Symbol rate

In digital transmission technology and communications technology, the symbol rate or baud rate is the number of symbols transmitted per time period. The unit “per second” of this rate is called baud , or Bd for short . B. to be distinguished from the bit rate .

Connections

In each of the time steps considered here ( symbol duration ), one of several possible symbols is transmitted. So the symbol rate is ${\displaystyle T_{s}}$

${\displaystyle f_{s}={\frac {1}{T_{s}}}\,.}$

If there are more than two different symbols, see line code , each symbol carries more than one bit of information and the bit rate is higher than the symbol rate.

With the frequency spreading method, the state changes several times per symbol, with the chip rate which is higher by the spreading factor . Without frequency spreading, the step speed is the same as the symbol rate. With carrier-based transmission methods , the step speed is also called the modulation rate .

Under certain conditions, the required bandwidth is roughly equal to the walking speed. The bandwidth actually required depends, among other things, on the type of pulse shaping and possibly on the type of modulation and the modulation index .

Examples

As the following table demonstrates, the data transmission rate and symbol rate can differ in both directions. Values ​​just below 0.001 to 15 occur in practice.

Transfers with many time steps compared to the number of bits in a coded word reduce the data transfer rate compared to the symbol rate. Many possible levels or states increase the data transmission rate compared to the symbol rate or reduce the necessary bandwidth. Forward error corrections permanently installed in encodings reduce the data transmission rate, but allow more constellations.

Modulations can partly be coded together, partly these are not explicitly stated (so there is a PAM-2 behind almost all codings with 2 levels).

Note: Information on the data transfer rate is often embellished for advertising purposes. Common are:

• The ratio of data rate to symbol rate is not taken into account.
• Any forward error corrections included are not taken into account.
• With full duplex transmissions, the outward and return directions are added together.

code	Levels / states	Data rate / symbol rate	max frequency / symbol rate	DC-free	Taktrück- recovery	Error detection	Error correction	application
RZ code	03	1/2	1/2	No	Yes	No	No
NRZ code (PAM-2)	02	1	1/2	No	No	No	No	natively connected with separate clock signal ( PCI , RAM ...)
PAM-4	04th	2	1/2	No	No	No	No	PCI Express 6.0, Ethernet over glass fiber with 100, 200 and 400 Gbit / s
PAM-16	16	4th	1/2	No	No	No	No	Ethernet 40GBASE-T
(0,1) -RLL	02	1/2	1/2	Yes	Yes	No	No	Floppy disks, hard drives
(0.2) -RLL	02	4/5	1/2	No	Yes	No	No
(1,7) -RLL	02	2/3	1/2	some	Yes	No	No
(2,7) -RLL	02	1/2	1/6	No	Yes	No	No	Hard drives
EFM	02	8/17	1/6	Yes	Yes	partially	No	compact disc
EFMplus	02	8/16	1/6	Yes	Yes	partially	No	DVD
Manchester code or 1B2B code	02	1/2	1/2	Yes	Yes	No	No
4B5B code	02	4/5	1/2	No	Yes	No	No
4B5B code + MLT-3 code	03	4/5	1/4	No	Yes	No	No	Fast Ethernet
8B10B code	02	8/10	1/2	Yes	Yes	partially	No	HDMI up to 2.0 , PCI Express 1.x and 2.x , Serial ATA , USB 3.0 , DisplayPort and many more
16b18b code	02	16/18	1/2	Yes	Yes	partially	No	HDMI 2.1
64b66b code	02	64/66	1/2	reduced	Yes	No	No	PCI Express 3.0 to 5.0
64b67b code	02	64/67	1/2	Yes	Yes	No	No
DSSS of the GPS	02	1/1023	1/2	Carrier tape	Yes	No	No	Global Positioning System
DSSS + DBPSK	02	1/11	1/2	Carrier tape	Yes	partially	No	Original WLAN 802.11
DSSS + DQPSK	04th	2/11	1/2	Carrier tape	Yes	partially	No	Original WLAN 802.11
BPSK	02	1	1/2	No	No	No	No	Bluetooth
QPSK	04th	2	1/2	No	No	No	No	DVB-S
8-PSK	08th	3	1/2	No	No	No	No	Bluetooth , DVB-S2
16-QAM	16	4th	1/2	No	No	No	No	DVB-C
64-QAM	64	6th	1/2	No	No	No	No
4096-QAM	4096	12	1/2	No	No	No	No	DVB-C2 (max.), ADSL
32768-QAM	32768	15th	1/2	No	No	No	No	ADSL (max.)

literature

• Gottapu Sasibhushana Rao: Cellular Mobile Communication . Pearson Education India, 2013, ISBN 81-317-9861-5 .
• Karl-Dirk Kammeyer: message transmission . 4th edition. Vieweg + Teubner, 2008, ISBN 978-3-8351-0179-1 .