Baseband vs. Synthesized Frequency Hopping in GSM

This article explores the two primary types of frequency hopping employed in GSM (Global System for Mobile Communications) to provide secure communication between a Mobile Station (MS) and a Base Station (BS). The core concept of frequency hopping aims to improve security and resilience against interference.

The primary goal of frequency hopping is to provide secured communication between GSM Mobile Station and Base Station.

Here’s a comparison of Baseband Frequency Hopping and Synthesized Frequency Hopping:

Fig:1 GSM TDMA Frame

In GSM, a 25 MHz bandwidth frequency band (890MHz to 915MHz for uplink and 935MHz to 960MHz for downlink) is divided into 124 carriers, each having a 200 KHz bandwidth. Each carrier is further divided into eight time slots to carry bursts. Eight time slots comprise one burst, which has a duration of 4.615ms. One burst in GSM carries 156.25 bits.

What is Frequency Hopping?

Frequency hopping is the change of RF (Radio Frequency) carrier frequency during the transmission of information. In GSM, the RF carrier is changed every 4.615ms, resulting in 217 hops per second. This means that in GSM, the RF carrier frequency is changed in each burst transmission. Hence, all the bits of each burst are transmitted on the same frequency. Frequency hopping helps to improve interference and frequency diversity.

There are two main types of frequency hopping based on the use of RF carrier frequencies for the RF transceivers during burst transmission in GSM:

1. Synthesized Frequency Hopping
2. Baseband Frequency Hopping

GSM Baseband Frequency Hopping

Fig:2 Baseband hopping

• In baseband frequency hopping, each transceiver is assigned one dedicated RF carrier frequency. The number of hopping frequencies is equal to the number of transceivers used in a single GSM cell.
• Bursts from transceiver controllers are routed to different transmitters via a bus interface (as illustrated in Figure 2).
• A narrow band filter combiner is used, which can connect up to 16 RF transceivers while incurring no more than 3 dB of loss.
• It is not possible to hop on frequencies more than the number of transceivers deployed.
• Handover of the baseband signal is done in baseband frequency hopping.

Since one RF transceiver is used for the entire duration of a call for all burst transmissions, a transceiver failure will affect all consecutive calls as well as the current call.

GSM Synthesized Frequency Hopping

Fig:3 FHSS (Frequency Hopping Spread Spectrum) Transmitter

• The number of hopping frequencies does not depend on the number of RF transceivers.
• More than one frequency is used for each transceiver for consecutive transmission of bursts.
• For each burst, there will be a new RF carrier for transmission through a single RF transceiver. Since multiple bursts need to be transmitted depending on the information (data/voice call) size, multiple RF carriers are required.
• Here, the number of hopping frequencies is greater than the number of transceivers in a cell.

The frequency of the RF synthesizer is changed according to a hopping sequence; hence, it’s known as synthesized frequency hopping. Figure 3 depicts a Frequency Hopping Spread Spectrum (FHSS) transmitter. A Pseudo-Noise (PN) sequence is used to prepare a random channel table, which is used to change the synthesizer output frequency. These different synthesizer output frequencies are used to generate different RF carrier frequencies using an RF mixer, as shown below.

Fig:4 GSM synthesizer hopping

As illustrated in Figure 4, RF transmitters change the frequency for each and every burst.

• It is possible to hop on multiple frequencies, even greater than the number of transceivers available in the system.
• Since hybrid combiners are used for connecting transmitters, the loss will be higher compared to baseband hopping.

Fig:5 GSM burst wise frequency hopping

For each burst, a new set of synthesizer frequencies is used to generate a new RF carrier frequency. The PN sequence is known to both the transmitter and receiver, making it easy to recover the data transmitted using the frequency hopping sequence. Each burst is transmitted on different unique frequencies in GSM synthesized FH type. This is shown in Figure 5.

Both baseband frequency hopping and synthesized frequency hopping fall under the “slow frequency hopping” type.

GSM Frequency Hopping Parameters

Fig:6 Use of HSN in GSM

The following are the frequency hopping (FH) parameters used in GSM:

• MA (Mobile Allocation): The MA list contains the RF frequencies assigned to one GSM sector. These channels are used for Circuit Switched (CS) and Packet Switched (PS) calls in GSM. There are a total of 63 frequencies in the MA list.
• HSN (Hopping Sequence Number): This number specifies the FH algorithm used in GSM, ranging from 0 to 63. Hence, there are 64 possible hopping algorithms. When HSN is equal to 0, there is no hopping, and the sequence is cyclic. When HSN is between 1 and 63, hopping is applied, and frequencies are chosen randomly from the list for transmission.
• MAIO (Mobile Allocation Index Offset): It sets the initial frequencies in the MA list. It ranges from 0 to (N-1), where N is the total number of frequencies in the MA list. An MAIO value of 0 for the RF transceiver refers to the first RF carrier (i.e., f1) to be used by it.