GSM Frequency Plan | Teleinf Edu

Why Frequency Allocation Matters in GSM

Wireless communication uses radio waves, a form of electromagnetic radiation. A frequency identifies the rate of oscillation of a radio signal, while a frequency band defines an interval that a service may use. Dividing the spectrum into such intervals lets independent systems operate in the same region with controlled separation and filtering.

A regulator assigns frequency ranges to services such as FM radio, television, and mobile networks. The allocation limits incompatible transmissions in the same band. GSM combines paired uplink and downlink ranges with 200 kHz radio channels and a frequency reuse plan, so the same channel set can serve separated cells. These assignments form the GSM frequency plan. A deployment must also account for channel numbering, transmitter power, receiver selectivity, and coordination with neighbouring systems.

Duplexing in GSM: Separating Uplink and Downlink

A bidirectional call requires the mobile station to transmit and receive concurrently. Its transmitter can be much stronger at the receiver input than the signal arriving from the base station, so isolation between the two directions is required. Without that separation, the mobile transmitter can desensitize its own receiver.

GSM solves this problem using a technique called . FDD assigns the two directions to separate paired frequency ranges.

Uplink (UL): This is the path from your Mobile Station (MS), your phone, up to the Base Transceiver Station (BTS), the cell tower. The lower frequency band is reserved for this.
Downlink (DL): This is the path from the cell tower down to your phone. The higher frequency band is reserved for this.

The frequency separation between an uplink channel and its corresponding downlink channel is called the or duplex offset. The separation gives the duplexer and receiver filters room to attenuate energy from the transmitter while both paths remain active.

GSM-900 Band and P-GSM Allocation

The first and most widely deployed version of GSM was designed to operate in the $900 \text{MHz}$ frequency band. The specific frequency blocks were carefully allocated to support the FDD scheme.

Frequency Allocation for GSM-900

The standard, often referred to as P-GSM (Primary GSM), was allocated a total of $50 \text{MHz}$ of spectrum, split into two separate blocks:

Uplink Band (Phone to Tower): $890 \text{MHz}$ to $915 \text{MHz}$ . This provides a total bandwidth of $25 \text{MHz}$ .
Downlink Band (Tower to Phone): $935 \text{MHz}$ to $960 \text{MHz}$ . This also provides a total bandwidth of $25 \text{MHz}$ .

The duplex spacing for GSM-900 is therefore a substantial $45 \text{MHz}$ ( $935 \text{MHz} - 890 \text{MHz} = 45 \text{MHz}$ ), providing excellent isolation between the transmitter and receiver.

Dividing the Band into Channels

To allow multiple users to communicate, the $25 \text{MHz}$ uplink and downlink bands are sliced into smaller channels. Each GSM radio channel has a width of $200 \text{kHz}$ ( $0.2 \text{MHz}$ ).

The ideal number of 200 kHz raster positions in one 25 MHz block can be calculated as: $\text{Total Bandwidth} / \text{Channel Width} = 25,000 \text{kHz} / 200 \text{kHz} = 125$

The standardized P-GSM radio channels use 1 through 124. The value 125 is an ideal bandwidth ratio; it is not a statement that ARFCN 0 is reserved for signaling. Each assigned ARFCN corresponds to a pair of uplink and downlink frequencies separated by the $45 \text{MHz}$ duplex spacing.

Additional GSM Capacity: DCS-1800 and PCS-1900

Operators also deployed higher-frequency GSM bands to add spectrum in dense areas. Their channel counts and propagation conditions differ from GSM-900, so the band plan and the radio environment both affect capacity.

DCS-1800 (Digital Cellular System at 1800 MHz)

In Europe and Asia, a new band around $1800 \text{MHz}$ was allocated. This system, also known as GSM-1800, had a larger paired spectrum allocation than GSM-900.

Uplink Band: $1710 \text{MHz}$ to $1785 \text{MHz}$ ( $75 \text{MHz}$ of bandwidth).
Downlink Band: $1805 \text{MHz}$ to $1880 \text{MHz}$ ( $75 \text{MHz}$ of bandwidth).
Duplex Spacing: $95 \text{MHz}$ .

With $75 \text{MHz}$ of bandwidth, the ideal ratio is $75,000 \text{kHz} / 200 \text{kHz} = 375$ . For the DCS-1800 band, the standardized ARFCN range 512 through 885 contains 374 numbered radio channels. A phone supporting both $900 \text{MHz}$ and $1800 \text{MHz}$ is called dual-band. The additional spectrum increases the number of frequencies that can be assigned, subject to the network plan and interference limits.

PCS-1900 (Personal Communications Service at 1900 MHz)

In North America, a similar approach was taken, but in the $1900 \text{MHz}$ band. This is known as PCS-1900 or GSM-1900.

Uplink Band: $1850 \text{MHz}$ to $1910 \text{MHz}$ ( $60 \text{MHz}$ of bandwidth).
Downlink Band: $1930 \text{MHz}$ to $1990 \text{MHz}$ ( $60 \text{MHz}$ of bandwidth).
Duplex Spacing: $80 \text{MHz}$ .

The ideal ratio is $60,000 \text{kHz} / 200 \text{kHz} = 300$ . The PCS-1900 ARFCN range 512 through 810 contains 299 numbered radio channels. Phones supporting several bands, for example 900, 1800, and 1900 MHz, are described as tri-band or quad-band devices; roaming still depends on operator agreements, supported channel ranges, and regional regulations.

Cellular Concept and Frequency Reuse

Channel availability alone does not determine coverage capacity. The cellular concept divides an area into cells and reuses a channel set at a controlled distance, allowing the available spectrum to serve multiple separated locations.

If two nearby cell towers (BTSs) use the same frequency, their signals can interfere. This effect is called . Frequency reuse planning keeps the received interference within the target level.

Divide the Area into Cells:The service area (like a city) is divided into a grid of smaller areas called cells. While a cell's actual coverage is an irregular shape, it's often modeled as a hexagon because hexagons can tile a flat plane perfectly without any gaps or overlaps, making them ideal for network planning diagrams. Each cell is served by a BTS located at its center.
Group Frequencies into Clusters: The total pool of available frequency channels (e.g., the 124 channels of GSM-900) is split into smaller, disjointed sets. A group of adjacent cells in which each cell uses a different set of these frequencies is called a .
Repeat the Pattern (Frequency Reuse): A cluster pattern can be repeated across the service area. The same set of frequencies can be reused in another cell when the separation is sufficient. The distance between cells using the same frequencies is called the frequency reuse distance. It must keep the resulting CCI within an acceptably low level.

The size of the cluster, denoted by $N$ , is a planning parameter. Common illustrative GSM cluster sizes are $N=3$ , $N=4$ , $N=7$ , and $N=12$ . The selected value balances capacity and interference:

Large Cluster Size (e.g., N=12): This increases the distance between cells reusing the same frequencies and can reduce co-channel interference. The total number of available channels is divided among more cells, so each cell receives fewer channels and its capacity falls.
Small Cluster Size (e.g., N=3): Each cell can receive more channels, calculated as the total channel count divided by 3, so capacity per cell can rise. The reuse distance is smaller, which raises the potential for co-channel interference and may degrade call quality.

Network engineers plan these reuse patterns by balancing traffic capacity, propagation, and the required signal quality. Spatial reuse lets a finite channel set serve many cells; the achievable subscriber and traffic capacity remains limited by the number of cells, assigned channels, and radio conditions.

Comparison of GSM-900, DCS-1800, and PCS-1900

Feature	GSM-900	DCS-1800 (Europe/Asia) / PCS-1900 (N. America)
Uplink Frequency Range	890 - 915 MHz	1710 - 1785 MHz / 1850 - 1910 MHz
Downlink Frequency Range	935 - 960 MHz	1805 - 1880 MHz / 1930 - 1990 MHz
Number of RF Channels	124	374 / 299
Duplex Spacing	45 MHz	95 MHz / 80 MHz
Max Mobile Station Power	2 W (Class 4)	1 W (Class 1)
Illustrative Planning Radius	~35 km	~5-10 km (typical)
Primary Use Case	Wide-area coverage (rural, highways)	High-capacity coverage (urban centers)