1、m 3404583 0073573 507 m 1 ETSI/TC SMG Released by : ETSI/PT 12 Release date: February 1992 RELEASE NOTE Recommendation GSM 05.04 Modulation Previously distributed version : 3.1.2 (Updated Release 1/90) New Released version February 92 : 3.1.2 (Release 92, Phase 1) 1. Reason for chancres No changes s
2、ince the previously distributed version. m 3404583 007L574 443 m ETSI/GSM GSM Recommendation: 05.04 version: 3.1.2 Title: MODULATION Date : February 1992 CONTENTS : 1. SCOPE 2. MODULATION FORMAT Number of pages: 3 Rec.05.04 (version 3.1.2) 1/2 bit+ r i The useful part i I &/2 bit I -3 k MODULATION 1
3、 SCOPE The modulator receives the bits from the encryption unit, see Rec.05.01, and produces an RF signal. The filtering of the RF signal necessary to obtain the spectral purity is not defined, neither are the tolerances associated with the theoretical filter requirements specified. These are contai
4、ned in Rec.05.05. 2 MODULATION FORMAT 2.1 Modulating bit rate: The modulating bit rate is 1/T = 1625/6 kbit/s (ie app. 270.833 kbit/s). 2.2 Start and stop of the burst: Before the first bit of the bursts as defined in 05.02 enters the modulator, the modulator has an internal state as if a modulating
5、 bit stream consisting of consecutive ones (di = i) had entered the differential encoder. Also after the last bit of the time slot, the modulator has an internal state as if a modulating bit stream consisting of consecutive ones (di = 1) had continued to enter the differential encoder. These bits ar
6、e called dummy bits and define the start and the stop of the active and the useful part of the burst as illustrated in Fig.1. Nothing is specified about the actual phase of the modulator output signal outside the useful part of the burst. dummy bits i3 tail bits . 111111,000 . I I 3 tail bits I dumm
7、y bits . 0001 111111 . Figure 1: Relation between active part of burst, tail bits and dummy bit?, For the normal burst the useful part lasts for 147 modulating bi t 3404583 0071576 21b Rec.05.04 (version 3.1.2) 2.3 Differential encoding: Each data value di = 0,1 is differentially encoded. The output
8、 of the differential encoder is: di = di di-1 (di E (0, 11) (1) where C denotes modulo 2 addition. The modulating data value ai input to the modulator is: h ai = 1 - 2di (ai E 1-11 +il) 2.4 Filtering: The modulating data values a, as represented by Dirac pulses excite a linear filter with impulse re
9、sponse defined by: g(t) = h(t) * rect(t/T) (3) where the function rect(x) is defined by: rect (t/T) = 1/T rect(t/T) = O for It1 T/2 otherwise and * means convolution. h(t) is defined by: h(t) = exp(-t2/ (2a2T2) ) / ( (&)aT) where 0 = dln(2) /SXBT and BT = 0.3 (4) where B is the 3 dB bandwidth of the
10、 filter with impulse response h(t) , and T is the duration of one input data bit. This theoretical filter is associated with tolerances defined in 05.05. 2.5 Output phase: The phase of the modulated signal is: t-iT cp(t) = ai x: h g(u)du 1 I -0 where the modulating index h is 1/2 (maximum phase change in radians is x/2 per data interval). 2.6 Modulation: The modulated RF carrier, except for start and stop of the TDMA burst may therefore be expressed as: where E, is the energy per modulating bit, fo is the centre frequency and qo is a random phase and is constant during one burst.
