RF over Fiber and Ka-Band Diversity
Demand for more capacity in satellite communication continues to drive the industry to use High Throughput Satellites (HTS) at higher frequencies such as Ka band. However, higher frequencies, are susceptible to rain attenuation – also known as rain fade. This article compares the following approaches to mitigate the impact of rain fade on a Ka band service:
- Uplink power control (UPC)
- Adaptive coding
- Antenna site diversity via RF over fiber.
What is rain fade?
Rain fade is considered to be either of the following:
- The rain droplet absorbs the signal energy passing through it
- The signal is refracted or diffracted therefore changing the direction of the radio wave.
Rain affects higher frequency signals typically >11GHz because these wavelengths are close to the size of a rain drop.
Solutions for dealing with rain fade:
(i) Uplink power control (UPC)
Increasing the power of the transmission is the simplest solution. However, there are a number of drawbacks:
- Risk of causing interference with other users
- Restriction on the amount of power that can be transmitted
- Transponder amplifiers operate near saturation so only a limited range of power variation is available
- Amplifiers are non-linear, therefore the required extra headroom requires over-sized amplifiers
- Higher power signals may overload receivers.
(ii) Adaptive Coding
DVB-S2 allows the coding of the data to be dynamically altered as rain affects the signal. This optimizes the use of available bandwidth. In practice, this means that higher order modulations schemes possible in good weather conditions support a high throughput but poor weather conditions will allow a low throughput. Whilst this doesn’t mitigate all the issues, it does allow the optimal use of the bandwidth available. In poor weather you will get at least some data.
(iii) Site Diversity
Geographic site diversity means a satellite operator choosing between sites for optimal transmission conditions. Weather affecting Ka-band signals is typically different to the weather 50-100km away at the same point in time. Even in large storms covering multiple sites, the look angles will be different – giving the option to choose the least affected site. Such antenna diversity can be achieved using long distance RF over Fiber. Due to the very low losses of optical fiber, a single L-Band HTS link can cover up to 100km. The use of Dense Wave Division Multiplexing (DWDM) means up to 80 signals can use the same fiber, though this does reduce the maximum distance without amplification.
Due to the distances involved and the possibility of the signals going over dark fiber, that may contain many patches and splices (which all add loss to the system) there is a need for high power lasers as well as EDFA’s to further increase the power into the receiver. Whilst it is not imperative that DWDM technology is used, most high power lasers are DWDM capable and the technology allows you to reduce the number of leased dark fibers to a single strand therefore saving the operator significant operating budget though lower leasing costs. Also the further the RF is carried over the fiber the more risk there is of chromatic dispersion, which can cause shifts in phase. Therefore a dispersion compensation technique is also needed to ensure the same signal is received at the distant end.
The system shown in the Ka-Band diversity diagram is bi-directional transmit and receive, which means that the EDFA’s need to be positioned after the demux due to them being uni-directional, the other key thing to note is that a delay technique is needed to ensure that – for loss and time continuity during switch over – both signals arrive at the receive end with the same loss and at the same time, regardless of path taken.
In areas of the world that are more prone to rain fade conditions such as Asia, where traditionally much lower frequencies have been used such as C-Band, it may also be necessary to increase the distance between the diverse locations up to 500km in the future as the RF over fiber manufactures increase their range.
Diverse antenna systems are arguably the most effective way to mitigate the effects of rain on a Ka-band satellite service. However this technique is only viable for larger operators with multiple sites and sufficient financial resources. Therefore, the most appropriate solution is dependent on the circumstances of the application and in practice all of these approaches are likely to be seen in the field.