A General Equation for the Blocking Problem

With a problem like blocking the engineer has to come up with an equation to fit the details of the problem. A good approach is to start off with a general equation and work from there. We can work out what the equation should look like by considering the parameters we need.

To do this I’ll start at the radio being blocked, the victim, and work backwards. At some particular power the performance of the victim receiver will fall due to blocking. Let’s call that X.

X = Blocking power in dB at the victim receiver which will affect the system performance.

How much power gets to the victim receiver though? Going from the victim receiver there will be a cable or feeding network, then an antenna.

L1 = Loss of circuits and interconnects between victim receiver and the antenna it is connected to.

Instead of going into great detail about the antennas here I’ll just define an aggregate figure…

C = Coupling between the antenna of the victim receiver and the antenna of the blocking transmitter. This is a coupling between the two antenna ports.

Then we have a loss corresponding to L2 at the other side…

L2 = Loss of circuits and interconnects between blocking transmitter and the antenna it is connected to.

And finally…

P = Power output of the blocking transmitter.

These parameters lead directly to the equation we need:
P – L2 – C – L1 = Y

If Y > X then there will be system performance degradation due to blocking.

The mismatch loss associated with the transmitter and receiver can be put into L1 and L2. All of the parameters I mention vary across frequency, the frequencies that are relevant here are those that the blocking transmitter produces.

The Problem with LTE

In my opinion the architects of LTE have made some mistakes, this isn’t uncommon, many new protocols have had problems in their early days. The system they have created is susceptible to front-end blocking in particular bands.

Consider the band layout:
* LTE band 7 will operate from 2500 MHz to 2690MHz. This is split into an uplink band from 2500MHz to 2570 MHz and a downlink band from 2620 MHz to 2690 MHz.
* LTE band 40 will operate from 2300 MHz to 2400 MHz.

In between these bands is the 2.4GHz ISM band, which runs from ~2.4GHz to ~2.5GHz, the exact frequencies of edges of the band depend on the country. Many systems use this ISM band, WiFi and Bluetooth use it. More obscures systems such as video transmission systems and proprietary wireless keyboard and mice use it.

The diagram below shows the situation:

Yesterday I described the front-end blocking problem in general terms. You don’t have to be an RF genius to understand the problem the LTE architects have created here. The front-end receivers for LTE band 7, LTE band 40, WLAN, Bluetooth are fairly wideband, they will each respond to signals from all of the bands I’ve shown in the diagram.

GSM and WCDMA are “one band at a time” radios. When a GSM radio is using the 900MHz band it doesn’t use the 1800MHz band, or any other band, at the same time. The baseband controllers switches the radio between the various bands, sometimes it does this so fast that it can receive from the 1800MHz band while in call on a 900MHz band. This is achieved by time-division though, the radio never uses both bands at the same time. LTE radios are the same, that means that if a device is transmitting on LTE band 7 then it will not be receiving on LTE band 40. There is little chance of blocking problems within the LTE system.

However, the other radio systems involved, such as Bluetooth and WLAN, are controlled by completely separate baseband controllers. Let’s suppose there is a cellphone that has LTE band 7 and Bluetooth. When the Bluetooth module transmits then that signal will be transmitted from the Bluetooth antenna and received by the LTE antenna from there it will then enter the LTE receiver. Let’s suppose the base-station is transmitting to that same LTE receiver at the time too. As I described in my earlier post on blocking the front-end is unlikely to be able to cope with both of these signals. This will probably result in the LTE information being destroyed. The same applies in the opposite direction, the LTE transmissions can block nearby WLAN and Bluetooth receivers, especially if both are integrated into the same device.

Front-End Blocking

If you’re an RF engineer and you haven’t heard of front-end blocking then you’re lucky. I suspect you will hear about it soon, during the last few years I’ve done a lot of work on this problem. The diagram below shows the basic problem. A radio receiver only needs to be sensitive to the band of frequencies that it must receive, there is no need for the radio to have any sensitivity to other frequencies. However, that doesn’t mean that a receiver will reject incoming signals that are out-of-band. Most designs of receiver front-end are sensitive to a much broader range of frequencies than the band in question.

Suppose that a signal is being transmitted in the frequency range marked in green on the picture above. That signal will be captured by the receiver front-end along with the wanted signal. Then the receiver may cause the two signals to interact garbling the information from the wanted signal. There are several different ways that this can happen in different parts of the receiver circuit, I may write about those another day.

In this scenario the interfering signal isn’t in-band, it’s out-of-band, but it still causes a problem. For the sensitive receivers used in modern digital communications systems the power of the interfering signal doesn’t have to be high to cause reduced performance or complete link failure.

There are six general approaches to mitigating this problem:

• Move the receiving antenna away from sources of interference
• Break the path between the source of interference and the receiving antenna
• Make the receiving antenna more narrowband
• Add in a band-limiting filter between the antenna and the receiver
• Make the receiver less sensitive to out-of-band signals
• Change the receiver design so that the wanted signal and out-of-band signals donâ€™t interact or interact less

I’ve used most of these strategies, in the future I’ll write about them in more detail.