FDMA vs FHMA | difference between FDMA and FHMA
This page compares FDMA vs FHMA and mentions difference between FDMA and FHMA. FDMA stands for Frequency Division Multiple Access and FHMA stands for Frequency Hopping Multiple Access.
FDMA | Frequency Division Multiple Access
It is analog digital multiple access technique.
As mentioned FDMA is the short form of Frequency Division Multiple Access.
Here entire band of frequencies is divided into multiple RF channels/carriers having 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.
The figure-1 depicts FDMA applied in the uplink (890-915) and downlink (935-960) of 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 digital multiple access technique where in PN (Pseudo Random) sequence is used for carrier frequency variation on which data is mapped before transmission over the air.The FHMA receiver uses same PN sequence as used by FHMA transmitter. Hence both are synchronized. Figure-2 depicts FHSS transmitter used in GSM. FHSS stands for Frequency Hopping Spread Spectrum. Refer GSM baseband vs synthesized frequency hopping.
Here carrier frequencies are varied within a wideband channel during the
digital data transmission period.
Following steps are followed by 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 total spread BW.
Change in carrier frequency rate is known as hopping rate. Figure-3 depicts frequency vs time plot of three channels being hopped in FHMA technique. There are two types of frequency hopping system viz. fast frequency hopping and slow frequency hopping.
The system in which carrier frequency rate (Fc) is higher than the symbol rate (Fs) ;
it is known as fast hopping system.
Here single bit is transmitted in multiple hops.
Fc > Fs ...Fast Hopping
The system in which carrier frequency rate is less than or equal;
it is known as slow hopping system.
Here multiple bits are transmitted in hopping period.
Fc <= Fs ...Slow Hopping
In GSM every 20ms of voice produces 456 bits of information which are carried over four consecutibe 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 constant RF frequency for transmission while FHMA uses range of frequencies as per hopping sequence. This means FHMA does not use single constant frequency as used by FDMA for transmission. GSM uses both FDMA and FHMA access techniques.
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