The Importance of Dynamic Range in Fiber Optic Links
Traditionally satellite transponders were 36 MHz wide and Spurious Free Dynamic Range (SFDR) of an RF over fiber link was less important. Ultra-wide dynamic range fiber optic links were a “nice-to-have” because the same product could be used for both the up and down links. However now, with High Throughput Satellites (HTS) which have transponder bandwidths of 500 or 800 MHz, the signals have increased hugely and consequently so have the demands on dynamic range. This means that ultra-wide dynamic range fiber optic links have become a “must-have” for anyone transporting HTS signals.
Within this document we compare the SFDR and real dynamic range obtained with HTS satellite signals, and demonstrate why RF over fiber links used in satellite teleports should have a SFDR of 110 dB/Hz2/3 or better for 500 MHz wide signals.
As the satellite industry moves towards increasing mobile data capacity, through the use of new satellites and wider channel bandwidths, the demand on higher link dynamic range has increased.
Calculating Spurious Free Dynamic Range
SFDR is a measure of the dynamic range of a fiber optic link and is relative to 1 Hz of bandwidth – which is not particularly relevant as most signals are much greater than 1 Hz.
To calculate SFDR you must use the thermal noise floor (kTB), the noise figure (nf), and the third order intercept point (IP3).
The thermal noise floor is -174 dBm/Hz.
The noise figure is the signal to noise ratio (SNR) degradation caused by the fiber optic link and is a product of the design of the link – it is usually stated in the supplier’s data along with the P1dB which will allow you to calculate SFDR.
The 1 dB compression point (P1dB) is the power level at which the gain drops by 1 dB from its small signal value.
The third order Intercept Point or IP3 is a theoretical point at which the third-order distortion signal amplitude equals the input signal. In a fiber optic link this is typically 12 dB above the P1dB. The calculation for SFDR is:
SFDR = 2/3((P1dB+12) – (Thermal Noise Floor + Noise Figure of the link))
ViaLiteSFDR = 2/3((-1+12) – (-174+20)) = 110 dB/Hz 2/3
Therefore the SFDR is affected by the P1dB and the nf of a fiber optic link and is a useful measure to compare the performance of fiber optic links, when looking at dynamic range.
The Dynamic Range (DR) is the difference between the minimum and maximum signal that you can put through a link for a given traffic bandwidth. The minimum detectable signal (MDS) is usually assumed to be just above the system noise floor and the maximum signal level is the largest peak power without distortion. Unlike gain or noise figure, dynamic range cannot be improved using amplification, but the window can be moved to suit higher or lower signals.
Calculating Minimum Detectable Signal (MDS)
The calculation for MDS is:
MDS = Thermal Noise Floor+10log(traffic bandwidth in Hertz) + Noise Figure of the link
Typically High Throughput Satellites (HTS) have a traffic bandwidth (BW) of 500 or 800 MHz. ViaLite MDS for a 500 MHz bandwidth is:
ViaLite MDS for 500 MHz BW = -174+(10log(500×106)+20 = -67 dBm
P1dB, Minimum Detectable Signal and Dynamic Range
As described above, the dynamic range is the difference between the MDS and the P1dB. So, for a ViaLite fiber optic link with a traffic bandwidth of 500 MHz the dynamic range is:
DR = P1dB-MDS
ViaLite DR = -1 dBm-(-67 dBm) = 66 dB
The dynamic range as described above does not take into consideration any Signal to Noise Ratio (SNR) or any back off (signal power reduction) to meet Intermodulation Ratio (IMR) requirements for higher order modulation schemes. These are normally customer or application specific but need to be taken into consideration – especially if the fiber optic link has a narrow dynamic range.
To calculate the true usable dynamic range, the SNR in dB needs to be added to the MDS and, to ensure no distortion occurs at the maximum input signal, signal levels may need to be reduced to meet IMR specifications – this further reduces the usable range.
If you take a typical IMR of 40 dBc, the maximum signal will be reduced by 8 dB and, at a typical SNR of 20 dB, the minimum input signal is increased by 20 dB as shown below:
Usable DR = (P1dB-back off) – (MDS+SNR)
ViaLite DR = (-1 dBm-8 dB) – (-67 dBm+20 db) = 38 dB (Usable Range – Max -9 dBm to Min-47 dBm)
The following table shows a comparison of Dynamic Range/Usable Dynamic Range based upon the SFDR of different brands of fiber optic link products.
|ViaLiteHD HWDR S2||ViaLite||Brand D||Brand E||Brand F|
|DR at 500 MHz*||73.3 dB||65.8 dB||61.3 dB||50.8 dB||32.8 dB|
|DR to meet 20 dB SNR and 40 dBc IMR**||45.3 dB||37.8 dB|
(-46.8 to -9 dBm)
|33.3 dB||22.8 dB|
(-40.8 to -18 dBm)
(-24 to -28.8 dBm)
** At 800 MHz the dynamic range is 2 dB narrower and 5 dB narrower for full-band (1500 MHz)
What does this mean for RF over Fiber links in HTS ground stations?
HTS satellites use more bandwidth and this means that there is a need for higher dynamic range RF over Fiber links. Fiber links that have an SFDR of 100 dB/Hz 2/3 or less are difficult or impossible to use in HTS applications.
The ViaLiteHD L-Band HTS Hyper Wide Dynamic Range (HWDR) links have been designed for those needing an even greater dynamic range. The Series 2 (S2) has an SFDR with an extra 5 dB/Hz over the standard ViaLite product. The increase in SFDR allows an extra 5 dB of dynamic range for High Throughput Satellites (HTS) transponder bandwidths of 500 MHz, 800 MHz or even 1500 MHz. This allows customers to improve intermodulation performance and reduce the minimum signal that can be detected – extremely important in HTS applications.
For further information on specific dynamic range requirements or calculations, please email: firstname.lastname@example.org.