June 22, 2016
The 600 MHz band plan is starting to take shape in North America. The spectrum, currently utilized by TV broadcasters, is being made available to mobile operators through a complicated reverse auction/forward auction process. For several years, various band configurations have been proposed and debated. On April 29th of this year, the Federal Communications Commission (FCC) released a document showing the initial spectrum clearing target of 126 MHz, which nets 100 MHz of paired spectrum for mobile operators. Figure 1 shows the band plan proposed by this latest FCC document.
An overview of the potential interference issues associated with the 600 MHz band were discussed in a recent post found on our sister blog, Interference Hunter. In this post, we are going to take a specific look at passive intermodulation (PIM) and its possible effect on this frequency spectrum. From a PIM perspective, operators in the 600 MHz band utilizing their own 10 MHz spectrum blocks (in isolation) should experience few PIM issues. If you look at the “I+J” block in figure 1, for example, the lowest order IM product falling in the I+J uplink band (generated by I+J transmit signals) would be IM13. In most cases, IM13 will be well below the receiver noise floor and not be strong enough to cause interference.
In today’s world, however, it is rare that individual operators are able to utilize their spectrum in isolation. Co-location on macro sites, as well as co-location within Distributed Antenna Systems (DAS), means multiple frequency bands are likely to be combined at the same site. As can be seen in Figure 2, IM3 signals that can cause significant interference are possible when multiple 600 MHz blocks are combined at the same site.
Interference Caused by IM3 Products
A possibly even greater concern with the new 600 MHz spectrum is IM3 products generated by a single operator may cause interference in the PCS 1900 and AWS 2100 MHz uplink bands. To understand this, we need to consider the math used for calculating the frequencies where IM products occur. Traditionally, we only consider the IM3 products that fall close to the transmit signals. In the case of the aforementioned I+J block, the IM3 products generated by this block fall in harmless locations, as shown in the table below.
When we account for all IM3 products, even those that fall far away from the transmit signals, the problem becomes evident. As shown in the tables below, a single 10 MHz block of 600 MHz spectrum is able to generate IM3 spanning 30 MHz in either the PCS or AWS receive bands!
Overcoming PIM Issues
In the field, there are two ways to deal with this interference. The first is to try to completely isolate the bands from each other. When following this approach, operators should not diplex 600 MHz signals onto existing PCS or AWS feed lines and should not deploy 600 MHz in DAS networks that already contain PCS or AWS. This approach is very limiting and will certainly lead to increased CAPX and OPEX cost for 600 MHz deployments.
The second approach is to carefully control the linearity of the system when building the site. PIM magnitude is only high when the system linearity is poor. A simple PIM test during construction will tell when the system is linear and good enough to allow multiple bands to peacefully co-exist on the same infrastructure. Instruments, such as the Anritsu PIM Master™ MW82119B battery-operated, high-power portable, passive intermodulation analyzer, are well suited for field PIM testing. It includes important features, such as distance-to-PIM (DTP), enabling technicians to quickly and efficiently verify system linearity.
Anritsu has published an application note, Understanding PIM, that outlines causes of passive intermodulation, its effects, and how to prevent it. This comprehensive guide is available through a free download.