What is Frequency Hopping? Why is it necessary?

A common transmission problem in GSM or any wireless telephony system is of Fading. This occurs as a result of the shadowing (fading) or multipath (fading). Shadowing effect is produced by buildings and natural obstacles such as hills located between the transmitting and receiving antennas of a Mobile Station (MS) and a Base Station (BS). As the MS moves around, the received signal strength increases and decreases as a function of the type of obstacles that are present at that moment between the transmitting and receiving antennas. Multipath effect occurs when the transmitted signal takes more than one path from the transmitting antenna to the receiving antenna so that the receiving antenna of the MS receives not just one signal (direct) but several copies of it (reflected and deviated from obstructions). The latter signals are delayed slightly in phase from one another. The reception of several versions of the same signal shifted in phase from one another results in the vector sum of the signals, the resultant composite signal being actually received at the receiving antenna. In some instances the vector sum of the received signal may be very low or even close to zero which can cause the received signal to virtually disappear.

In addition to fading, a dense GSM network suffers with co-channel interference. This means that a phone call is interfered with calls in another site operating on the same physical channel and time slot.

To compensate for above transmission difficulties in GSM, a technique called Frequency Hopping is used . Frequency Hopping is the technique of improving the signal to noise ratio in a link by adding frequency diversity. It means that MS sequentially communicates with a BS on different frequencies. The BS commands the MS to activate frequency hopping as the MS moves towards the edge of a cell or into an area of high interference. When frequency hopping is activated in the MS, the BS assigns the MS a set of RF channels, rather than a single RF channel. A frequency hopping algorithm is also assigned to the mobile and is used to inform the mobile of the pattern of available frequencies it has to use.

How frequency hopping compensates for co-channel interference and fading?
Consider co-channel interference. Not all of the slots are in use on all of the physical channels on each site where they are reused, so although slot 4 on channel 555 might be clobbered by another conversation, slot 7 on channel 522 probably isn’t. So, if we can take each caller on a particular sector and jump them from slot to slot, and from frequency to frequency, then each user runs a far lower risk of suffering from co-channel interference. And when such interference does occur, chances are good that the error correction algorithms can take care of it.

Now consider multipath. Due to the very high frequency of GSM service the wavelength of the signals is extremely short (only a few inches in fact). That means the phase difference on one channel will be quite different than on another. By jumping from frequency to frequency we may only experience problematic multipath for very short periods of time, once again giving the error correction algorithms the chance to clean it up.

Frequency hopping can be of two types: fast and slow. For fast frequency hopping, the rate at which the frequency changes is higher than the signal modulation rate. In the GSM system, the frequency is required to remain unchanged during a burst period. The frequency hopping in the GSM system is slow frequency hopping.

There are two kinds of hopping algorithm:

• Cyclic hopping: The transceiver hops through a fixed repeated pattern of frequencies
  
• Pseudo-random hopping: The transceiver hops through the list of frequencies in a random manner.

Specific parameters of the channel, defined in the channel assignment message that MS uses during Frequency Hopping are:

• MA: Mobile allocation of radio frequency channels, defines the set of radio frequency channels to be used in the mobiles hopping sequence. The MA contains N radio frequency channels, where 1 ≤ N ≤ 64.
• MAIO: Mobile allocation index offset (0 to N-1, 6 bits).
• HSN: Hopping sequence (generator) number (0 to 63, 6 bits).

There are a total of 64 different frequency patterns. The hopping sequence the MS uses depends on the HSN; a HSN of 0 corresponds to Cyclic hopping sequence and values 1 to 63 correspond to pseudo random patterns. The ARFCNs used in the hopping sequence pattern are determined by the contents of MA Table. The entry of the MA Table at which the hopping sequence begins is called the MAIO. An MAIO of 0 corresponds to the first entry (lowest ARFCN) of the MA Table.

Frequency Hopping in BTS

There are two types of frequency hopping method available for the BTS: RF frequency hopping and Baseband frequency hopping.

• RF (synthesizer) Frequency Hopping - In this method, the BTS-TRX itself changes frequencies according to the hopping sequence. So, one TRX would hop between multiple frequencies in the same sequence that the MS is required to.
• Baseband Frequency Hopping - In this method, BTS has several TRXs and each TRX is modulated to a fixed frequency and allocated with a fixed ID. Each TRX would be assigned a single time slot within a TDMA frame. For example, time slot 1 might be assigned to TRX 2 in one TDMA frame and in the next TDMA frame it would be assigned to TRX 3, and the next frame would be TRX 3. So, the data on each time slot would be sent on a different frequency each frame, but the TRXs on the BTS do not change frequency. The BTS simply routes the data to the appropriate TRX, and the MS knows which TRX to be on for any given TDMA frame.

Note: On the RF channel carrying a BCCH (C0), frequency hopping is not permitted on any timeslot supporting a BCCH. A non-hopping radio frequency channel sequence is characterized by a mobile allocation consisting of only one radio frequency channel, i.e. with N=1, MAIO=0. In this instance sequence generation is unaffected by the value of the value HSN.