FACH (Forward Access Channel)

Last updated on Apr 4, 2023

The Forward Access Channel (FACH) is a wireless communication channel used in Universal Mobile Telecommunications System (UMTS) networks. It is a dedicated uplink channel, meaning that it is used by mobile devices to send data to the network, such as voice calls, video calls, text messages, and internet data.

In this article, we will discuss the technical aspects of FACH, its features, functions, and uses, and how it differs from other uplink channels used in UMTS networks.

Technical Aspects of FACH:

The FACH is a Time Division Duplex (TDD) channel, which means that it operates in the same frequency band as the downlink channel but uses a different time slot to transmit data. This is in contrast to Frequency Division Duplex (FDD) channels, which use separate frequency bands for the uplink and downlink channels.

The FACH operates at a fixed data rate of 384 kbps and uses Quadrature Phase Shift Keying (QPSK) modulation. The channel bandwidth is 5 MHz, which is divided into time slots of 10 milliseconds each. Each time slot is further divided into two parts, a transmission part and a reception part.

The FACH uses a Transmission Time Interval (TTI) of 10 milliseconds, which is the time interval between the start of two consecutive FACH transmissions. During this time interval, the mobile device sends data to the network using the uplink channel.

Features of FACH:

The FACH has several features that make it a reliable and efficient uplink channel for UMTS networks. These features include:

a. Constant Data Rate: The FACH operates at a fixed data rate of 384 kbps, which provides a consistent and reliable data transmission speed. This is important for applications that require a continuous data stream, such as video calls and live streaming.

b. Low Latency: The FACH has a low latency, which is the time delay between the mobile device sending data and the network receiving it. This is important for applications that require real-time communication, such as voice calls and online gaming.

c. Efficient Power Usage: The FACH uses power efficiently, which means that it consumes less battery power than other uplink channels. This is important for mobile devices, which have limited battery life.

d. Low Interference: The FACH has low interference with other channels, which reduces the risk of data transmission errors and dropped calls.

Functions of FACH:

The FACH has several functions in a UMTS network. These functions include:

a. Initial Access: The FACH is used by mobile devices to initiate communication with the network. When a mobile device is turned on or enters a new cell, it sends a synchronization signal on the FACH to synchronize with the network.

b. Idle Mode Signaling: The FACH is used by the mobile device to send signaling messages to the network when it is in idle mode. This includes location updates, paging requests, and cell updates.

c. Dedicated Signaling: The FACH is used by the network to send dedicated signaling messages to the mobile device. This includes call setup and teardown messages, handover commands, and resource allocation messages.

d. User Data: The FACH is used to transmit user data, such as voice calls, video calls, text messages, and internet data. This data is transmitted using Packet Switched (PS) or Circuit Switched (CS) mode, depending on the type of data being transmitted.

Uses of FACH:

The FACH is used in several applications in UMTS networks. These applications include:

a. Voice Calls: The FACH is used to transmit voice calls between mobile devices and the network. This is done using Circuit Switched (CS) mode, where a dedicated channel is allocated to the voice call for the duration of the call.

b. Video Calls: The FACH is used to transmit video calls between mobile devices and the network. This is done using Packet Switched (PS) mode, where the video data is transmitted in packets over the network.

c. Text Messages: The FACH is used to transmit text messages between mobile devices and the network. This is done using PS mode, where the text message is transmitted in packets over the network.

d. Internet Data: The FACH is used to transmit internet data between mobile devices and the network. This is done using PS mode, where the internet data is transmitted in packets over the network.

Differences between FACH and other uplink channels:

There are several uplink channels used in UMTS networks, including the Dedicated Channel (DCH), the Random Access Channel (RACH), and the Enhanced Dedicated Channel (E-DCH). Each of these channels has its own features, functions, and uses that make them suitable for different types of communication.

a. DCH: The DCH is a high-speed uplink channel that is used for real-time applications, such as video calls and online gaming. It operates at a variable data rate, which means that the data rate can be adjusted based on the requirements of the application. Unlike the FACH, the DCH is not a dedicated channel, meaning that it can be used by multiple mobile devices at the same time.

b. RACH: The RACH is a low-speed uplink channel that is used for initial access and random access to the network. It operates at a fixed data rate of 2 kbps and uses Slotted Aloha multiple access, which means that multiple mobile devices can use the channel at the same time. Unlike the FACH, the RACH does not require synchronization with the network before use.

c. E-DCH: The E-DCH is an enhanced uplink channel that is used for high-speed data transmission, such as file downloads and uploads. It operates at a variable data rate and uses a more advanced modulation scheme than the FACH, which allows for higher data rates. Unlike the FACH, the E-DCH is a dedicated channel, meaning that it can only be used by one mobile device at a time.

Conclusion:

The Forward Access Channel (FACH) is a dedicated uplink channel used in UMTS networks for communication between mobile devices and the network. It operates at a fixed data rate of 384 kbps and uses Quadrature Phase Shift Keying (QPSK) modulation. The FACH has several features that make it a reliable and efficient uplink channel, including constant data rate, low latency, efficient power usage, and low interference. The FACH has several functions in a UMTS network, including initial access, idle mode signaling, dedicated signaling, and user data transmission. The FACH is used in several applications, including voice calls, video calls, text messages, and internet data transmission. The FACH differs from other uplink channels, such as the Dedicated Channel (DCH), the Random Access Channel (RACH), and the Enhanced Dedicated Channel (E-DCH), in terms of their features, functions, and uses.