GSM Radio Frequency (RF) Planning: BCCH, Bands, and ARFCN

GSM RF planning is crucial for the effective operation of mobile networks, ensuring optimal frequency allocation, minimizing interference, and maximizing coverage. A critical aspect of RF planning is the allocation of the Broadcast Control Channel (BCCH) frequency. This channel carries vital network information to mobile devices, enabling them to connect to the network. This guide will cover the core principles of GSM RF planning, the role of BCCH frequencies, and how planners design robust systems to meet the growing communication demands.

There are various GSM frequency bands. For our discussion of GSM RF (Radio Frequency) planning, we will focus on GSM 900’s P-GSM frequency band.

GSM Frequency Bands vs ARFCN

Here’s a table summarizing GSM frequency bands and their corresponding ARFCN (Absolute Radio Frequency Channel Number) ranges:

As shown in the table, there are approximately 124 channels in P-GSM, 174 ARFCNs in E-GSM, and 374 ARFCNs in DCS1800.

Let’s see how these ARFCNs are derived using the following formulas:

• Uplink Frequency: $F(Uplink) = 890 + 0.2 * n$, where $n$ varies from 1 to 124 (ARFCN - Absolute Radio Frequency Channel Number).

• Downlink Frequency: $F(Downlink) = F(Uplink) + 45 MHz$

The 45 MHz difference is known as the duplex spacing, representing the separation between uplink and downlink frequency channels.

What is BCCH Frequency in GSM?

In GSM, the BCCH frequency refers to the specific frequency used by the Broadcast Control Channel (BCCH). This is a crucial logical channel within the GSM network. The BCCH broadcasts essential information to all mobile devices within a cell, including details about the network, available services, and system parameters. This information helps mobile devices synchronize with the network and initiate communication.

Key Points About BCCH Frequency:

• Purpose: The BCCH transmits crucial information, such as cell identity, network codes, frequency hopping sequences, and location area information.
• Allocation: A fixed radio frequency within the GSM spectrum is dedicated to the BCCH in each cell. This ensures that mobile devices can always listen to it, even before establishing a full connection with the network.
• Non-Hopping Channel: The BCCH frequency is typically a non-hopping frequency. This means it remains constant and does not change as part of the frequency hopping scheme used for other traffic channels.
• The BCCH frequency is essential for enabling mobile devices to detect the presence of a GSM cell, select the best available cell, and begin the process of network entry, including authentication and channel assignment.

GSM Radio Frequency Planning

As the frequency spectrum is a limited resource, it must be used optimally in any cellular network, including GSM. Proper radio frequency planning is required to meet the communication demands.

First, areas are divided into cells. These cells can use omnidirectional or directional antennas to broadcast signals to mobile users. For this discussion, we’ll assume that each cell is divided into three sectors.

Each cell uses one Base Transceiver Station (BTS). If there are N BTSs allocated and each cell is allocated M channels, the total ARFCNs (T) allocated in the GSM system can be calculated as follows:

$T = 3 * M * N$

Frequency reuse can be implemented during radio frequency planning to efficiently utilize the channels (ARFCNs) in deploying the GSM network. ARFCNs (RF Frequencies) are reused in non-adjacent cells.

The frequencies used in GSM radio frequency planning are divided among different frequency groups. GSM commonly uses reuse patterns of 4/12 and 3/9.

• 4/12: Refers to 12 frequency groups and 4 base stations. The available GSM network frequencies are divided into 12 frequency groups across 4 BS sites. It is assumed that there are 3 cells interfaced with each BS. The frequency groups are designated as A1, B1, C1, D1, A2, B2, C2, D2, A3, B3, C3, D3.

The figure above illustrates a 4/12 GSM frequency layout.

Frequency Allocation to GSM Channels

In GSM, there are two primary categories of channels: signaling and traffic.

• Signaling channels: These channels help the network maintain resources, allocate resources (frequency and time slot), establish and release calls, and convey system-related critical information (in the form of System Information messages) to the mobiles within the cell.

• Traffic channels: These channels carry user voice/data (email, SMS, browsing).

The signaling channel category is referred to as the BCCH frequency, while the traffic channel category is referred to as the TCH frequency (non-BCCH frequency).

To fulfill these purposes, each cell is assigned one BCCH frequency (ARFCN) in the downlink and one BCCH frequency in the uplink. Each cell is also assigned one or more TCH frequencies in the downlink and uplink, depending on the capacity required in the cell.

Conclusion

Effective GSM RF planning ensures seamless operation of mobile networks. This involves balancing capacity and coverage while minimizing interference. BCCH frequency allocation is a key element, enabling mobile devices to receive vital network information and maintain connectivity. By mastering the principles of RF planning and understanding the significance of BCCH, network engineers can optimize network performance and ensure reliable communication services across various environments.