xMax and its deployment (part II)

A reader asked me a very interesting question. It seems that the xG Technology handsets are planned to be dual-mode and will have both xMax and WiFi. This is stated on the company web page among other places. So the question was: “How will one be able to tell if the handsets work on the xMax link or the WiFi link?” The direct answer is that the users will not know, unless the handset was designed to give an indication of which radio link is being used. Even then, most users will not pay any attention because they do not care how they are connected. However, what makes this question really interesting is what happens if a WiFi radio were to be co-located with the xG Technology basestation! The reason why this is so interesting is as follows.

In previous posts I have attempted to explain why xMax does not have a performance advantage over conventional communication systems. There is every reason to believe that the link budget of xMax is comparable to that of WiFi. Furthermore, contrary to popular opinion, a WiFi transmission can reach a very long distance under favorable propagation conditions (such as being placed on an 850 foot tower). In fact, all things being equal (transmit power, antennas, location, etc.), WiFi and xMax should have roughly the same range.

Consider the case where co-located with an xMax basestation there is a WiFi access point operating at maximum power. In this case an xMax handset may be as likely to receive the WiFi signal as it is the xMax signal. In fact, because the handset may lock on to one of the many local WiFi transmitters (hot-spots, residential access points etc.) , it may be much more likely that it will operate on the WiFi radio than on the xMax radio. This raises the intriguing possibility that a handset with the xMax radio completely turned off, may function nearly as well (or as poorly) as a handset in which the xMax radio is on! Unless the operator of the network releases statistics which report separately the performance of the xMax link and the WiFi link, it will be impossible to tell what is going on.

All of this makes me very curious as to the following: (i) Will xG Technology deploy a WiFi basestation co-located with each xMax basestation? (ii) Do the xMax handsets have an indicator showing which radio is being used? (iii) Will the operator of the network release information which will allow evaluation of the xMax performance? I do not know the answers to these questions, If any readers have some relevant information, please drop me a note.

The main point here is that it is may be possible to have an xMax deployment which produces no useful information about the performance of xMax! This it yet another compelling reason why xMax should be evaluated by standard laboratory tests, such as measuring the BER vs. Eb/N0 curve.

Let me emphasize again that I am talking specifically about evaluating the claim that “xMax can use 1,000 to 100,000 times less power than comparable transmission technologies.” What I am saying is that an xMax deployment in itself is not likely to provide a clear validation (or rather falsification) of this claim, because it involves too many unknown and uncontrolled variables. A simple laboratory test is a far more reliable way to get a definitive answer.

A technical comment to those interested.

A standard WiFi radio will not work over long distances even if the signal to noise ratio is high. This is because the transmitter expects an acknowledgment from the receiver within a time window which is set to accommodate short delays corresponding to the shorter ranges over which WiFi is normally used. However, it is straightforward to fix this by changing certain software settings, and this has been done many times to allow long range WiFi operation. The current record for WiFi is a 237 mile link! Because xG Technology designs both sides of the link, they could if they wanted to, allow the WiFi radio to operate over much longer distances than the standard WiFi you get in your local store. All it takes is a software change. I have no idea whether or not they are doing this. I certainly would expect them to do so if they intend to place a WiFi radio next to the xMax baseststion or as part of it.

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4 responses to “xMax and its deployment (part II)

  1. xMax would have one operational advantage over WiFi were its base stations co-located with WiFi base stations. xMax operates on the 902-928 MHz band, while WiFi operates on 2400-2483 MHz, and the path loss between isotropic (0 dBi gain) antennas is 8.5 dB less on 902 MHz than on 2400 MHz. The antennas used on most mobile phones are close to isotropic in performance. Base stations can use better antennas, but their gains are also limited if the provider wants to cover a wide area.

    So xMax has an advantage simply because of the frequency band on which it operates. However, my link budget calculations show that xMax probably requires a significantly higher Eb/N0 ratio to operate, so in the end the two systems on the two bands may be comparable.

  2. JimDeGries@gmail.com

    Yes, I realized the difference in propagation loss, and was assuming that xMax is probably less efficient. For example, reading the parts of Bobier’s patents which are related to the Wavelet Pass Filter makes it clear that he is not using a matched filter, which is bound to cause a loss of a few dB. However, my main point is that because the systems have similar performance (give or take a few dB) it will be very difficult to learn about the performance of xMax from anecdotal information about what happens in a deployment. A deployment is not likely to prove/falsify the statement “xMax can use 1,000 to 100,000 times less power than comparable transmission technologies.” A simple lab test will.

  3. Diedrik Bosmans

    >> A deployment is not likely to prove/falsify the statement
    I would think that those who have invested (and other potentially interested parties) will get full access with test equipment and engineers to verify performance.. From drive tests to verify radio planning to actually manipulating the handsets in a real life trial when others are in full control at the network side..
    But indeed, a simple lab test is the logical approach.

  4. JimDeGries@gmail.com

    It is, of course, possible to do careful tests on a deployed system which will provide very useful performance results. Whether or not these results will be publicly available is an open question at this time. My point was that having a deployment where users report that they are able to make calls, does not in itself provide a useful assessment of the xMax claim.

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