Spread Spectrum Multiple Access

Introduction

Spread Spectrum Multiple Access (SSMA) is a technique used in mobile communication systems to allow multiple users to share the same frequency band simultaneously. It provides increased capacity, improved security, and resistance to interference. This topic will cover the fundamentals of SSMA and discuss two popular SSMA techniques: Frequency Hopped Multiple Access (FHMA) and Code Division Multiple Access (CDMA).

Importance of Spread Spectrum Multiple Access

SSMA plays a crucial role in mobile communication systems due to the following reasons:

• Increased Capacity: SSMA allows multiple users to share the same frequency band, significantly increasing the capacity of the system.
• Improved Security: SSMA spreads the signal over a wide frequency band, making it difficult for unauthorized users to intercept or jam the communication.
• Resistance to Interference: SSMA spreads the signal energy over a wide frequency band, reducing the impact of narrowband interference.

Fundamentals of Spread Spectrum Multiple Access

SSMA is based on the concept of spreading the signal over a wider bandwidth than the minimum required for transmission. This spreading is achieved by using specific spreading codes that are unique to each user. These spreading codes allow multiple users to transmit and receive signals simultaneously without interfering with each other.

Different Spreading Codes

There are two main types of spreading codes used in SSMA: Frequency Hopped Multiple Access (FHMA) and Code Division Multiple Access (CDMA).

Frequency Hopped Multiple Access (FHMA)

FHMA is a spread spectrum technique where the carrier frequency is rapidly changed during transmission. This hopping is done according to a predefined hopping sequence known to both the transmitter and receiver.

How FHMA Works

In FHMA, the data signal is multiplied with a hopping sequence that determines the carrier frequency at each time interval. The hopping sequence is typically a pseudo-random sequence generated by a hopping code generator. The receiver uses the same hopping sequence to synchronize with the transmitter and demodulate the received signal.

Advantages and Disadvantages of FHMA

Advantages of FHMA:

• Improved resistance to interference: The frequency hopping nature of FHMA makes it more resilient to narrowband interference.
• Enhanced security: The hopping sequence adds an additional layer of security, as unauthorized users would need to know the hopping pattern to intercept the communication.

Disadvantages of FHMA:

• Lower capacity: FHMA requires a larger bandwidth to accommodate the hopping sequence, reducing the overall capacity of the system.
• Synchronization challenges: The transmitter and receiver need to be synchronized to the same hopping sequence, which can be challenging in certain scenarios.

Code Division Multiple Access (CDMA)

CDMA is a spread spectrum technique where each user is assigned a unique spreading code. The spreading codes are orthogonal to each other, meaning they have minimal interference between them.

How CDMA Works

In CDMA, the data signal is multiplied with the spreading code assigned to the user. The resulting spread spectrum signal is then transmitted over the shared frequency band. At the receiver, the received signal is despread using the same spreading code to recover the original data signal.

Advantages and Disadvantages of CDMA

Advantages of CDMA:

• High capacity: CDMA allows multiple users to share the same frequency band simultaneously, significantly increasing the capacity of the system.
• Robustness to interference: CDMA spreads the signal energy over a wide frequency band, reducing the impact of narrowband interference.

Disadvantages of CDMA:

• Complexity: CDMA requires sophisticated signal processing techniques to separate the signals from different users.
• Near-far problem: CDMA systems can experience the near-far problem, where a strong signal from one user can interfere with the reception of weaker signals from other users.

CDMA Digital Cellular System

The CDMA digital cellular system is a widely used mobile communication system that employs CDMA as the SSMA technique. It consists of two main channels: the forward channel and the reverse channel.

Overview of CDMA Digital Cellular System

The CDMA digital cellular system allows multiple users to communicate simultaneously by assigning unique spreading codes to each user. It provides high capacity, improved call quality, and efficient spectrum utilization.

Forward Channel

The forward channel in CDMA is responsible for transmitting signals from the base station to the mobile devices. It consists of several components, including the pilot channel, synchronization channel, paging channel, and traffic channels.

Detailed Description of Forward Channel in CDMA

• Pilot Channel: The pilot channel is a reference signal transmitted by the base station. It helps mobile devices synchronize with the base station and estimate the signal strength.
• Synchronization Channel: The synchronization channel provides timing and synchronization information to the mobile devices.
• Paging Channel: The paging channel is used by the base station to broadcast messages to specific mobile devices.
• Traffic Channels: The traffic channels carry the actual voice or data traffic between the base station and the mobile devices.

Different Standards Used in Forward Channel

There are several standards used in the forward channel of CDMA systems, including IS-95, cdma2000, and WCDMA.

Real-World Applications and Examples of Forward Channel

The forward channel is used in various real-world applications, such as mobile phone networks, wireless internet access, and satellite communication systems.

Reverse Channel

The reverse channel in CDMA is responsible for transmitting signals from the mobile devices to the base station. It also consists of several components, including the access channel and the traffic channels.

Detailed Description of Reverse Channel in CDMA

• Access Channel: The access channel is used by mobile devices to request access to the network and initiate calls.
• Traffic Channels: The traffic channels carry the actual voice or data traffic from the mobile devices to the base station.

Different Standards Used in Reverse Channel

Similar to the forward channel, the reverse channel also has different standards, including IS-95, cdma2000, and WCDMA.

Real-World Applications and Examples of Reverse Channel

The reverse channel is used in various real-world applications, such as mobile phone networks, wireless internet access, and satellite communication systems.

Capacity of Cellular Systems

Capacity refers to the maximum number of users that can be supported by a cellular system. In CDMA systems, capacity is determined by several factors.

Definition and Explanation of Capacity in Cellular Systems

Capacity in cellular systems is the maximum number of users that can be supported while maintaining a certain level of service quality. It is influenced by factors such as available bandwidth, signal-to-noise ratio, and interference levels.

Factors Affecting Capacity

Several factors affect the capacity of CDMA systems:

• Bandwidth: The available bandwidth determines the number of users that can be accommodated in the system.
• Signal-to-Noise Ratio (SNR): A higher SNR allows for more efficient use of the available bandwidth, increasing the system capacity.
• Interference Levels: Higher interference levels reduce the capacity of the system as they degrade the quality of the received signals.

Calculation of Capacity in CDMA Systems

The capacity of CDMA systems can be calculated using various formulas and models, taking into account the available bandwidth, SNR, and interference levels.

Advantages and Disadvantages of CDMA Capacity

Advantages of CDMA capacity:

• High capacity: CDMA systems can support a large number of users simultaneously, making them suitable for densely populated areas.
• Efficient spectrum utilization: CDMA allows for efficient use of the available spectrum by sharing it among multiple users.

Disadvantages of CDMA capacity:

• Near-far problem: CDMA systems can experience the near-far problem, where a strong signal from one user can interfere with the reception of weaker signals from other users.
• Complexity: CDMA systems require complex signal processing algorithms to separate the signals from different users, increasing the system complexity.

Conclusion

Spread Spectrum Multiple Access (SSMA) is a fundamental technique in mobile communication systems that allows multiple users to share the same frequency band simultaneously. Two popular SSMA techniques, Frequency Hopped Multiple Access (FHMA) and Code Division Multiple Access (CDMA), were discussed in this topic. CDMA, in particular, is widely used in digital cellular systems due to its high capacity, improved security, and resistance to interference.

In summary, SSMA provides increased capacity, improved security, and resistance to interference in mobile communication systems. FHMA and CDMA are two popular SSMA techniques, each with its own advantages and disadvantages. The CDMA digital cellular system utilizes CDMA as the SSMA technique and consists of the forward channel and the reverse channel. Capacity in cellular systems is influenced by factors such as available bandwidth, SNR, and interference levels. CDMA systems offer high capacity and efficient spectrum utilization, but they also face challenges such as the near-far problem and system complexity.

Summary

Spread Spectrum Multiple Access (SSMA) is a technique used in mobile communication systems to allow multiple users to share the same frequency band simultaneously. It provides increased capacity, improved security, and resistance to interference. There are two main types of spreading codes used in SSMA: Frequency Hopped Multiple Access (FHMA) and Code Division Multiple Access (CDMA). FHMA is a spread spectrum technique where the carrier frequency is rapidly changed during transmission. CDMA is a spread spectrum technique where each user is assigned a unique spreading code. The CDMA digital cellular system is a widely used mobile communication system that employs CDMA as the SSMA technique. It consists of the forward channel and the reverse channel. Capacity in cellular systems is influenced by factors such as available bandwidth, SNR, and interference levels. CDMA systems offer high capacity and efficient spectrum utilization, but they also face challenges such as the near-far problem and system complexity.

Analogy

Imagine a crowded room where people are having conversations. Spread Spectrum Multiple Access (SSMA) is like a technique that allows multiple conversations to happen simultaneously without interfering with each other. It's like using different languages or secret codes to communicate, so each conversation remains private and secure. Frequency Hopped Multiple Access (FHMA) is like changing the language or secret code rapidly during the conversation, making it difficult for others to understand. Code Division Multiple Access (CDMA) is like assigning a unique secret code to each person, so they can communicate without interference, even if they are speaking at the same time. The CDMA digital cellular system is like a network of interconnected rooms, where people can move freely and have conversations using their assigned secret codes. Capacity in cellular systems is like the maximum number of people that can have conversations in each room without causing overcrowding or degradation of the conversation quality.