Frequency Hopping

Frequency Hopping is an old technique introduced firstly in military transmission system to ensure the secrecy of communications and combat jamming.  Frequency Hopping is mechanism in which the system changes the frequency (uplink and downlink) during transmission at regular intervals.  It allows the RF channel used for signaling channel (SDCCH) timeslot or traffic channel (TCH) timeslots, to change frequency every TDMA frame (4.615 ms).  The frequency is changed on a per burst basis, which means that all the bits in a burst are transmitted in the same frequency. 

Advantages of Frequency Hopping

1. Frequency Diversity

In cellular urban environment, multi-path propagation exists in most cases.  Due to Rayleigh fading, short-term variations in received level are frequently observed.  This mainly affects stationary or quasi-stationary mobiles.  For a fast moving mobile, the fading situation can be avoided from one burst to another because it also depends on the position of the mobile so the problem is not so serious.  Frequency Hopping is able to take the advantage due to frequency selective nature of fading to decrease the number of errors and at the same time they are temporally spread.  As a result, the decoding and de-interleaving processes can more effectively remove bit errors caused by bursts received whilst on fading frequencies (errors will be randomly distributed instead of having long bursts of errors).  This increase in effectiveness leads to a transmission quality improvement of the same proportion.

·         Frame Erasure Rate reduces due to 6 dB to 8 dB gain.

·         Number of reports with rxqual 6 and 7 reduce.

·         Reported values of rxlev are more concentrated around mean.

2. Interference Averaging

Interference Averaging means spreading raw bit errors (BER caused by the interference) in order to have random distribution of errors instead of having burst of errors, and therefore, enhance the effectiveness of decoding and de-interleaving process to cope with the BER and lead to better value of FER. 

With hopping, the set of interfering calls will be continually changing and the effect is that all the calls experience average quality rather than extreme situations of either good or bad quality.  All the calls suffer from controlled interference but only for short and distant periods of time, not for all the duration of the call.

·         For the same capacity, Frequency Hopping improves quality and for a given average quality Frequency Hopping makes possible increase in capacity.

·         When more than 3 % of the reports have rxqual of 6 or 7 then voice quality disturbances start to appear.

·         Gains (reduction in the C/I value needed to satisfy the quality requirements involved in the criterion) from hopping relative to fixed frequency operation can be achieved.

1/3 interference: 1 dB gain 

i.e. if 1 out of 3 frequencies are experiencing a continuous interference a gain of 1 dB in C/I requirement is obtained.

Similarly,

1/4 interference: 4 dB gain

1/5 interference: 6 dB gain

2/4 interference: 0 dB gain

2/5 interference: 4 dB gain

The effective gain obtained with Frequency Hopping is due to the fact that the interference effect is minimized and it is easier to keep it under control.

                                       Parameter

SQIà +18 above is good (19 is ok)

C/Ià  +9 above good in case of hopping(particular channel 200khz/total interference)

C/Ià +12 above good in case of Non hopping(particular channel 200khz/total interference) this is case of co-channel interference

C/Aà -9 above good (e.g -8,-7,-6) this carrier upon adjacent channel interference

Co-channel interference;- 

(1) Level is high but call is drop due to handoff problem and this is due to interference on same channel in which call is latch.

(2) problem in access.

Adjacent  interference;- check by quality , Rx level is good

Path Balance:- path balance gives the difference between uplink reception and down link reception

Path balance =  path Loss +110

Path Loss = Up path Loss  – DL path Loss

Where

Uplink Path Loss = Actual Ms Tx power --- Rx Lev_UL

Downlink Path Loss = Actual BTS Tx power --- Rx Lev_dL

+- 10 db is acceptable in link budget

negative value means your BTS Rx is bad

Positive value means your  BTS Tx is bad


 Effect of Frequency Hopping

1.      Handovers: 

   When SFH is implemented, BCCH plan is done using lesser number of  frequencies as compared to fixed frequency plan.  This may result in quality degradation.  However quality of hopping carriers improves than before.  Also, quality threshold for handovers on hopping carrier should be increased as compared to fixed frequency plan. In the present version (GSR3), different quality threshold settings are set BCCH and NBCCH.  By setting lower quality thresholds for BCCH as compared to NBCCH, number of dropped calls can be controlled.  Handover Success Rate may go down because of the BCCH replan (less frequencies).  This reduction may get compensated due to improvement in quality of hopping carriers (improvement in TCH assignment success rate).

2.      Call setup:

    In call setup, SDCCH hopping is also possible.  There are no separate settings required for SDCCH hopping. Since GSR3 allows control over SDCCH configuration (location of SDCCH on timeslot basis), SDCCH hopping depends on the location of SDCCH.  In case of SFH (with BCCH not included in MA list), if SDCCHs are on BCCH carrier they will not hop whereas SDCCHs on NBCCH carriers may hop.  Generally it is preferred to keep SDCCHs on BCCH carrier as SDCCH timeslot is used continuously and it will increase interference on hopping carriers.  Call success rate will depend on the cleanliness of BCCH carriers.  Call Success Rate may reduce after BCCH replan.  This reduction may fet compensated due to improvement in quality of hopping carriers (improvement in TCH assignment success rate).

3.      Frame Erasure Rate (FER): 

     FER indicates the number of TDMA frames that could not be decoded by the mobile due to interference.  This parameter gives the indication of hit-rate.  FER improves (gain of 6 to 8 dB) after implementation of frequency hopping.  FER is represented in percentage terms.  FER less than 10% is considered to be good.  But this is a subjective issue and good value should be decided by doing multiple drives.  In future Motorola is planning to include FER as a statistics in the OMCR.