Bit rate duration is determined from physical channel throughput for particular modulation schema. GMSK modulation is used with constant BT = 0.3, where B is the 3dB bandwidth of the theoretical Gaussian filter and T is duration of input data bit. Knowing B, we calculate 1/T = 1625/6 ksymb/s or approximately 270.833 ksymb/s. From here bit duration is 3.69.. μ s.
Voice signal is processed in time intervals of 20ms = s/50. For that period of time throughput is [(1625/6) x (1000/50)] symbols. Because this number is fractional, let look at 6 times longer time interval. For time interval 120ms throughput is [1625 x 20] symbols or [13 x 125 x 20] symbols or [26 x 1250] symbols or [26 x 8 x 156.25] symbols. >From last result we can identify: Number of symbols in a burst = 156.25 symbols, and burst time 156.25 x 3.69... = 576,92μs Number of symbols in TDMA frame is 8 x 156.25 = 1250 symbols, Number of symbols in Multiframe is 26 x 1250 symbols. On Fri, Jan 7, 2011 at 8:51 AM, Nataraju A B <natarajuab.t...@gmail.com> wrote: > > Hi Andreas, > Thanks for the speedy reply. > The link which sent you is definitely useful. But the very basic question I > wanted to get a clarification was that, what is the drive behind selection of > burst time to be 577us. > For example the audio frequency can range upto 4Khz. The sampling frequency > should be 8Khz. If each sample is of 8 bit accuracy. This lead to 64K bits of > data. ...... If we explore further on these lines we should be able to > correlate to the time 577us for each burst. > I wanted some more info these lines. What was/were the drives behind > selection of this burst time. Otherwise what are the other end requirements > led to selection of this burst time and in turn TDMA frame hierarchy. > Thanks, > Nataraju A B > On Fri, Jan 7, 2011 at 7:01 PM, Andreas.Eversberg > <andreas.eversb...@versatel.de> wrote: >> >> in addition: >> >> Traffic Multiframe Structures - The 26 traffic multiframe structure is >> used to send information on the traffic channel. The 26 traffic >> multiframe structure is used to combine user data (traffic), slow >> control signaling (SACCH), and idle time period. The idle time period >> allows a mobile device to perform other necessary operations such as >> monitoring the radio signal strength level of a beacon channel from >> other cells. The time interval of a 26 frame traffic multiframe is 6 >> blocks of speech coder data (120 msec). >> (http://www.althos.com/tutorial/GSM-tutorial-frame-structure.html) >> >> one encoded speech block lasts 20ms. >> >> >> 576,92307692307692307692307692308us per slot >> 4615,3846153846153846153846153846 per 8 slots (1 frame) >> 120ms per 26 frames (6 speech blocks) >> >> >> > > > > -- > Thanks, > Nataraju A B
