CDMA TECHNOLOGY PDF
CDMA Technology for. Intelligent Transportation Systems. Rabindranath Bera 1, Jitendranath Bera2, Sanjib Sil 3,. Dipak Mondal 1, Sourav Dhar1 & Debdatta. Each station is assigned a unique m-bit code (chip sequence). ○ To send bit 1, station sends chip sequence. ○ To send bit 0, station sends the. CDMA. Code Division Multiple Access or CDMA has it's roots in WWII era spread spectrum technology. CDMA can be implemented in several ways, two of.
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Code Division Multiple Access (CDMA) is a channel access method normally used by CDMA technology has given significant advantages over other parallel . Many Digital cellular and mobile systems are in use today. Analog cellular systems use Frequency Division Multiple Access (FDMA) technique. Digital systems. PDF | Code Division Multiple Access (CDMA) is technology for digital transmission of radio signal in telecommunication systems.
Original aerospace telemetry systems used an FDMA system to accommodate multiple sensor data on a single radio channel. Today, all of these applications use TDMA digital techniques. Each time slot is used to transmit one byte or another digital segment of each signal in sequential serial data format.
Key elements of CDMA
A good example is the widely used T1 transmission system, which has been used for years in the telecom industry. T1 lines carry up to 24 individual voice telephone calls on a single line Fig. Each voice signal usually covers Hz to Hz and is digitized at an 8-kHz rate, which is just a bit more than the minimal Nyquist rate of two times the highest-frequency component needed to retain all the analog content.
Each time slot is allocated to one user. The high data rate makes the user unaware of the lack of simultaneity. The digitized voice appears as individual serial bytes that occur at a kHz rate, and 24 of these bytes are interleaved, producing one T1 frame of data.
The frame occurs at a 1. A single synchronizing bit is added for timing purposes for an overall data rate of 1.
At the receiving end, the individual voice bytes are recovered at the kHz rate and passed through a digital-to-analog converter DAC that reproduces the analog voice. It divides up the radio spectrum into kHz bands and then uses time division techniques to put eight voice calls into one channel.
The eight time slots can be voice signals or data such as texts or e-mails. This GSM digital cellular method shows how up to eight users can share a kHz channel in different time slots within a frame of bits.
It is also known as spread spectrum because it takes the digitized version of an analog signal and spreads it out over a wider bandwidth at a lower power level. The digitized and compressed voice signal in serial data form is spread by processing it in an XOR circuit along with a chipping signal at a much higher frequency.
In the cdma IS standard, a 1. Spread spectrum is the technique of CDMA. The compressed and digitized voice signal is processed in an XOR logic circuit along with a higher-frequency coded chipping signal.
The result is that the digital voice is spread over a much wider bandwidth that can be shared with other users using different codes. The chipping signal is derived from a pseudorandom code generator that assigns a unique code to each user of the channel. This code spreads the voice signal over a bandwidth of 1.
The resulting signal is at a low power level and appears more like noise. 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. 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[ edit ] Asynchronous CDMA offers a key advantage in the flexible allocation of resources i. 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. 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[ edit ] 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. The technology of CDMA was used in , 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. In , Kupriyanovich made the new experimental "pocket" model of mobile phone. This phone weighed 0. To serve more customers, Kupriyanovich proposed the device, which he called "correlator.
It was placed in the trunk of the vehicles of high-ranking officials and used a standard handset in the passenger compartment.
CDMA is a spread-spectrum multiple-access  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. The figure shows how a spread-spectrum signal is generated.
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. 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.
CDMA (Code-Division Multiple Access)
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. 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: 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 is represented by the vector 1, 0, 1, 1. If the dot product is zero, the two vectors are said to be orthogonal to each other. 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, 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. 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. 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. Because signal0 and signal1 are transmitted at the same time into the air, they add to produce the raw signal. 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:.It allows multiple users of the network to access the single cable for transmission. Contains many new and previously unpublished research results. Spread spectrum in digital communications. The frequencies used in different cells must be planned carefully to ensure signals from different cells do not interfere with each other.
Fundamentals of Communications Access Technologies: FDMA, TDMA, CDMA, OFDMA, AND SDMA
The number of users for which the CDMA fluctuates due to the fact that the reverse link which the blocking probability equals a certain quality of service capacity depends on is interference-limited [Viterbi and value is defined as Erlang capacity of the system and is Viterbi, ]. This is achieved by modelling telephone traffic using statistical approach to generate a CDMA blocking probability that is adapted into Erlang B formula for capacity calculations and matlab is used to model the blocking probability formula.
All forms of CDMA use spread-spectrum process gain to allow receivers to partially discriminate against unwanted signals.
But with digital techniques, multiple TV channels may share a single band today thanks to compression and multiplexing techniques used in each channel. Artech characterized using statistical means.