Symbol Rate (Custom IQ)

The Symbol Rate parameter sets the symbol rate for the VSA's custom IQ demodulator. Set this parameter to match the symbol rate of the system.

A symbol may consist of one or more bits. For example, in a BPSK Binary phase shift keying - A type of phase modulation using 2 distinct carrier phases to signal ones and zeros. system, each symbol represents 1 bit; in a QPSK Quadrature phase shift keying system, each symbol represents 2 bits.

Symbols are valid only at the timing instant when the receiver interprets the signal. This timing instant is called the detection-decision point.

The VSA's demodulator uses the Symbol Rate to determine the frequency of the detection-decision points. It is important that the Symbol Rate is set to match exactly the symbol rate of the system because the symbol clock frequency is not estimated.

For some satellite formats, there are different symbol rates transmitted on I vs. Q. For analysis of such a signal, you should enter the highest symbol rate. Currently separate demodulation of I vs. Q is not supported by option BHK.

The more complex the constellation, the more critical it is that the Symbol Rate be exact. Specifying an incorrect symbol rate introduces errors into the demodulation process.

If a Symbol Rate that is slightly different than the symbol rate of your signal is entered, the EVM Error vector magnitude (EVM): A quality metric in digital communication systems. See the EVM metric in the Error Summary Table topic in each demodulator for more information on how EVM is calculated for that modulation format. (error vector magnitude) is typically small at the center of the Result Length and increases linearly towards the ends of the Result Length.

Symbol Rate and Frequency Span

The Symbol Rate determines the maximum Frequency Span (information bandwidth) that can be measured. For QAM Quadrature Amplitude Modulation and PSK Phase Shift Keying: A broad classification of modulation techniques where the information to be transmitted is contained in the phase of the carrier wave. signals, the Symbol Rate also determines the minimum Frequency Span that achieves published specifications.

The minimum span is dependent on the filtering. The absolute minimum span is about 1.1 * symbolRate for a signal that is filtered to limit out-of-band emissions – such as common cellular and terrestrial formats such as WiFi, GSM Global System for Mobile Communications: Originally developed as a pan-European standard for digital mobile telephony, GSM has become the world’s most widely used mobile system. It is used on the 900 MHz and 1800 MHz frequencies in Europe, Asia and Australia, and the 1900 MHz frequency in North America and Latin America., CDMA Code Division Multiple Access: One of several digital wireless transmission methods in which signals are encoded using a specific pseudo-random sequence, or code, to define a communication channel. A receiver, knowing the code, can use it to decode the received signal in the presence of other signals in the channel. This is one of several "spread spectrum" techniques, which allows multiple users to share the same radio frequency spectrum by assigning each active user an unique code. CDMA offers improved spectral efficiency over analog transmission in that it allows for greater frequency reuse. Other characteristics of CDMA systems reduce dropped calls, increase battery life and offer more secure transmission. See also IS-95., CDMA2000, 1XEVDO, WCDMA, LTE. The recommended span ratio is approximately1.25 * symbolRate for these cases.

For typical satellite transmissions where filtering is less important, the minimum span ratio may be many multiples of the symbol rate. In general the span should encompass 99% of the transmit energy (99% OBW Occupied bandwidth is closely related to channel power. It indicates how much frequency spectrum is covered by some given percentage (often 99%) of the total power of the modulated signal.).

Larger spans are better in general

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where: alpha is the roll-off factor set with Alpha / BT

For all other signals, there is no "easy" formula to help determine the minimum span that achieves published specifications. However, it can be ensured that the measurement achieves published specifications by selecting a Frequency Span that contains all energy of the transmitted signal.

To see if your signal falls within the current frequency span, view the spectrum of the signal. If necessary, modify the Frequency Span until it includes all of the signal's energy.

To achieve best results, set the span to capture all of the signal’s energy.