw signal

(1, −1, −1, 1, 1, −1, 1, −1) + (−1, −1, −1, −1, 1, 1, 1, 1) = (0, −2, −2, 0, 2, 0, 2, 0).

This raw signal is called an interference pattern. The receiver then extracts an intelligible signal for any known sender by combining the sender's code with the interference pattern. The following table explains how this works and shows that the signals do not interfere with one another:

Step Decode sender0 Decode sender1
0 code0 = (1, −1), signal = (0, −2, −2, 0, 2, 0, 2, 0) code1 = (1, 1), signal = (0, −2, −2, 0, 2, 0, 2, 0)
1 decode0 = pattern.vector0 decode1 = pattern.vector1
2 decode0 = ((0, −2), (−2, 0), (2, 0), (2, 0)) · (1, −1) decode1 = ((0, −2), (−2, 0), (2, 0), (2, 0)) · (1, 1)
3 decode0 = ((0 + 2), (−2 + 0), (2 + 0), (2 + 0)) decode1 = ((0 − 2), (−2 + 0), (2 + 0), (2 + 0))
4 data0=(2, −2, 2, 2), meaning (1, 0, 1, 1) data1=(−2, −2, 2, 2), meaning (0, 0, 1, 1)

Further, after decoding, all values greater than 0 are interpreted as 1, while all values less than zero are interpreted as 0. For example, after decoding, data0 is (2, −2, 2, 2), but the receiver interprets this as (1, 0, 1, 1). Values of exactly 0 means that the sender did not transmit any data, as in the following example:

Assume signal0 = (1, −1, −1, 1, 1, −1, 1, −1) is transmitted alone. The following table shows the decode at the receiver:

Step Decode sender0 Decode sender1
0 code0 = (1, −1), signal = (1, −1, −1, 1, 1, −1, 1, −1) code1 = (1, 1), signal = (1, −1, −1, 1, 1, −1, 1, −1)
1 decode0 = pattern.vector0 decode1 = pattern.vector1
2 decode0 = ((1, −1), (−1, 1), (1, −1), (1, −1)) · (1, −1) decode1 = ((1, −1), (−1, 1), (1, −1), (1, −1)) · (1, 1)
3 decode0 = ((1 + 1), (−1 − 1), (1 + 1), (1 + 1)) decode1 = ((1 − 1), (−1 + 1), (1 − 1), (1 − 1))
4 data0 = (2, −2, 2, 2), meaning (1, 0, 1, 1) data1 = (0, 0, 0, 0), meaning no data

When the receiver attempts to decode the signal using sender1's code, the data is all zeros, therefore the cross-correlation is equal to zero and it is clear that sender1 did not transmit any data.

Asynchronous CDMA

When mobile-to-base links cannot be precisely coordinated, particularly due to the mobility of the handsets, a different approach is required. Since it is not mathematically possible to create signature sequences that are both orthogonal for arbitrarily random starting points and which make full use of the code space, unique "pseudo-random" or "pseudo-noise" (PN) sequences are used in asynchronous CDMA systems. A PN code is a binary sequence that appears random but can be reproduced in a deterministic manner by intended receivers. These PN codes are used to encode and decode a user's signal in asynchronous CDMA in the same manner as the orthogonal codes in synchronous CDMA (shown in the example above). 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 (following the central limit theorem in statistics). Gold codes are an example of a PN suitable for this purpose, as there is low correlation between the codes. 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. In other words, unlike synchronous CDMA, the signals of other users will appear as noise to the signal of interest and interfere slightly with the desired signal in proportion to number of users.

All forms of CDMA use spread-spectrum process gain to allow receivers to partially discriminate against unwanted signals. Signals encoded with the specified PN sequence (code) are received, while signals with different codes (or the same 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 CDM (synchronous 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 asynchronous 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 asynchronous 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.

Advantages of asynchronous CDMA over other techniques

Efficient practical utilization of the fixed frequency spectrum

In theory CDMA, TDMA and FDMA have exactly the same spectral efficiency, but, in practice, each has its own challenges – power control in the case of CDMA, timing in the case of TDMA, and frequency generation/filtering in the case of FDMA.

TDMA systems must carefully synchronize the transmission times of all the users to ensure that they are received in the correct time slot and do not cause interference. Since this cannot be perfectly controlled in a mobile environment, each time slot 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 unpredictable Doppler shift of the signal spectrum because of user mobility. The guard bands will reduce the probability that adjacent channels will interfere, but decrease the utilization of the spectrum.

Flexible allocation of resources

Asynchronous CDMA offers a key advantage in the flexible allocation of resources i.e. allocation of PN codes to active users. In the case of CDM (synchronous CDMA), TDMA, and FDMA the number of simultaneous orthogonal codes, time slots, and frequency slots respectively are fixed, hence the capacity in terms of the number of simultaneous users is limited. There are a fixed number of orthogonal codes, time slots or frequency bands that can be allocated for CDM, 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 an asynchronous 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 asynchronous 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 accommodated 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, asynchronous 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 (synchronous CDMA), 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 time slots. Furthermore, it would require significant overhead to continually allocate and deallocate the orthogonal-code, time-slot or frequency-channel resources. By comparison, asynchronous 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.

Spread-spectrum characteristics of CDMA

Most modulation schemes try to minimize the bandwidth of this signal since bandwidth is a limited resource. However, spread-spectrum techniques use a transmission bandwidth that is several orders of magnitude greater than the minimum required signal bandwidth. One of the initial reasons for doing this was military applications including guidance and communication systems. These systems were designed using spread spectrum because of its security and resistance to jamming. Asynchronous CDMA has some level of privacy built in because the signal is spread using a pseudo-random code; this code makes the spread-spectrum signals appear random or have noise-like properties. A receiver cannot demodulate this transmission without knowledge of the pseudo-random sequence used to encode the data. CDMA is also resistant to jamming. A jamming signal only has a finite amount of power available to jam the signal. The jammer can either spread its energy over the entire bandwidth of the signal or jam only part of the entire signal.[11]

CDMA can also effectively reject narrow-band interference. Since narrow-band interference affects only a small portion of the spread-spectrum signal, it can easily be removed through notch filtering without much loss of information. Convolution encoding and interleaving can be used to assist in recovering this lost data. CDMA signals are also resistant to multipath fading. Since the spread-spectrum signal occupies a large bandwidth, only a small portion of this will undergo fading due to multipath at any given time. Like the narrow-band interference, this will result in only a small loss of data and can be overcome.

Another reason CDMA is resistant to multipath interference is because the delayed versions of the transmitted pseudo-random codes will have poor correlation with the original pseudo-random code, and will thus appear as another user, which is ignored at the receiver. In other words, as long as the multipath channel induces at least one chip of delay, the multipath signals will arrive at the receiver such that they are shifted in time by at least one chip from the intended signal. The correlation properties of the pseudo-random codes are such that this slight delay causes the multipath to appear uncorrelated with the intended signal, and it is thus ignored.

Some CDMA devices use a rake receiver, which exploits multipath delay components to improve the performance of the system. A rake receiver combines the information from several correlators, each one tuned to a different path delay, producing a stronger version of the signal than a simple receiver with a single correlation tuned to the path delay of the strongest signal.[12]

Frequency reuse is the ability to reuse the same radio channel frequency at other cell sites within a cellular system. In the FDMA and TDMA systems, frequency planning is an important consideration. The frequencies used in different cells must be planned carefully to ensure signals from different cells do not interfere with each other. In a CDMA system, the same frequency can be used in every cell, because channelization is done using the pseudo-random codes. Reusing the same frequency in every cell eliminates the need for frequency planning in a CDMA system; however, planning of the different pseudo-random sequences must be done to ensure that the received signal from one cell does not correlate with the signal from a nearby cell.[13]

Since adjacent cells use the same frequencies, CDMA systems have the ability to perform soft hand-offs. Soft hand-offs allow the mobile telephone to communicate simultaneously with two or more cells. The best signal quality is selected until the hand-off is complete. This is different from hard hand-offs utilized in other cellular systems. In a hard-hand-off situation, as the mobile telephone approaches a hand-off, signal strength may vary abruptly. In contrast, CDMA systems use the soft hand-off, which is undetectable and provides a more reliable and higher-quality signal.[13]

Collaborative CDMA

In a recent study, a novel collaborative multi-user transmission and detection scheme called collaborative CDMA[14] has been investigated for the uplink that exploits the differences between users' fading channel signatures to increase the user capacity well beyond the spreading length in the MAI-limited environment. The authors show that it is possible to achieve this increase at a low complexity and high bit error rate performance in flat fading channels, which is a major research challenge for overloaded CDMA systems. In this approach, instead of using one sequence per user as in conventional CDMA, the authors group a small number of users to share the same spreading sequence and enable group spreading and despreading operations. The new collaborative multi-user receiver consists of two stages: group multi-user detection (MUD) stage to suppress the MAI between the groups and a low-complexity maximum-likelihood detection stage to recover jointly the co-spread users' data using minimal Euclidean-distance measure and users' channel-gain coefficients.

See also

References

  1. ^ a b c Guowang Miao; Jens Zander; Ki Won Sung; Ben Slimane (2016). Fundamentals of Mobile Data Networks. Cambridge University Press. ISBN 1107143217.
  2. ^ Ageev, D. V. (1935). "Bases of the Theory of Linear Selection. Code Demultiplexing". Proceedings of the Leningrad Experimental Institute of Communication: 3–35.
  3. ^ Nauka i Zhizn 8, 1957, p. 49.
  4. ^ Yuniy technik 7, 1957, p. 43–44.
  5. ^ Nauka i Zhizn 10, 1958, p. 66.
  6. ^ Tekhnika Molodezhi 2, 1959, p. 18–19.
  7. ^ "First Russian Mobile Phone". September 18, 2006.
  8. ^ Ipatov, Valeri (2000). Spread Spectrum and CDMA. John Wiley & Sons, Ltd.
  9. ^ Dubendorf, Vern A. (2003). Wireless Data Technologies. John Wiley & Sons, Ltd.
  10. ^ "CDMA Spectrum". Retrieved 2008-04-29.
  11. ^ Skylar, Bernard (2001). Digital Communications: Fundamentals and Applications (Second ed.). Prentice-Hall PTR.
  12. ^ Rapporteur, Theodore S. (2002). Wireless Communications, Principles and Practice. Prentice-Hall, Inc.
  13. ^ a b Harte, Levine, Kikta, Lawrence, Richard, Romans (2002). 3G Wireless Demystified. McGowan-Hill.CS1 maint: Multiple names: authors list (link)
  14. ^ Shakya, Indu L. (2011). "High User Capacity Collaborative CDMA". IET Communications.

Further reading

External links

3rd Generation Partnership Project 2

The 3rd Generation Partnership Project 2 (3GPP2) is a collaboration between telecommunications associations to make a globally applicable third generation (3G) mobile phone system specification within the scope of the ITU's IMT-2000 project. In practice, 3GPP2 is the standardization group for CDMA2000, the set of 3G standards based on the earlier cdmaOne 2G CDMA technology.

The participating associations are ARIB/TTC (Japan), China Communications Standards Association, Telecommunications Industry Association (North America) and Telecommunications Technology Association (South Korea).

The agreement was established in December 1998.

Ultra Mobile Broadband (UMB) was a 3GPP2 project to develop a fourth-generation successor to CDMA2000. In November 2008, Qualcomm, UMB's lead sponsor, announced it was ending development of the technology, favoring LTE instead.3GPP2 should not be confused with 3GPP; 3GPP is the standard body behind the Universal Mobile Telecommunications System (UMTS) that is the 3G upgrade to GSM networks, while 3GPP2 is the standard body behind the competing 3G standard CDMA2000 that is the 3G upgrade to cdmaOne networks used mostly in the United States (and to some extent also in Japan, China, Canada, South Korea and India).

GSM/GPRS/EDGE/W-CDMA is the most widespread wireless standard in the world. A few countries (such as China, the United States, Canada, Ukraine, Trinidad and Tobago, India, South Korea and Japan) use both sets of standards, but most countries use only the GSM family.

AAA (computer security)

AAA refers to Authentication, Authorization and Accounting. It is used to refer to a family of protocols that mediate network access.

Two network protocols providing this functionality are particularly popular: the RADIUS protocol, and its newer Diameter counterpart.Further explanations of Authentication, Authorization, and Accounting are available on external sites.

Authentication and Key Agreement

Authentication and Key Agreement (AKA) is a security protocol used in 3G networks. AKA is also used for one-time password generation mechanism for digest access authentication. AKA is a challenge-response based mechanism that uses symmetric cryptography.

CAVE-based authentication

CAVE-based Authentication (a.k.a. HLR Authentication, 2G Authentication, Access Authentication) is an access authentication protocol used in CDMA/1xRTT computer network systems.

CDMA2000

CDMA2000 (also known as C2K or IMT Multi‑Carrier (IMT‑MC)) is a family of 3G mobile technology standards for sending voice, data, and signaling data between mobile phones and cell sites. It is developed by 3GPP2 as a backwards-compatible successor to second-generation cdmaOne (IS-95) set of standards and used especially in North America and South Korea.

CDMA2000 compares to UMTS, a competing set of 3G standards, which is developed by 3GPP and used in Europe, Japan, and China.

The name CDMA2000 denotes a family of standards that represent the successive, evolutionary stages of the underlying technology. These are:

Voice: CDMA2000 1xRTT, 1X Advanced

Data: CDMA2000 1xEV-DO (Evolution-Data Optimized): Release 0, Revision A, Revision B, Ultra Mobile Broadband (UMB)All are approved radio interfaces for the ITU's IMT-2000. In the United States, CDMA2000 is a registered trademark of the Telecommunications Industry Association (TIA-USA).

Channel access method

In telecommunications and computer networks, a channel access method or multiple access method allows more than two terminals connected to the same transmission medium to transmit over it and to share its capacity. Examples of shared physical media are wireless networks, bus networks, ring networks and point-to-point links operating in half-duplex mode.

A channel access method is based on multiplexing, that allows several data streams or signals to share the same communication channel or transmission medium. In this context, multiplexing is provided by the physical layer.

A channel access method is also based on a multiple access protocol and control mechanism, also known as medium access control (MAC). Medium access control deals with issues such as addressing, assigning multiplex channels to different users, and avoiding collisions. Media access control is a sub-layer in the data link layer of the OSI model and a component of the link layer of the TCP/IP model.

Chip (CDMA)

In digital communications, a chip is a pulse of a direct-sequence spread spectrum (DSSS) code, such as a Pseudo-random Noise (PN) code sequence used in direct-sequence code division multiple access (CDMA) channel access techniques.

In a binary direct-sequence system, each chip is typically a rectangular pulse of +1 or –1 amplitude, which is multiplied by a data sequence (similarly +1 or –1 representing the message bits) and by a carrier waveform to make the transmitted signal. The chips are therefore just the bit sequence out of the code generator; they are called chips to avoid confusing them with message bits.

The chip rate of a code is the number of pulses per second (chips per second) at which the code is transmitted (or received). The chip rate is larger than the symbol rate, meaning that one symbol is represented by multiple chips. The ratio is known as the spreading factor (SF) or processing gain:

Direct-sequence spread spectrum

In telecommunications, direct-sequence spread spectrum (DSSS) is a spread spectrum modulation technique used to reduce overall signal interference. The spreading of this signal makes the resulting wideband channel more noisy, allowing for greater resistance to unintentional and intentional interference.A method of achieving the spreading of a given signal is provided by the modulation scheme. With DSSS, the message signal is used to modulate a bit sequence known as a Pseudo Noise (PN) code; this PN code consists of a radio pulse that is much shorter in duration (larger bandwidth) than the original message signal. This modulation of the message signal scrambles and spreads the pieces of data, and thereby resulting in a bandwidth size nearly identical to that of the PN sequence. In this context, the duration of the radio pulse for the PN code is referred to as the chip duration. The smaller this duration, the larger the bandwidth of the resulting DSSS signal; more bandwidth multiplexed to the message signal results in better resistance against interference.Some practical and effective uses of DSSS include the Code Division Multiple Access (CDMA) channel access method and the IEEE 802.11b specification used in Wi-Fi networks.

Evolution-Data Optimized

Evolution-Data Optimized (EV-DO, EVDO, etc.) is a telecommunications standard for the wireless transmission of data through radio signals, typically for broadband Internet access. EV-DO is an evolution of the CDMA2000 (IS-2000) standard which supports high data rates and can be deployed alongside a wireless carrier's voice services. It uses advanced multiplexing techniques including code division multiple access (CDMA) as well as time division multiplexing (TDM) to maximize throughput. It is a part of the CDMA2000 family of standards and has been adopted by many mobile phone service providers around the world particularly those previously employing CDMA networks. It is also used on the Globalstar satellite phone network.EV-DO service has been or will be discontinued in much of Canada in 2015.An EV-DO channel has a bandwidth of 1.25 MHz, the same bandwidth size that IS-95A (IS-95) and IS-2000 (1xRTT) use, though the channel structure is very different. The back-end network is entirely packet-based, and is not constrained by restrictions typically present on a circuit switched network.

The EV-DO feature of CDMA2000 networks provides access to mobile devices with forward link air interface speeds of up to 2.4 Mbit/s with Rel. 0 and up to 3.1 Mbit/s with Rev. A. The reverse link rate for Rel. 0 can operate up to 153 kbit/s, while Rev. A can operate at up to 1.8 Mbit/s. It was designed to be operated end-to-end as an IP based network, and can support any application which can operate on such a network and bit rate constraints.

Frequency-hopping spread spectrum

Frequency-hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver. It is used as a multiple access method in the code division multiple access (CDMA) scheme frequency-hopping code division multiple access (FH-CDMA).

Each available frequency band is divided into sub-frequencies. Signals rapidly change ("hop") among these in a predetermined order. Interference at a specific frequency will only affect the signal during that short interval. FHSS can, however, cause interference with adjacent direct-sequence spread spectrum (DSSS) systems.

Adaptive frequency-hopping spread spectrum (AFH), a specific type of FHSS, is used in Bluetooth wireless data transfer.

Metro by T-Mobile

Metro by T-Mobile (formerly known as MetroPCS and also known simply as Metro) is a prepaid wireless carrier brand owned by T-Mobile US. It previously operated the fifth largest mobile telecommunications network in the United States using code division multiple access. In 2013, the carrier engaged in a reverse merger with T-Mobile USA; post-merger, its services were merged under T-Mobile's 4G and LTE network.

Multi-carrier code-division multiple access

Multi-carrier code-division multiple access (MC-CDMA) is a multiple access scheme used in OFDM-based telecommunication systems, allowing the system to support multiple users at the same time over same frequency band.

MC-CDMA spreads each user symbol in the frequency domain. That is, each user symbol is carried over multiple parallel subcarriers, but it is phase-shifted (typically 0 or 180 degrees) according to a code value. The code values differ per subcarrier and per user. The receiver combines all subcarrier signals, by weighing these to compensate varying signal strengths and undo the code shift. The receiver can separate signals of different users, because these have different (e.g. orthogonal) code values.

Since each data symbol occupies a much wider bandwidth (in hertz) than the data rate (in bit/s), a ratio of signal to noise-plus-interference (if defined as signal power divided by total noise plus interference power in the entire transmission band) of less than 0 dB is feasible.

One way of interpreting MC-CDMA is to regard it as a direct-sequence CDMA signal (DS-CDMA), which is transmitted after it has been fed through an inverse FFT (fast Fourier transform).

Near–far problem

The near–far problem or hearability problem is the effect of a strong signal from a near signal source in making it hard for a receiver to hear a weaker signal from a further source due to adjacent-channel interference, co-channel interference, distortion, capture effect, dynamic range limitation, or the like. Such a situation is common in wireless communication systems, in particular CDMA. In some signal jamming techniques, the near–far problem is exploited to disrupt ("jam") communications.

PowerSource (phone brand)

PowerSource, or "hybrid" phones, are specialized cellular devices used by customers of the American telecommunications company Sprint-Nextel. They are distinct from other mobile phones in that they make use of two cellular networks instead of a single one.

PowerSource phones currently include the ic402, ic502, ic602 and ic902, all manufactured by Motorola and available only through Sprint-Nextel in the United States.

Rake receiver

A rake receiver is a radio receiver designed to counter the effects of multipath fading. It does this by using several "sub-receivers" called fingers, that is, several correlators each assigned to a different multipath component. Each finger independently decodes a single multipath component; at a later stage the contribution of all fingers are combined in order to make the most use of the different transmission characteristics of each transmission path. This could very well result in higher signal-to-noise ratio (or Eb/N0) in a multipath environment than in a "clean" environment.

The multipath channel through which a radio wave transmits can be viewed as transmitting the original (line of sight) wave pulse through a number of multipath components. Multipath components are delayed copies of the original transmitted wave traveling through a different echo path, each with a different magnitude and time-of-arrival at the receiver. Since each component contains the original information, if the magnitude and time-of-arrival (phase) of each component is computed at the receiver (through a process called channel estimation), then all the components can be added coherently to improve the information reliability.

It was patented by Robert Price and Paul E. Green in July 1956.

Sasatel (Dovetel)

Sasatel (Dovetel) was a telecommunications company with a unified national licence in Tanzania. Their licence was later revoked. The company rolled out a 3G wireless (based on Code Division Multiple Access (CDMA)) national network benefiting from the demand for data and broadband services to both high-end residential and corporate customers. ZTE (a Chinese multinational telecommunications equipment and systems company) is the supplier of the network infrastructure. Dovetel bundles its broadband offering with fixed voice services and offers limited mobility voice services to the low-end of the residential market in order to increase penetration beyond the traditional GSM target market for mobile voice. In addition to CDMA modems and routers, Sasatel offers WiMAX solutions providing greater bandwidth.

Selectable Mode Vocoder

Selectable Mode Vocoder (SMV) is variable bitrate speech coding standard used in CDMA2000 networks. SMV provides multiple modes of operation that are selected based on input speech characteristics.

The SMV for Wideband CDMA is based on 4 codecs: full rate at 8.5 kbit/s, half rate at 4 kbit/s, quarter rate at 2 kbit/s, and eighth rate at 800 bit/s. The full rate and half rate are based on the CELP algorithm that is based on a combined closed-loop-open-loop-analysis (COLA). In SMV the signal frames are first classified as:

Silence/Background noise

Non-stationary unvoiced

Stationary unvoiced

Onset

Non-stationary voiced

Stationary voicedThe algorithm includes voice activity detection (VAD) followed by an elaborate frame classification scheme. Silence/background noise and stationary unvoiced frames are represented by spectrum-modulated noise and coded at 1/4 or 1/8 rate. The SMV uses 4 subframes for full rate and two/three subframes for half rate. The stochastic (fixed) codebook structure is also elaborate and uses sub-codebooks each tuned for a particular type of speech. The sub-codebooks have different degrees of pulse sparseness (more sparse for noise like excitation). SMV scores a high of 3.6 MOS at full rate with clean speech.

The coder works on a frame of 160 speech samples (20 ms) and requires a look ahead of 80 samples (10 ms) if noise-suppression option B is used. An additional 24 samples of look ahead is required if noise-suppression option A is used. So the algorithmic delay for the coder is 30 ms with noise-suppression option B and 33 ms with noise-suppression option A.

The next evolution of CDMA speech codecs is VMR-WB which provides much higher speech quality with wideband while fitting to the same networks.

SMV can be also used in 3GPP2 container file format - 3G2.

UMTS

The Universal Mobile Telecommunications System (UMTS) is a third generation mobile cellular system for networks based on the GSM standard. Developed and maintained by the 3GPP (3rd Generation Partnership Project), UMTS is a component of the International Telecommunications Union IMT-2000 standard set and compares with the CDMA2000 standard set for networks based on the competing cdmaOne technology. UMTS uses wideband code division multiple access (W-CDMA) radio access technology to offer greater spectral efficiency and bandwidth to mobile network operators.

UMTS specifies a complete network system, which includes the radio access network (UMTS Terrestrial Radio Access Network, or UTRAN), the core network (Mobile Application Part, or MAP) and the authentication of users via SIM (subscriber identity module) cards.

The technology described in UMTS is sometimes also referred to as Freedom of Mobile Multimedia Access (FOMA) or 3GSM.

Unlike EDGE (IMT Single-Carrier, based on GSM) and CDMA2000 (IMT Multi-Carrier), UMTS requires new base stations and new frequency allocations.

UMTS Terrestrial Radio Access Network

UTRAN (short for "UMTS

Terrestrial Radio Access Network") is a collective term for the network and equipment that connects mobile handsets to the public telephone network or the Internet. It contains the base stations, which are called Node B's and Radio Network Controllers (RNCs) which make up the UMTS radio access network. This communications network, commonly referred to as 3G (for 3rd Generation Wireless Mobile Communication Technology), can carry many traffic types from real-time Circuit Switched to IP based Packet Switched. The UTRAN allows connectivity between the UE (user equipment) and the core network.

The RNC provides control functionalities for one or more Node Bs. A Node B and an RNC can be the same device, although typical implementations have a separate RNC located in a central office serving multiple Node Bs. Despite the fact that they do not have to be physically separated, there is a logical interface between them known as the Iub. The RNC and its corresponding Node Bs are called the Radio Network Subsystem (RNS). There can be more than one RNS present in a UTRAN.

There are four interfaces connecting the UTRAN internally or externally to other functional entities: Iu, Uu, Iub and Iur. The Iu interface is an external interface that connects the RNC to the Core Network (CN). The Uu is also external, connecting the Node B with the User Equipment (UE). The Iub is an internal interface connecting the RNC with the Node B. And at last there is the Iur interface which is an internal interface most of the time, but can, exceptionally be an external interface too for some network architectures. The Iur connects two RNCs with each other.

Spread spectrum in digital communications
Main articles
Spread spectrum methods
CDMA schemes
Major implementations
Major concepts
Channel-based
Packet-based
Duplexing methods

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