by Douglas Chun, Norsat International
C-band on the Rise
Satellite services are being severely disrupted by interference from new Broadband Wireless Access (BWA) networks operating in the C-band frequencies from 3.4-4.2 GHz. Traditionally, this range of C-band is used for satellite services, radar, and microwave links; however, with the terrestrial wireless industry’s planned introduction of new mobile phone networks (such as LTE, Wi-Max, and 5G), these C-band frequencies will now be shared amongst a wider range of services.
The popularity of C-band networks is already on the rise, with mobile and base stations for terrestrial wireless applications emitting signals simultaneously from multiple locations and in all directions. These signals are powerful enough to saturate the sensitive C-band satellite receiving systems, causing a potential for total loss of service. It was reported by the Office of the Telecommunications Authority in Hong Kong that the use of frequencies for terrestrial wireless services in C-band was not practical due to interference risk. The Asia-Pacific Broadcasting Union, a regional broadcasting professional association, has warned that, “BWA is a promising technology. However, if implemented in the same frequency bands as the satellite downlinks, it will have an adverse impact and may make satellite operation in the entire C-band unworkable.” With the recent boom in mobile phones and Wi-Max equipment, what are the implications for C-band?
In 2008, the Satellite Users Interference Reduction Group (SUIRG) conducted formal field tests on the compatibility of fixed satellite services and Wi-Max services sharing the C-band spectrum. Wi-Max equipment operates in the 3.65-3.7 GHz band, which borders the lower end of the satellite C-band frequencies. The studies showed that most interference in C-band occurs in the 3.4-3.7 GHz range. The testing also showed that the Wi-Max transmit signal could cause significant problems to a satellite digital signal at a distance of more than 12 kilometers. Frequency Selective Surface antennas cannot coexist with Wi-Max system ranges between 50 and 200 kilometers. The affected distance depends on the local terrain and the Wi-Max output levels.
Sources of Interference
C-band frequencies have been assigned for satellite downlinks for decades. With new BWA networks sharing the same C-band spectrum, thousands of VSAT systems may suffer substantial interference. The services in large areas covering intercontinental and global communications provide a wide range of services for distance learning, telemedicine, universal access, disaster recovery, national security, air navigation and safety, e-government, and television transmission in many tropical regions, such as India, Indonesia, and other South East Asian countries. The interference caused along the C-band frequency has already impacted several service providers who had stable transmissions but now must deal with the newly created satellite interference. Since BWA shares the same frequency band as C-band satellite downlinks, the satellite service operators sometimes avoid using frequency spectrum below 3.7 GHz. However, even with this change, the complete issue of interference in the receiving system cannot be resolved as the BWA signal can still induce interference into the LNB/LNA (Low Noise Block Down Converter/Low Noise Amplifier).
Not only does the addition of BWA interfere with satellite downlink signals, it also affects aircraft radar altimeters as it is assigned the frequency band 4.2-4.4 GHz in the microwave C-band (as shown in Figure 1).This frequency band typically has a center frequency of 4300 MHz which is easily picked up by VSAT ground networks located by airports. With new players on the C-band spectrum, satellite and radar users must find ways to reduce the satellite interference and continue to provide effective services.
Reducing Satellite Interference using Bandpass Filters
A C-band bandpass filter is commonly used by satellite receivers to remove sources of interference from signals at frequencies in the C-band which are close to its own frequency of operation. Examples of interferers are the Wi-Max signals and signals from the C-band radar. If these spurious signals are not removed, they can cause problems such as intermodulation or in some cases, LNB/LNA saturation. In the worst case, an LNB/LNA can be permanently damaged by strong spurious signals.
Simple filters often have inadequate performance, while more complex filters can be too difficult to properly specify, fabricate, and tune. A good filter is built with numerous stringent filter requirements to separate channels or to block adjacent bands. These filters may require a broad passband with low insertion loss, steep skirts that roll off quickly, and high stopband rejection, as shown in Figure 2. Ideally, the filter has close to zero dB loss over a broad passband and more than 80 dB of stopband rejection. It is always a great challenge for a designer to make a filter with 0 dB loss and steep skirts cut off beyond the passband. Advanced filter design techniques have been developed to meet these requirements. For instance, elliptical filters allow for the adjustment of both the passband ripple and stop band attenuation as a means of decreasing the transition bandwidth. Chebyshev filters have steeper skirts at the cost of some ripple in the passband loss. Its transition band can be made narrower by adding ripples to the pass band.
Despite much filter design methodology, there are few waveguide C-band bandpass filters available in the market which are used to eliminate spurious signals. Waveguide filters represent sustainable solutions for the systems where high power and low losses are required. Waveguide filters can be implemented in different ways depending on their waveguide structure modification. It is up to the user to carefully choose the right filter for the application.
Evaluating Bandpass Filters
The performance of the bandpass filter can be characterized by three major parameters (see Figure 2): center frequency, insertion loss, and bandwidth. Center frequency is the frequency at which the amplitude is at maximum; insertion loss is the maximum amplitude response occurring at the center frequency; and bandwidth (or passband) is the frequency range between the -3 dB points located on either side of the center frequency. Table 1 shows the typical specifications of the C-band filter of 3.7-4.2 GHz pass band. Figure 3 is the physical representation of a C-band filter. Figure 4 is a graphical plot of a typical C-band bandpass filter frequency response.
Since the interference can occur at any frequency point within the range 3.4-4.2 GHz, the problem cannot be resolved by simply using an off-the-shelf filter. A custom design filter is recommended as it can be designed to fit the specific requirements of that system. As well, the three critical parameters—center frequency, bandwidth, and insertion loss—must be well defined before the design process begins.
Other considerations include choosing the filter’s length and weight. The filter must be installed between the feed horn and the LNB, meaning a long-length filter would need to adjust the boom arm distance to align with the antenna focal point. Also, weight must be considered when choosing a filter as the heavier a filter becomes, the increased likelihood of the boom arm to be overloaded.
There are only a few manufacturers that make C-band LNBs with an input frequency range from 3.7-4.2 GHz. Examples include Norsat’s 8000R, 5000R and 3000C series (Figure 5). This type of LNB has a built-in filter to eliminate spurious frequency below 3.7 GHz, and while this is a cost-saving solution, adding an external C-band filter may still be inevitable for dealing with strong interference signals.
Selecting Bandpass Filters
When selecting a C-band bandpass filter for satellites, the main considerations are as follows:
The center frequency of the filter should be close to the mid receiving frequency of the system
The insertion loss should be as low as possible. Narrow bandwidth makes it more difficult to achieve low insertion loss.
The filter must be able to completely block the signals from Wi-Max, radar and other unwanted frequencies
Reasonable length and weight of the filter
The waveguide connection dimensions match with the LNB input port
The filter must be powder coated in either white or a specific color, depending on the environment
It must be sealed and moisture resistant
The interference frequency must be known and measured beforehand to determine the specifications for center frequency, bandwidth, and insertion loss.
Reducing Satellite Interference Using Other Methods
Occasionally, interference can occur at any frequency point. Spurious frequencies generated by machines, radio transmissions, and power lines can also seriously affect the intermediate frequency (950-2150 MHz) of the LNB. In this case, intermediate frequency filters (L-band filter, Figure 6) are used to eliminate the unwanted frequencies. Figure 6 shows one type of ceramic L-band bandpass filter. Typically, these filters are a custom design but they are becoming more prominent in the market due to the fact that interference can occur in any environment and condition, such as improper placement of the equipment, and poor cable grounding. These conditions can cause spurious signal leaking into the equipment in the absence of a proper L-band bandpass filter.
There are many different types of L-band bandpass filters in the market today. Depending on the application, one design of an L-band filter may be better than another. For example, L-band cavity filters are designed to function in applications where low insertion loss and superb performance are required. L-band filters with cavity structures are typically known for low insertion loss, but they should also be known for their high power handling ability. The design of these filters helps reduce overall size and increases its peak power handling.
C-band satellite services are popular because of their lower overall equipment cost and minimal susceptibility to rain fade, especially during disaster recovery in tropical areas. For these reasons, satellite C-band will continue to play an important role in long-range communications and will likely not be replaceable by other frequency bands. Protecting the C-band spectrum from the threat of interference posed by the shared usage of broadband wireless access services is of paramount importance. In order to keep satellite signals clear, the VSAT locations need to be restricted in order to prevent interference with the terrestrial microwave communication system. With the current advantages of C-band VSAT and the techniques to prevent satellite interference, C-band VSAT is anticipated to continue growing in this spectrum. Although there is no clear answer to how this space should be used, it is known that C-band networks will remain a common choice for satellite users.
About the author:
Douglas Chun of Norsat International has over 30 years of experience in the RF/microwave field. He started his career as an Associate RF Engineer in Motorola’s Semiconductor Components Group, where he worked alongside both technical engineers and business managers. Douglas soon became the Strategic Business Manager of the RF Division and oversaw product development, manufacturing, and production control. In 1994, Douglas moved from Hong Kong to Canada and joined the Norsat International team.