Difference between FDMA, TDMA and CDMA

In this tutorial, you will learn the basic concepts of channelization protocols such as FDMA, TDMA, and CDMA. After reading this tutorial, you will be able to describe the working principles, advantages, and disadvantages of these channelization protocols, as well as the differences between FDMA, TDMA, and CDMA.

Contents:

1. What is Channelization?
2. Frequency-Division Multiple Access (FDMA)
3. Frequency Division in FDMA Protocol
4. Time-Division Multiple Access (TDMA)
5. Time-Division in TDMA Protocol
6. Code-Division Multiple Access (CDMA)
7. Chips in CDMA
8. Encoding, Decoding, and Signal Level in CDMA
9. Difference between FDMA, TDMA and CDMA

What is Channelization?

Channelization is a multiple-access method used to divide the bandwidth of a channel between devices so that signals avoid collisions with each other.

• The bandwidth of the channel is shared between different stations through time, frequency or code.
• Multiple devices are communicating on a channel and transmitting signals simultaneously over a wire of a channel. But still, they can communicate without interference as all signals are transmitted precisely on the same channel.
• There are many channels available on a network, and all channels transmit different types of data.
• There are mainly three types of channelization protocols used on the network.
1. Frequency-Division Multiple Access (FDMA)
2. Time-Division Multiple Access (TDMA)
3. Code-Division Multiple Access (CDMA)

Frequency-Division Multiple Access (FDMA)

In FDMA, the channel’s available bandwidth is divided into the frequency bands among the stations. Here, each band of frequency is reserved for a specific station, and it is permanent.

• The station communicates in its own band at all times. It cannot interfere with another station’s band.
• To avoid collisions on the channel, the station’s frequency band is separated from the other device’s frequency bands by a small guard band.
• The station transmits data in the form of an analog signal, and it produces sine waves. Communication takes place only in fixed frequency bands.
• The physical layer takes the responsibility of converting the data received from the data link layer into a band-pass (frequency range) signal. No multiplexers are used to physically create the signal. Signals created at each station are band-pass filtered automatically.
• Band-pass signals are generated by the physical layer when they are traveling on a common channel.

Frequency Division in FDMA Protocol

As we learned that bandwidth is divided into frequency bands that are separated by guard bands among devices. Now, let’s understand frequency division.

The diagram below explains the frequency division.

• As shown in the figure, there are four stations, and each station gets a frequency band for communication.
• Station-1, Station-2, Station-3, and Station-4 are assigned the f-1, f-2, f-3, and f-4 frequency bands, respectively.
• Each station is sending data on its own frequency band. To prevent interference of signals, all frequency bands are separated by guard bands.
• A well-known example of FDMA is FM radio, on which a separate bandwidth is allocated to each station.
• 91.1 MHz, 92.7 MHz, 94.3 MHz, etc., are radio stations with different frequency bands separated by guard bands.

Time-Division Multiple Access (TDMA)

In time-division multiple access, the channel bandwidth is shared between stations through time. Bandwidth is divided into time slots, meaning that each station is allocated a time slot for sending data.

• In TDMA, the allotted frequency band is not permanent for the station. When a station completes communication at a given time, the channel removes the frequency band and re-allocates it to another station.
• Here, the main problem with TDMA is synchronization because the station needs to know the beginning of the slot and the location of its slot.
• If the stations are spread over a large area, the synchronization problem can be difficult. To overcome this problem, we can add guard time.
• So, synchronization will be achieved by having preamble bits at the beginning of each slot.
• The data link layer of the station communicates with the physical layer and asks it to use the allotted time slot.
• In TDMA, a channel can carry data from 64 Kbps to 120 Mbps. The Global System for Mobile Communications (GSM network) is an example of a TDMA method.

Time-Division in TDMA Protocol

When the frequency is divided between devices on the channel for a specific amount of time, each device adds a bit in the data for synchronization purposes. Sender and receiver devices can synchronize their clocks at regular times with the help of a synchronization bit. The bandwidth of one channel is divided in such a way that it cannot interfere with the signal of another station.

The below diagram explains the time division in TDMA protocol.

• As shown in the figure, there are four stations on a channel.
• The channel’s bandwidth is divided into four stations, and each station gets a particular time slot in which it can communicate.
• As you can see in the figure, TS-1, TS-2, TS-3, and TS-4 are assigned to Station-1, Station-2, Station-3, and Station-4, respectively.
• Here, when a particular station is transmitting on one channel, the other station also gets to know which station is transmitting data at a particular time slot.
• We can say that in TDMA, bandwidth is just one channel that is time-shared between different stations.

Code-Division Multiple Access (CDMA)

In CDMA, the entire bandwidth of the link is occupied by one channel, and all stations can send data simultaneously because there is no timesharing. The channel shared between devices can easily allow communication between multiple pairs but in different languages such as codes.

The below diagram explains the CDMA method.

• There are four stations 1, 2, 3, 4 as shown in the figure. Station-1, Station-2, Station-3 and Station-4 have d1, d2, d3, and d4 data, and codes c1, c2, c3 and c4, respectively.
• Here, we have assumed that if we multiply each code by the other, we get 0, and if we multiply each code by itself, we get 4, which is the number of stations.
• Now, station-1 multiplies its data d1 by the code c1, which makes d1*c1. Similarly, station-2, station-3 and station-4 form d2*c2, d3*c3, and d4*c4 respectively.
• So, the sum of all the station’s data will be broadcast on a single channel.
• Suppose, Station-1 and Station-2 are communicating, and Station-2 wants to hear what Station-1 is saying. It multiplies the data by the code c1 of station-1 on the channel. Then, station-2 will divide the result by 4 to get the data from station-1.

Chips in CDMA

As we learned that CDMA is a coding-based principle in which a code is assigned to each station. Codes are a sequence of numbers, known as chips.

The diagram below explains the chip sequence.

As shown in the figure, each code consists of a sequence of numbers known as chips. In addition, the chips are not chosen at random. Chip sequences are called orthogonal sequences and are generated by the Walsh table.

The chips have the following properties:

• In each code, the sequence is made up of N (number of stations) elements.
• If we multiply a sequence by a scalar, each element is multiplied by that number.
• Example: 2*[+1 -1 -1 +1] = [+2 -2 -2 +2]
• If we multiply two identical sequence elements element by element, we get the total number of stations present on the channel.
• Example: [+1 +1 -1 -1]*[+1 +1 -1 -1] = 1 + 1 + 1 + 1 = 4
• If we multiply and sum two different chip sequence elements element by element, we get 0.
• Example: : [+1 +1 +1 +1]*[+1 +1 -1 -1] = 1 + 1 – 1 – 1 = 0
• If we add two different chip sequences, it creates another sequence.
• Example: : [+1 +1 -1 -1] + [+1 -1 -1 +1] = [+2 0 -2 0]

Encoding, Decoding, and Signal Level in CDMA

Encoding, decoding, and signal level are important terms in CDMA. Using them, we can represent the data sent by the station.

Encoding and Decoding: The encoding method describes the rules for encoding data. If the station sends a bit 0 and 1, the method encodes it as -1 and +1, respectively. When a channel is idle, it does not send any signal displayed as 0.

The figure below explains the encoding and decoding method in CDMA.

• As shown in the figure, there are four stations, Station-1, Station-2, Station-3, and Station-4, communicating on a common channel.
• Suppose station-1 and station-2 are sending 0 bits, and station-4 is sending 1 bit, but station-3 is silent. Therefore, the data 0, 0, silent, and 1 will be encoded as -1, -1, 0, and +1, respectively, according to the encoding rule.
• After that, each station multiplies the number corresponding to its chip, which is unique to a station. The sequence [-1 -1 -3 +1] generated on a channel is the sum of all four sequences.
• Now suppose station-3 is listening to station-2 and wants to know what station-2 is saying, then it will decode the data. So, it multiplies the total data on the channel by the code of station-2 and divides the result by the number of stations (N),
• [-1 -1 -3 +1] * [+1 -1 +1 -1] = -4
• -4/N = -4/4 = -1

Signal Level: We can understand the process better if we show the digital signal that was produced by each station and the data received at the destination.

The figure below shows the digital signal produced by each station in CDMA.

As shown in the figure above, each station has a different chip sequence and accordingly generates a digital signal on a channel. It also shows that the sum of all the station’s chip sequences is data on a single channel.

Difference between FDMA, TDMA and CDMA

Here is a comparison of FDMA, TDMA, and CDMA.