FDMA vs FHMA: Understanding the Key Differences

This page compares FDMA vs FHMA and mentions the difference between FDMA and FHMA.

• FDMA stands for Frequency Division Multiple Access.
• FHMA stands for Frequency Hopping Multiple Access.

FDMA | Frequency Division Multiple Access

It is an analog digital multiple access technique.

As mentioned, FDMA is the short form of Frequency Division Multiple Access. Here, the entire band of frequencies is divided into multiple RF channels/carriers having a certain bandwidth. Each RF carrier is allocated to different users or subscribers.

Example: 890 to 915 MHz band having 25 MHz bandwidth in GSM is divided into 124 RF carriers of 200KHz BW each.

Figure 1 depicts FDMA applied in the uplink (890-915) and downlink (935-960) of the GSM900 frequency band. There are two types of FDMA viz. fixed assignment and demand assignment type as per the allocations to the subscribers or users of the network.

FHMA | Frequency Hopping Multiple Access

As mentioned, FHMA is the short form of Frequency Hopping Multiple Access. It is a digital multiple access technique wherein a PN (Pseudo Random) sequence is used for carrier frequency variation on which data is mapped before transmission over the air. The FHMA receiver uses the same PN sequence as used by the FHMA transmitter. Hence, both are synchronized.

Figure 2 depicts the FHSS transmitter used in GSM. FHSS stands for Frequency Hopping Spread Spectrum.

In GSM baseband vs synthesized frequency hopping, carrier frequencies are varied within a wideband channel during the digital data transmission period.

Following steps are followed by the FHMA transmitter.

• The digital data is divided into uniform-sized bursts which are transmitted on different RF carrier frequencies.
• The instantaneous BW (bandwidth) of any one transmission burst is much smaller than the total spread BW.

Change in carrier frequency rate is known as the hopping rate.

Figure 3 depicts a frequency vs time plot of three channels being hopped in FHMA technique.

There are two types of frequency hopping systems viz. fast frequency hopping and slow frequency hopping.

The system in which carrier frequency rate (Fc) is higher than the symbol rate (Fs) is known as a fast hopping system. Here, a single bit is transmitted in multiple hops.

Fc > Fs ...Fast Hopping

The system in which the carrier frequency rate is less than or equal is known as a slow hopping system. Here, multiple bits are transmitted in the hopping period.

Fc <= Fs ...Slow Hopping

In GSM, every 20ms of voice produces 456 bits of information which are carried over four consecutive bursts. This is due to the fact that one GSM burst consists of 156.25 bits carrying two blocks, each block having 57 bits of speech data.

Each burst in GSM is transmitted on different unique RF carrier frequencies as shown in the figure 4.

The advantages of FHMA are as follows.

• It provides secure communication.
• It is immune to fading conditions.

The major difference between FDMA and FHMA is that FDMA uses a constant RF frequency for transmission while FHMA uses a range of frequencies as per the hopping sequence. This means FHMA does not use a single constant frequency as used by FDMA for transmission.

GSM uses both FDMA and FHMA access techniques.