New article: "GMSK and OQPSK"

It's a small article, which dealt with the problem of identifying GMSK and OQPSK.
We hope this will be useful...


It's a small fragment...


OQPSK and GMSK are tightly connected, as well as SDPSK and CPFSK, as well as pi/4 DQPSK and C4FM.


Such "dualism" is spread enough, and connected/linked by the fact that often some modes of frequency shift keying at some conditions, smoothly flowing and/or aspire to acquire the shape of phase manipulation.


*** GMSK ***


The general features of  GMSK manipulation formation, we had already considered in the article FSK, GFSK, MSK, GMSK and some other modes . Also it was marked, that application of the filter of the Gauss allows to narrow the main spectrum of a signal considerably.


The filter of the Gauss is characterized by parameter BT, interested ones can independently learn this subject, us interested in the practical for the analysis moments.


The value range of parameter BT lies within 1 > BT > 0 limits. The lesser BT value, the more compact spectrum at resultant signal GMSK.




GMSK with the filter of the Gauss with BT = 1, is turning into classical MSK.


For formation of GSM signals the filter of the Gauss with BT = 0.3 is standardly used, and on an example of the synthesized signals it is possible to look as it looks from positions of tasks of the analysis.




Some preliminary remarks.


As, the finished result of concerning of GMSK nuances will be some conclusions about signal TETRAPOL

, then the synthesized signals have the same speed of manipulation equal 8000 Hz.

Besides, sometimes I will add a bit of white noise in the synthesized signals, because ideal signals though look beautifully, nevertheless, having absolute accuracy, in some cases these ideal signals conceal/hide the general details or make these details badly watched. This moment should be well clear from a picture below.

The full description here.

PSK: TETRAPOL

TETRAPOL

The trunking system for VHF, Br - 8000, mode GMSK

Author: SergUA6 

Band Width ~9000-1000 Hz

Baud Rate 8000 Hz

n-Ary (PSK/MPSK) GMSK

ACF  20 ms

RX mode  source I/Q record

Perfect by the quality example of TETRAPOL. Sent by foregin collegues.

An example of TETRAPOL (two channeles). Sent by foregin collegues.

Sonograms:

pic.1 General view

pic.2 Manipulation speed

Diagrams:

pic.3 The graphical spectrum

pic.4 Allocation of the channeles

 Pictures:

pic. 5 Phase constellation

 pic.6  Transitions in the constellation

pic.7 The signal's ACF

TETRAPOL - an old enough system of digital trunking communication/connection, which had gotten wide enough spreading. The standard is developed in the mid-eighties the last century. Usage of GMSK modulation with the shaping/forming filter of the Gauss with BT = 0.25 is very interesting. I Will remind, GSM uses the filter of the Gauss with BT = 0.3 and has compact enough spectrum, reduction of this parameter allows to narrow further the spectrum, but as a whole, it does already noticeably reduce interference immunity or restricts/reduces the zone of confident communication. It Was unexpected enough to find out, that on the signals sent by foreign colleagues BT is even less than 0.25, that even more worses interference immunity.


Nevertheless, in TETRAPOL standard it is marked, that it is possible use both channeles as with band 12.5 kHz (most likely it is with BT = 0.25), so in case of need, with a band 10 kHz, and it already implies smaller BT values.


Considering that the standard isn't oriented on commercial networks, and intended for the organization of communication services of public safety, then some lowering of a confident cover zone isn't serious lack according to the developers opinion.