GSM物理层协议

1.           Physical Resource

Ref : 45.002, 45.005

1.1.    Frequency allocation

 

GSM 850	824 - 849 MHz (uplink) with 869 - 894 MHz (downlink)
GSM900	890 - 915 MHz (uplink) with 935 - 960 MHz (downlink)	125-1 = 124 carriers
GSM1800	1710 - 1785 MHz (uplink) with 1805 - 1880 MHz (downlink)
GSM1900	1850 - 1910 MHz (uplink) with 1930 - 1990 MHz (downlink)

 

 

 

Duplexing : FDD (Frequency division duplex)

Channel spacing : 200 kHz

Modulation : GMSK

 

 

 

 

 

1.2.    Multiple Access Scheme

Ref : 45.002, 45.010

 

GSM/GPRS standard uses a FDMA/TDMA multiple access scheme, combined with frequency hopping.

 

TDMA Frame : basic unit for definition of logical channels. Contains 8 timeslots

 

Timeslots : contains a burst

 

Burst : 156.25 symbol periods

 

Symbol : in GMSK 1 symbol = 1 bit ( for 8PSK(EDGE) 1 symbol = 3 bits )

 

Full burst : 147 symbols useful duration (NB, FB, SB)

 

Short burst : 87 symbols useful duration (AB) sent only by the MS

 

Physical channel : combination of a timeslot number, a TDMA frame number sequence, and a frequency hopping sequence.

 

Channel rate : 270.8 kbps

1.3.    TDMA Frames

 

 

 

 

1.4.    Burst Types

 

Bursts are made of 156.25 symbols. Each symbol has a duration of 48/13 μs (3.69 μs) or 48 cycles at a 13 MHz frequency.

1.4.1.      Normal Burst (NB)

 

 

Normal and dummy bursts. Used on all channels (except RACH) on both directions.

1.4.2.      Frequency Correction Burst (FB)

 

Used on the FCCH channel (downlink only, slot 0 only). Essentially an empty frame. Important marker in call processing.

1.4.3.      Synchronization Burst (SB)

 

Used on the SCH channel (downlink only, slot 0 only).

1.4.4.      Access Burst (AB)

 

Used on the RACH Common Control Channel, and on TCHs, SDCCHs…during handover  (uplink only).

 

1.4.5.      Dummy Burst (DB)

 

Emitted on RF channel C0 (the one where BCCH is located) when a timeslot is not allocated for any other purpose(CCCH…). The BS must continuously emit on that channel to allow the MSs to make Rx measurements.