GPRS Definitions

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Timing advance

Timing advance (TA) is a control parameter ordered by the base station, which tries to compensate the length of time a signal takes to reach the BTS from a mobile phone.

A burst arrives on time when it appers centered on the timeslot. Due to the delay introduced by the distance between the MS and the BTS (which is reported through the timing error), timing advance tries to adjust the time at which the transmitter(the MS) sends bursts in order to compensate the delay at the receiver (BTS). This is performed to prevent collisions with bursts from adjacent timeslots. Thus, TA will tell the MS by how much to advance in time its transmission.

The range of the parameter is 0..63 and it is passed to the MS through messages such as: ImmediateAssignment, PacketUplinkAssignment, PacketUplinkAckNack. Each step represents about an advance of one bit period (~3.69 microseconds). This means that one TA step represents a change in distance of ~1100meters. Taking into account that the delay is round-trip, a TA step means an ~550meters change in the distance between the MS and BTS. So, a TA of 3 means that the MS must be ~1.6km away from the BTS.

Timing error

Timing error (TE) is a calculated value at transceiver level, which determines the time delay in arrival of a burst at the receiver. It is the difference between the moment the burst should have arrived and the moment the burst actually arrived (centered in the timeslot). The TA compensates this delay.

Timing errors are usually positive, meaning that the burst arrived late. It uses the same scale as timing advance, meaning that one step corresponds to ~550m distance between the MS and the BTS. If the TE is negative, it means that the MS is very close to the BTS, and that cannot be compensated through a TA value.

When the TA is ordered accordingly, the TE should be ~0.

C value

C value is the normalized received signal strength at the MS. Through this value, the MS reports the power of the BTS as seen at its end (see 3GPP TS 45.008, section 10.2.3.1).

It is reported in a range of 0..63 and it is mapped according to the following formula (see TS 45.008, section 8.1.4):

C = -111dBm + CV. 

I_LEVEL

I_LEVEL represents the the interference level observed on a timeslot compared to the C value (see 3GPP TS 45.008, section 10.3).

The range is 0..15. I_LEVEL represents the mapping of the interference level compared to the C value. Mapping is done like this:

for I_LEVEL = 1..14: C - 2  * I_LEVEL <= interference <= C - 2 * (I_LEVEL - 1)
for I_LEVEL = 0:  interference >= C
for I_LEVEL = 15: interference <= C - 28dB

RXQUAL

RXQUAL represents the BER (bit error rate) detected for the channel by the MS (see 3GPP TS 45.008, section 8.2.3.1 and 8.2.4).

Range is 0..7 and it’s interpreted like this:

RXQUALBER is greater than (%)Assumed BER value (%)BER is less than (%)
00.140.2
10.20.280.4
20.40.570.8
30.41.131.6
41.62.263.2
53.24.536.4
66.49.0512.8
712.818.10

Power control loop

The open power control loop used for GPRS tries to adjust the MS radio power output in order for bursts to reach the BTS. The MS cannot transmit a GPRS burst at full power at all times because it will saturate the BTS’s receiver (and will also consume more power). To counter this inability, the MS adjust its output power taking into account the measured signal strength received from the BTS and the commands received from the BTS.

According to 3GPP TS 45.008, section 10.2.1, the MS output power is determined by:

Pch = min(Gamma0 - GammaCh - Alpha * (C + 48), PMAX)

According to the Annex B, all power calculations are in dBm. The parameters of the formula are:

  • PMAX:
    • is GPRS_MS_TXPWR_MAX_CCH if PBCCH is present
    • is MS_TXPWR_MAX_CCH, if no PBCCH is present (the case of mbts)
    • has values in range between 5-43 dBm (see Table 1 is spec)
  • Gamma0 is a set in stone parameter with the following values:
    • 39 dBm for GSM400, GSM850, GSM900
    • 36 dBm for DCS1800 and PCS1900
  • C is the C Value
  • Alpha:
    • is a system parameter broadcast on PBCCH (if present – not in the mbts case), and ordered by the BTS to the MS through some downlink RLC control messages.
    • is encoded on range of 0…10 corresponding to values of 0.1…1.0 in 0.1 steps
    • serves as a factor which decides the weight of the measured level of the received BTS signal in the output power
  • GammaCh:
    • is an MS (our case) and channel specific parameter sent to the MS in several downlink RLC control messages
    • is encoded on a range of 0…31 corresponding to 0…62dB, in 2dBm steps
    • is a parameter which tells the MS by how much to turn down the power.

Looking closely to the formula, and knowing that the the MS usually measures received signal levels (the C value) between -111dBm…-48dBm, the conclusion is that the whole (C + 48) expression is negative. Combined with the negative sign in front of Alpha, it results that ALPHA DECIDES BY HOW MUCH THE MS SHOULD TURN UP ITS POWER.

Because it is always positive, GAMMACH DECIDES BY HOW MUCH THE MS SHOULD TURN DOWN ITS POWER.

Setting ALPHA to 1 will allow the MS to increase its power with the difference between the received signal and the -48 dBm threshold. In theory, if GammaCh is also set to 0, the power control is given completely to the MS. But, as PMAX = MS_TXPWR_MAX_CCH is usually +33 / +30dBm (equivalent to 2/1W transmit power),setting GammaCh to 0, will have no effect, as Gamma0 is already 6dBm greater than PMAX, so the transmit power will always be PMAX.

Setting ALPHA to 0 and network adjusting of GammaCh based on the MS received signal strength allows the network to take control of the power loop for the MS.

Below are some examples of the effects of the power control loop on the transmit power of the MS.

Case study 1: Alpha = 10 and GammaCh = 31

Pch = 39 - 2 * 31 - 1.0 * (C + 48) (dBm) = -23 - (C + 48) (dBm)
        = -71 - C (dBm)
    For:
        C = -111 dBm -> Pch = 40 dBm, but PMAX = 33/30 dBm so it will trasmit with PMAX
        C = -104 dBm -> Pch = 33 dBm (same as PMAX = 33 dBm for a 2W MS in GSM850/GSM900)
        C = -101 dBm -> Pch = 30 dBm (same as PMAX = 30 dBm for a 1W MS in DCS1800/PCS1900)
        C = -76 dBm -> Pch = 5 dBm (usually the minimum transmit power of 3mW)
        C = -48 dBm -> Pch = -23 dBm (but limited by the minimum 5 dBm transmit power)
    So, Pch varies only for value of C in range -104...-76 dBm for GSM850/GSM900, -101...-76 dBm for DCS1800/PCS1900.

Case study 2: Alpha = 10 and GammaCh = 17

    Pch = 5 - 1.0 (C + 48) (dBm) = -43 -C (dBm)
    For:
        C = -76 dBm -> Pch = 33 dBm (same as PMAX in GSM850/GSM900)
        C = -73 dBm -> Pch = 30 dBm (same as PMAX in DCS1800/PCS1900)
        C = -48 dBm -> Pch = 5 dBm ( minimum of 3mW transmit power)
    So, Pch varies only for values of C in range -76...-48 dbm for GSM850/GSM900, -73...-48 dBm for DCS1800/PCS1900.

Case study 3: Covering the entire range of C with adjustments of Pch

C has a range of 62dB and Pch has a typical range of 28 or 25 dB.
   That gives Alpha = 4 and GammaCh = 17.
   Pch = 5 - 0.4 (C + 48) (dBm) = -14 -0.4*C (dBm)
       C = -111 dBm -> Pch = 30 dBm
       C = -48 dBm -> Pch = 5 dBm
   For GSM850 and GSM900, setting Alpha = 5 makes it possible to exploit the larger MS_TXPWR_MAX_CCH.

See 3GPP TS 45 008 V12.3.0, Annex B for further explanations.

It is desirable to try to keep the MS output power at the minimum required for the burst to be received at the BTS. If the bursts where to be transmitted at full power, it can saturate the receiver and the burst will be dropped. The same if the transmit power is too low, the burst might not be detected at receiver.

Interpreting the parameters

  • Correlation between RSSI, Gamma and CV/RXLev:
    • if the RSSI is high and the power control cannot order turning down the power to an even lower value (meaning Gamma = 31), it could mean one or all of the following situations:
      • the MS is quite close to base station;
      • the BTS is transmitting at a lower power (it can be seen at what power the MS see it through CV and/or RXLev), and the MS thinks that it is far from the BTS and will transmit at a higher power. In this case, an attenuator on the receiver might be needed;
      • both situations will lead to high power burst arriving at the BTS. A high RSSI for a burst increases the chances that bursts are being dropped due to receiver saturation and, therefore, have the appearance that nothing is being received from MS.
    • if the RSSI is low and the power cannot be turned up (Gamma = 0), it could mean that the MS is close to being out the range of the BTS. This can be checked by looking at CV/RXlev to see at what level the MS is seeing the BTS signal. A value of -100dBm is low, and at -110dBm, MSs consider the BTS signal lost. Increasing (if possible) the transmit power of the BTS should solve the problem.
    • depending of the range wished to be covered, setting an appropriate Alpha for the power control is important. For a short range, you’d want Alpha 0 so that the whole output power of the MS is not influenced by the BTS transmit power, and only by Gamma.
  • Correlation between BTS output power, MS output power
    • the MS power is inversely correlated with the BTS output power. This is designed so that the MS will transmit at low power when it is close to the BTS, and high when it’s far because the MS assumes that BTS transmits at a certain power as to cover a certain distance.
    • assuming that the MS stays in place and the BTS turns down its power, this will cause the MS to increase its transmit power. The BTS will be able to turn down the MS power to a certain point (until Gamma is 31), but if after that the MS signal is still to strong, you can either adjust Alpha (make it smaller), either turn up the BTS power at least to where the power loop can compensate or either install an attenuator on the receiver of the BTS.
  • Interpreting RXQUAL
    • a high value of RXQUAL indicates that the MS is receiving a percentage of burst with the BER as indicated by the mapping in the RXQUAL definition. A high BER could indicate interference with the BTS signal or possibly that the BTS is transmitting with too low/too high amplification.
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