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GPRS - general rules for successful communication

Antenna installation

Proper antenna placement is essential for proper operation. The decisive factors are:
- the signal strength of the received GSM cell, and
- the quality of this signal.

The signal strength can be measured using the AT commands AT ^ MONI and AT ^ MONP. The limit value at which the networks are designed by the operator is 92 dBm, but this value applies in open country. In large cities or in places with industrial interference, a higher level is needed. For design, it is also necessary to add a reserve in the signal strength for signal fluctuations caused by various influences. These are mainly the influence of weather (snow, icing, very heavy rain), vegetation (growth of trees and shrubs, deciduous leaves), construction of buildings and changes in the topology of the operator's network. A reserve of 10dB seems to be a reasonable minimum.

The second component for assessing the suitability of a signal is its quality. At present, it is not possible to measure this parameter directly. As an orientation measurement, a test using the ping function and then finding out the transmission statistics is offered. Depending on the number of repeated and untransmitted messages, it can be judged whether the signal on the connected station is OK.

To achieve quality GPRS transmission, the following conditions must be ensured:
- the signal level from the serving cell must be better than 82dBm,
- if a signal is present on an adjacent channel next to the received signal, it must be ensured that its level is at least 12dB lower, otherwise interference may occur,
- if different near-level signals differing by less than 3 dB are received, there is a risk of reselection (switching between cells, which causes a degraded response and potential communication failure due to reconfiguration of the MS path), therefore it is necessary to increase the signal spacing.

Example:
Serving Cell I Dedicated channel
chann rs dBm PLMN LAC cell NCC BCC PWR RXLev C1 I chann TS timAdv PWR dBm Q ChMod
107 18 -77 23001 352A 6E91 5 6 33 -106 29 I No connection

chann rs dBm PLMN BCC C1 C2
76 18 -78 23001 1 28 28
106 17 -79 23001 3 27 27
33 17 -80 23001 7 26 26
77 14 -85 23001 3 21 21
30 14 -85 23001 1 21 21

From the above statement you can read:
- the module receives cell number 6E91 on channel 107 with a level of 77dBm,
- the signal may be interfered with by interference from channel 106, which is received at a level of 79dBm,
- during operation, reselection may occur between channels 76, 106, 107 and 33 because their signal is very close.

Ways of solution:
These shortcomings can be overcome by repositioning the antenna or replacing the omnidirectional antenna with a directional antenna, such as a yagi. After this adjustment, the received signals must be measured again.
 

Abbreviations for command AT^MONI:

      Serving Cell:

  • chann ARFCN (Absolute Frequency Channel Number) of the BCCH carrier
  • rs RSSI value 0 . 63 (RSSI = Received signal strength indication)
  • dBm receiving level of the BCCH carrier in dBm
  • PLMN PLMN ID code
  • LAC location area code
  • cell cell ID
  • NCC PLMN colour code
  • BCC base station colour code
  • PWR maximal power level used on RACH channel in dBm
  • RXLev minimal receiving level (in dBm) to allow registration
  • C1 coefficient for base station selection

    Dedicated channel:

  • chann ARFCN (Absolute Frequency Channel Number) of the TCH carrier
  • Note: = h indicates frequency hopping.
  • TS timeslot number
  • timAdv timing advance in bits
  • PWR current power level
  • dBm receiving level of the traffic channel carrier in dBm
  • Q receiving quality (0.7)
  • ChMod channel mode (S_HR: Half rate, S_FR: Full rate, S_EFR: Enhanced Full
    Rate)

Abbreviations for command AT^MONP:

  • Chann ARFCN (Absolute Frequency Channel Number) of the BCCH carrier
  • rs RSSI value 0 . 63 (RSSI = Received signal strength indication)
  • dBm Receiving level in dBm
  • PLMN PLMN ID code
  • BCC Base Station colour code
  • C1 coefficient for base station selection
  • C2 coefficient for base station reselection