Channelization Protocols

FDMA (Frequency Division Multiple Access)

In this technique, the bandwidth is divided into frequency bands, and each frequency band is allocated to a particular station to transmit its data. The frequency band distributed to the stations becomes reserved. Each station uses a band-pass filter to confine their data transmission into their assigned frequency band. Each frequency band has some gap in-between to prevent interference of multiple bands, and these are called guard bands.

Merits and Limitations of FDMA Protocols

The merits of FDMA Protocols are:

• Simplicity: FDMA is relatively simple to implement compared to some other multiple access techniques like CDMA (Code Division Multiple Access) or TDMA (Time Division Multiple Access). It involves straightforward frequency allocation without requiring complex synchronization mechanisms.
• Predictable Performance: Since each user is allocated a dedicated frequency band, FDMA provides predictable performance for individual users. This predictability can be advantageous for applications requiring guaranteed bandwidth or quality of service (QoS).
• Efficient Use of Frequency Spectrum: FDMA efficiently utilizes the available frequency spectrum by dividing it into non-overlapping frequency bands. This helps in reducing interference between users and maximizing the capacity of the system.
• Suitable for Continuous Data Streams: FDMA is well-suited for applications with continuous data streams, such as voice communication or streaming media, where a constant bandwidth allocation is required.

The limitations of FDMA Protocols are:

• Fixed Bandwidth Allocation: FDMA allocates a fixed bandwidth to each user, regardless of their actual communication needs. This can lead to inefficient use of bandwidth, especially if some users require more bandwidth at certain times while others remain idle.
• Limited Scalability: As the number of users increases, the frequency spectrum needs to be divided into smaller and smaller bands to accommodate them, which can lead to reduced spectral efficiency and increased interference.
• Vulnerable to Interference: FDMA systems are susceptible to interference from neighboring cells or users operating on adjacent frequency bands. This interference can degrade the quality of communication and reduce the overall capacity of the system.
• Inflexibility: FDMA systems may lack flexibility in adapting to changing traffic patterns or varying user requirements since bandwidth allocation is fixed.

Frequency Allocation Schemes in FDMA Protocols

Frequency allocation schemes in FDMA protocols determine how the available frequency spectrum is divided and assigned to individual users. Common frequency allocation schemes include:

• Fixed Frequency Allocation: In this scheme, each user is assigned a fixed frequency band that remains constant over time. This approach is simple and suitable for applications with predictable bandwidth requirements but may lead to inefficient spectrum utilization.
• Dynamic Frequency Allocation: Dynamic frequency allocation dynamically assigns frequency bands to users based on their communication needs and network conditions. This approach improves spectral efficiency by allocating bandwidth where it is needed most but requires more complex coordination and signaling mechanisms.
• Orthogonal Frequency Division Multiple Access (OFDMA): OFDMA is an advanced frequency allocation scheme that divides the frequency spectrum into multiple orthogonal subcarriers. These subcarriers can be dynamically allocated to users based on their bandwidth requirements, allowing for efficient spectrum utilization and improved resistance to interference.
• Fractional Frequency Reuse (FFR): FFR is a technique used in cellular networks where the available frequency spectrum is divided into multiple regions, and each region is reused in a controlled manner to minimize interference between neighboring cells. This approach enhances the overall capacity and performance of the network.

Advantages of FDMA

• FDMA system is easy to implement, and it's not very complex.
• Frequency bands ensure continuous transmission, saving the bits used for synchronization and framing.
• When the traffic is uniform, FDMA becomes very efficient due to its separate frequency band for each station.
• All stations can run simultaneously at all times without waiting for their turn.
• If the channel is not being used, then it sits idle.
• There is no restriction regarding the baseband or modulation.

Disadvantages of FDMA

• The bandwidth channel is narrow.
• The planning of the network and spectrum is very time-consuming.
• The presence of a guard band reduces the bandwidth available for use.
• Bandwidth is assigned permanently to each station which reduces its flexibility.

TDMA (Time Domain Multiple Access)

TDMA is another technique to enable multiple access in a shared medium. In this, the stations share the channel's bandwidth time-wise. Every station is allocated a fixed time to transmit its signal. The data link layer tells its physical layer to use the allotted time. TDMA requires synchronization between stations. There is a time gap between the time intervals, called guard time, which is assigned for the synchronization between stations. The rate of data in TDMA is greater than FDMA but lesser than CDMA.

Advantages of TDMA

• TDMA separates users according to time, and this ensures that there is no interference from the simultaneous transmissions
• No frequency guard band is required in TDMA
• It shares a single carrier frequency with multiple users.
• It saves power as the user is only active while transmitting in its allotted time frame.
• There is no need for precise, narrow band filters as there is no division in the frequency range.

Disadvantages of TDMA 

• If the stations are spread over a wide area, there is a propagation delay, and we use guard time to counter this.
• Slot allocation in TDMA is complex.
• Synchronization between different channels is difficult to achieve. Each station has to know the beginning of its slot and its location.
• The stations configured according to TDMA demand high peak power during uplink in their allotted time slot.

CDMA (Code Division Multiple Access)

In the CDMA technique, communication happens using codes. Using this technique, different stations can transmit their signal on the same channel using other codes. There is only one channel in CDMA that carries all the signals. CDMA is based on the coding technique, where each station is assigned a code (a sequence of numbers called chips). It differs from TDMA as all the stations can transmit simultaneously in the channel as there is no time sharing. And it differs from FDMA as only one channel occupies the whole bandwidth.

Advantages of CDMA

• CDMA operates at low power than FDMA and TDMA
• The capacity of a CDMA system is higher than FDMA and TDMA
• CDMA is cost-effective
• It provides high voice quality than TDMA and FDMA.
• It has the most outstanding spectrum efficiency.
• Hackers can't decode the communication on CDMA

Disadvantages of CDMA

• The performance of CDMA decreases with the increasing number of users.
• The cost of CDMA is higher due to the requirement of types of equipment.
• Incorrect code selection can induce delay.

Source: reseachgate.net

The figure given above summarizes all the three channelization protocols. Fig.(a) is FDMA, Fig,(b) is TDMA, and Fig.(c) is CDMA.

You can read related articles such as Congestion Control in Computer Networks here.

Space division multiple access (SDMA)

Space Division Multiple Access (SDMA) is a multiple access technique used in telecommunications where each user is allocated a unique physical space within the communication medium, typically achieved through the use of directional antennas or spatial processing techniques. In SDMA, different users occupy different spatial regions, allowing simultaneous communication without interference.

Advantages of SDMA

• Increased Capacity: SDMA allows for increased capacity by enabling multiple users to communicate simultaneously within the same frequency band without causing interference.
• Improved Signal Quality: Since users are spatially separated, the signal quality for each user can be optimized, resulting in improved communication performance.
• Enhanced Security: Spatial separation of users adds an extra layer of security as it reduces the chances of unauthorized interception of communications.
• Flexibility: SDMA is flexible and can adapt to changing network conditions and user requirements by dynamically allocating spatial resources.

Disadvantages of SDMA

• Complexity: Implementing SDMA requires sophisticated spatial processing techniques and hardware such as directional antennas, which can increase system complexity and cost.
• Limited Coverage: SDMA may face challenges in providing uniform coverage across large areas, especially in environments with obstacles or terrain variations.
• Interference Management: While SDMA reduces interference between spatially separated users, it may still face challenges in managing interference from users occupying adjacent spatial regions.

Orthogonal Frequency division multiple access (OFDMA)

Orthogonal Frequency Division Multiple Access (OFDMA) is a multiple access technique that combines the principles of Orthogonal Frequency Division Multiplexing (OFDM) with multiple access methods. In OFDMA, the available frequency spectrum is divided into multiple orthogonal subcarriers, and each subcarrier is assigned to individual users or user groups for simultaneous communication.

Advantages of OFDMA

• High Spectral Efficiency: OFDMA enables efficient utilization of the frequency spectrum by dividing it into orthogonal subcarriers, allowing multiple users to transmit simultaneously without causing interference.
• Flexibility: OFDMA is highly flexible and adaptable to various communication environments and user requirements. It can efficiently support both high data rate and low data rate applications.
• Robustness to Frequency Selective Fading: OFDMA's use of orthogonal subcarriers makes it robust against frequency selective fading, a common issue in wireless communication systems.
• Resource Allocation: OFDMA allows dynamic allocation of subcarriers to users based on their communication needs and channel conditions, optimizing resource utilization and improving overall system capacity.

Disadvantages of OFDMA

• Complex Signal Processing: Implementing OFDMA requires complex signal processing algorithms and hardware, especially in handling synchronization, subcarrier allocation, and interference management.
• PAPR (Peak-to-Average Power Ratio): OFDMA signals can suffer from high Peak-to-Average Power Ratio (PAPR), which may require additional power amplifiers and may cause nonlinear distortion in the transmitter.
• Interference from Adjacent Subcarriers: While OFDMA reduces inter-user interference, it may still face interference from adjacent subcarriers, especially in environments with severe frequency-selective fading or adjacent channel interference.

Frequently Asked Questions

What is the channel access protocol?

A channel access protocol is a set of rules governing how devices share and utilize a communication channel for transmitting data.

Channelization Protocols are implemented on which network layer?

Datalink layer

What is channelization code?

Channelization codes are generated recursively to form a binary tree structure.

Conclusion

In this article, we have learned about different types of channelization protocols. I hope you have learned something from this article. Don't stop here; learn more about computer networks, check out - 

• Controlled Access Protocol
• HDLC Protocol
• Introduction to computer networks
• Types of area network
• Routing algorithms in Computer Networks
• Transmission media in Computer Networks
• Line configuration
• Introduction to HTTP/2
• Go-Back-N ARQ Protocol
• Stop and Wait Protocol 
• Lock based protocol in DBMS
• Device Manager in Computer

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