1. Introduction to CDMA: Code Division Multiple Access (CDMA) is a digital cellular technology that allows multiple users to share the same frequency band simultaneously. It achieves this by assigning a unique code to each user, enabling their signals to coexist and be separated at the receiver end.
2. Basic Principle: The fundamental principle of CDMA is based on spreading the user’s signal over a wider bandwidth using a unique code. Each user’s signal is multiplied by a code, which allows multiple signals to occupy the same frequency band simultaneously. The receiver then uses the corresponding code to separate and recover the desired signal.
3. Spreading and Multiplexing: CDMA uses a process called spreading to encode each user’s signal. Spreading involves multiplying the original signal with a unique code that has a wider bandwidth than the original signal. This spreading process spreads the signal across a larger frequency range, allowing multiple signals to be transmitted concurrently.
4. Orthogonal Codes: CDMA utilizes orthogonal codes that are mathematically designed to be mutually orthogonal, meaning they are perfectly uncorrelated with each other. Orthogonal codes have the property that their cross-correlation is zero, enabling the receiver to isolate and recover the intended signal while rejecting interference from other users’ signals.
5. Signal Separation at the Receiver: At the receiver end, the received signal, which consists of multiple overlapping signals, is multiplied by the corresponding code of the desired user. The multiplication process effectively despreads the desired user’s signal, while the signals from other users appear as noise. The receiver then filters out the noise and extracts the desired user’s signal.
6. Soft Handoff: CDMA allows for soft handoff, which is a seamless transition as a mobile device moves between different cells. During handoff, the mobile device is connected to multiple base stations simultaneously, and the signals from all connected base stations are combined at the receiver end. This results in improved call quality and increased system capacity.
7. Power Control: CDMA employs power control techniques to mitigate the near-far problem, where users located at different distances from the base station transmit signals with varying power levels. Power control adjusts the transmission power of each user dynamically, ensuring that all users maintain a consistent signal strength at the base station, improving system capacity and reducing interference.
8. Capacity and Spectral Efficiency: CDMA offers high capacity and spectral efficiency compared to other multiple access technologies. By allowing multiple users to share the same frequency band simultaneously, CDMA maximizes the utilization of the available spectrum. It achieves higher capacity by exploiting the orthogonality of the codes and using advanced signal processing techniques for interference rejection.
9. Improved Signal Quality: CDMA provides better signal quality and resistance to fading and interference. The spreading and despreading process in CDMA helps to combat multipath fading, where signals arrive at the receiver via multiple paths, resulting in interference. CDMA can recover signals even in the presence of multipath propagation, resulting in improved call quality.
10. Security and Privacy: CDMA offers inherent security and privacy features. The use of unique codes for each user makes it difficult for unauthorized users to intercept or decode the signals. Additionally, CDMA signals appear as noise-like signals to users without the corresponding codes, providing a level of privacy.
11. Applications of CDMA:

• Cellular Networks: CDMA is widely used in cellular networks, particularly in the 3G (Third Generation) and 4G (Fourth Generation) systems. It provides efficient spectrum utilization and high capacity for voice and data services.
• Satellite Communication: CDMA is utilized in satellite communication systems, allowing multiple users to share the limited frequency resources efficiently. It enables global coverage and supports various applications such as voice, data, and video transmission.
• Wireless Local Area Networks (WLANs): CDMA is employed in WLANs, particularly in the IEEE 802.11 family of standards. It enables multiple users to access the WLAN simultaneously, providing improved capacity and performance.
• Personal Communication Systems (PCS): CDMA is used in PCS networks, providing wireless communication services for personal devices such as smartphones, tablets, and laptops. It enables high-quality voice calls and high-speed data transmission.

In summary, Code Division Multiple Access (CDMA) is a digital cellular technology that allows multiple users to share the same frequency band simultaneously by using unique codes to spread and separate signals at the receiver end. CDMA offers benefits such as increased capacity, improved signal quality, security, and privacy. It finds applications in cellular networks, satellite communication, WLANs, and PCS. CDMA’s efficient spectrum utilization and robustness against interference make it a widely adopted multiple access technology in various wireless communication systems.

Code Division Multiple Access (CDMA)

CDMA uses the principle of spread spectrum. Due to this, the various signals are modulated after which a single signal is transmitted and is correlated at the receiving end using the spreading function to get the actual data.

CDMA actually came into existence in the 1940s. But after technological advancements, it has been used for military purposes. As transmitted signal appears as noise to unauthorized user thus decoding is quite difficult without the knowledge of the right spreading code.

Spread spectrum is a method that offers a way to transmit a signal over a communication channel with a bandwidth that is comparatively wider than the bandwidth which is actually needed to transmit the signal. More simply, in this technique, the bandwidth of the signal is increased intentionally in the frequency domain during transmission.

This raises the robustness of the transmitted signal. The reason for this is that by increasing the bandwidth component, interference level can be reduced because for wider bandwidth interference is comparatively lower than for smaller bandwidth. So, spread spectrum allows conversion of baseband signal into a modulated signal of wider bandwidth.

CDMA is sometimes referred to as spread spectrum multiple access (SSMA) because the insertion of spreading code increases the transmission bandwidth.

Introduction to Code Division Multiple Access

The code division multiple access is a type of multiple access technique that offers resource sharing. Before elaborating on CDMA, it is important to know –

What is Multiple Access?

Whenever we talk about any satellite-based system, then it is obvious that multiple users form connections with a single satellite transponder for the purpose of communication on a daily basis. Hence, this requires that the available resources must be shared by each of them without hampering each other’s data. So, to achieve this multiple access technique is used.

This content is all about code division multiple access, in which, unlike TDMA and FDMA, neither time nor frequency slots, respectively are required to be divided amongst the various users. As by assigning separate unique codes to each user, the data is combinedly transmitted over the channel, and at the other end, the receiving stations use the respective codes that were used by the transmitting stations to retrieve the actual message signal of each station.

Hence, in CDMA, with the help of unique spreading codes, multiple users simultaneously access the RF bandwidth for signal transmission.

It is to be noted here that, in this approach, the codes to be used are selected in a way that there must be the least possible correlation between them.

There are two ways in which CDMA is implemented, namely,

• Direct Sequence/ Sequencing
• Frequency Hopping

These two are different in a way that direct sequence spread spectrum makes use of high-speed spreading code to have wider bandwidth while in frequency hopping technique carrier frequency is shifted to obtain the same.

Direct Sequencing CDMA

The principle of operation of DS-CDMA is such that two or even more signals of the same bandwidth, get individually spread by a user-specific orthogonal code. Over the communication channel, the signals are mixed and sent combinedly. Here the energy for transmission will remain the same however, bandwidth requirement will be more. At the receiving end, de-spreading of signals is done using the replica of the orthogonal code.

The figure below shows the block diagram representation of DS-CDMA for transmission and reception of the signal:

Initially, for transmission, the pseudorandom code generator generates a unique spreading code. This random code sequence is multiplied by the input data stream that the user actually wants to transmit. For each individual user, willing to transmit over that channel, a specific PN code will be generated for spreading the bit sequence.

Further, the modulator (PSK type) performs the modulation of obtained bit sequence with a carrier signal thereby providing a BPSK modulated signal as output. This signal is then broadcasted using an antenna.

The waveform representation is shown below clearly represents the generation of BPSK modulated signal:

Here, the product will be 1 only when both data and code bit will be the same i.e., either 0 or 1 otherwise product will be 0.

At the time of signal reception, first, the obtained signal is amplified to raise its level. Then demodulation of the signal is performed using radio-frequency carrier. Here the obtained sequence appears nothing more than noise. However, the replica of code produced by the pseudo code generator is multiplied with the data stream obtained after demodulation.

To get the original data stream (i.e., for de-spreading) the PN code generated at the receiver end must be same as that produced as the transmitting end for each individual user. After multiplication with the de-spreading code, original data is retrieved by the receiving station.

Suppose a₁(t) data is multiplied with c₁(t) code to produce b₁(t) = a₁(t)c₁(t) as output. Similarly, a₂(t) and code c₂(t) gives b₂(t) = a₂(t)c₂(t) as output. The received signal will be:

R(t) = Σ b₁(t) + b₂(t)

So, to retrieve actual information, data is to be multiplied with the respective code of each user. In this way using direct spreading, signals can be transmitted over a channel at the same time.

Frequency Hopping CDMA

The word hopping corresponds to jumping or switching. The frequency hopping type of CDMA technique is based on accessing different frequency slots of a complete channel bandwidth at different instants of time by multiple users. It is not FDMA or TDMA because in that case a fixed frequency or time slot respectively is provided to different users. However, here each specific user can access to same frequency slot in different time instants.

Consider the graphical representation shown below-representing slotting of available frequency band and various users are accessing the bands in different time slots:

Here, in time slot t₄, the signal is transmitting in band f₅, likewise in t₂, band f₄ is transmitting, and so on. Also, in slot t₃, band f₄ is again transmitting. Thus, the frequency and time slots are showing variable nature. The time interval between two slots is called chirp duration or hopping interval and is denoted by T_c.

To understand how this happens, consider the block diagram shown below:

Here frequency synthesizer is used to change the carrier frequency. The message signal is first fed to the modulator, according to a pre-determined sequence, the carrier frequency is generated that hops in steps. Basically, the overall satellite bandwidth is divided into series of frequency slots with which the encoded carrier sequentially hops here. This hopping sequence must not be easily predictable like the PN code of DS-CDMA. Externally, it appears that a single signal is occupying the complete bandwidth but in actuality, it is occupying a single slot at a time.

At the receiving end, a similar frequency synthesizer must be present for generating a replica of carrier frequency, which is mixed with the received signal, and the output is filtered for noise removal. This provides a fixed intermediate frequency signal which is demodulated to get the actual message signal.

Example for DS-CDMA

Now, consider an example to understand signal transmission and reception through CDMA.

Suppose there are four stations M, N, O, and P individually transmitting 1, 0, 1, 1. And each one is having a unique code sequence (C₁, C₂, C₃, C₄) where the codes are of orthogonal nature.

To represent data bits and code bits we will use polar signaling thus,

• Binary 0 will be represented as -1 and
• Binary 1 will be represented as +1 (or 1)

Thus, data vector i.e., (M, N, O, P) will be(1, -1, 1, 1).

Parameter for choosing codes:

• The sum of resultant bits obtained from the multiplication of codes of any two stations must be 0.

It is to be noted here that always while finding the product of two data sequences, 1^st bit of one sequence is multiplied with the first bit of another sequence. Likewise, 2^nd bit with 2^nd bit and so on.

Suppose here, C₁*C₄ = (1, 1, -1, -1).(1, -1, 1, -1) = (1,-1, -1, 1)

On addition of all 4 bits of resultant, we will get 0. Thus, codes are of orthogonal nature.

• The sum of resultant obtained when a code sequence is multiplied with itself must indicate the total number of stations transmitting.

Suppose, C₂*C₂ = (1, -1, -1, 1). (1, -1, -1, 1) = (1, 1, 1, 1)

So, 1+1+1+1 will give 4 as output. Hence, verifying that there are 4 stations transmitting at a time.

Transmission: We have discussed previously that, to perform DS-CDMA, first, data bits are to be multiplied separately with their respective code.

Hence, the resultant of product of data bit and code bit will be:

Now, over the channel, the bits will be transmitted combinedly.

The complete bit sequence to be transmitted will be produced by adding the bits according to their positional sequence:

The sequence transmitted over the channel will be: 2, 2, 2, -2.

Reception: The receiver will get the above sequence. Now, to retrieve the actual information from this received (coded form) data, each receiving station must have the code sequence of their respective transmitting station.

Here each receiver will get the original data sequence by multiplying the received bit sequence with its respective code stream.

R₁  = (2, 2, -2, 2)

R₂ = (2, -2, -2, -2)

R₃ = (2, 2, 2, -2)

R₄ = (2, -2, 2, 2)

Hence, by summing every bit of the sequence and dividing it will the total number of transmitting stations one can get the originally transmitted data bit. So,

calculating for each receiving station, we will get:

R₁ = [2 + 2 + (-2) + 2]/Number of stations = 4/4 = 1

R₂ = [2 + (-2) + (-2) + (-2)]/Number of stations = -4/4 = -1

R₃ = [2 + 2 + 2 + (-2)]/Number of stations = 4/4 = 1

R₄ = [2 + (-2) + 2 + 2]/Number of stations = 4/4 = 1

According to polar signalling 1 denotes binary 1 and -1 denotes binary 0. Therefore, the data bits received at each receiving station will be 1, 0, 1, 1.

It can be clearly checked that the received bits are exactly the same as the one which was transmitted from the transmitting stations. Hence, in this way CDMA can be implemented.

Advantages

1. It offers a considerable increase in user capacity than TDMA and FDMA.
2. Low cost than GSM.
3. Highly secured way of information transmission.
4. Quite beneficial for voice and data communication.
5. The chances of interference are low.
6. It offers the use of complete channel spectrum.

Disadvantages

1. The system operation is complicated.
2. The original data can only be recovered when orthogonal codes are used in a synchronous manner.
3. The loss of orthogonality between codes leads to self-jamming.

Applications of CDMA

The high capacity and security offered by CDMA make it worthy for use in the field of wireless technology such as mobile telephony, radar, and navigation systems. The combined use of CDMA with GSM technology has provided high-speed internet services like 4G.