ReFLEX™ frequencies

I used to conduct training for my marketing staff every week while I was managing Motorola's international paging market development group. They all told me that they loved that hour that we spent together. I really don't think it was just the box of donuts that I usually brought, I think it was the "non-threatening" environment that we met in. These were professionals — most of them with MBAs — so they certainly weren't dumb, they just didn't have a background in technical topics. The problem was, they were embarrassed sometimes to ask a technical question in a room full of engineers. No one likes to hear, "What! — you mean you don't know that?" So my first goal was to make them feel comfortable asking anything about paging. I said that if I couldn't answer the question, someone in the building could. (This was the building where a large portion of the world's pagers were produced. Later torn down.)

One day, the market specialist for Latin America asked, "Brad I hear people talking about microwave equipment being used in paging and I always wonder what they are talking about. My wife has a microwave in the kitchen but I don't think they mean the same thing. Could you explain how microwave equipment is used in paging?" Well, I gave him a simple explanation and from then on he didn't have to feel embarrassed when the engineers talked about microwave links.

So my point is, there seems to still be a lot of confusion about how frequencies are assigned and used in ReFLEX two-way paging systems, and some of you may be too embarrassed to ask about this. Since you are not dumb, I will give you a fairly non-technical review of how this all works. First of all, the FCC and others refer to ReFLEX as "narrowband" PCS and this is confusing to paging people because the two-way channel assignments are usually wider than the one-way paging channels. They are "narrowband" only when you compare them with "wideband" PCS (telephone) channels, not to one-way paging channels.

The next confusing thing is how you should refer to the frequencies that ReFLEX uses. But first, let's review what radio frequencies are—just the basics—nothing too complicated. A radio frequency, or one cycle of a radio wave is measured in hertz, named after Heinrich Rudolf Hertz , the German physicist who first proved that electricity could be transmitted in electromagnetic waves. Back when I started in radio, we used the terms "cycles per second" or "kilocycles" or "megacycles." (I also played football with a leather helmet. Ha!) Now-a-days, the correct terms are "hertz" or "kilohertz" or "megahertz." In the following example, you can see how these three terms are related:

The frequency 929,662,500 hertz can also be stated as 929,662.5 kilohertz, or 929.6625 megahertz. They all mean the same thing; the decimal point just gets moved over three places as the name charges. To put all this in perspective, look at some examples: 1 megahertz is a frequency that you can receive on your AM car-radio, and 100 megahertz is a frequency you can receive on your FM car-radio. TV stations operate all over the place between 54 MHz and 806 MHz. Microwave ovens operate on 2,500 megahertz (2.5 gigahertz). Cellphones operate around 850 and 1,700 MHz. Now you know a little more about radio frequencies.

What then is the right way to specify a ReFLEX radio frequency? There are two ways you can do this. The first and traditional way is to refer to the center frequency of a radio channel. This is simply the middle frequency between the upper and lower channel limits. The other way is to specify the range by stating the lower frequency followed by the upper frequency. For example, following below you will find a graphical illustration of a typical 50 KHz ReFLEX forward channel. Forward means that this range of frequencies is used to transmit from a base station transmitter (some people say "the tower") out to the ReFLEX pager or telemetry device. So, you can say that this is a 50 kilohertz channel centered on 940.275 megahertz or you can say that this is a 940.250 to 940.300 MHz (megahertz) channel. Either way is correct.

ReFLEX technology puts three forward sub-channels into 50 KHz of bandwidth by using 12.5 KHz spacing instead of the traditional 25 KHz spacing used on most of the radio channels here in the USA. They leave a little empty space in the upper and lower limits of the channel to help avoid interference (splatter) on the adjacent channels. On the reverse channels, they squeeze 4 sub-channels into the same amount of bandwidth since these are the frequencies that the pagers transmit on, and their power is much less than a base station — so the chance of interference to other radios is much less.

The forward channel is the one the two-way pager listens to, and the reverse channel is the one it talks back on. Easy! OK, so here is how the forward channel is set up:

Now for the reverse channel, where four sub-channels are used instead of three. The correct way to refer to this channel is, "50 KHz centered on 901.275 MHz" or just "901.250 - 901.300." Either one is OK.

			Modulation speeds
			Forward channel:	6400 bps
			Reverse channel options
			Option 1:	800 bps
			Option 2:	1600 bps
			Option 3:	6400 bps
			Option 4:	9600 bps

(The following clarification was added.)

ReFLEX™ tutorial continued

(I get a little help from my friends.)

Following is some excellent supplementary information on ReFLEX. Several readers have sent in additional information and positive comments on the previous issue of TECH TIPS. We are fortunate that some of the industry's leading engineers, who work with these issues on a daily basis, have taken the time to contribute — for the better understanding of us all. I haven't included their names since I want them all to feel free to send in more comments in the future.

The previous information on ReFLEX was all about ReFLEX²⁵ since that is what I am most familiar with. I should have pointed out some of the main differences between ReFLEX²⁵ and ReFLEX⁵⁰ , but I have to be careful because both of these protocols are proprietary to Motorola and Motorola/SkyTel respectively. So, I am only reporting details that I believe to be in the public domain. I think both names, "ReFLEX²⁵ " and "ReFLEX⁵⁰ " were unfortunate choices . They should have been called "ReFLEX 12.5" and "ReFLEX 10" which more correctly describe their basic differences. In other words, ReFLEX²⁵ uses 12.5 kHz channel spacing and ReFLEX⁵⁰ uses 10 kHz channel spacing. Following are some clarifications to last week's information as adapted from an e-mail sent by an authority on ReFLEX who was kind enough to check my work:

Like FLEX, ReFLEX²⁵ can operate using the same three forward channel rates and two types of modulation. A difference between FLEX and ReFLEX, however, is that the maximum FLEX deviations are ±4800 Hz from the center frequency, while, as shown last week, the ReFLEX maximum deviations are ±2400 Hz. Because of these narrower deviations, three ReFLEX channels will fit within the NBPCS mask — also shown above. FLEX and ReFLEX both use the same interleaved (31,21,2) BCH coding*.

The reverse channel maximum deviations are the same as those of the forward channel: ±2400 Hz from the center frequency. The error correction on the reverse channel uses a shortened (31,23) Reed-Solomon code*.

The reverse channels for ReFLEX⁵⁰ are very similar to ReFLEX however, there are some differences. I can't say too much about ReFLEX⁵⁰ because it is proprietary to SkyTel and I have never had access to the details.

* Bose-Chauhuri-Hochquenghem (BCH) and Reed-Solomon (RS) are mathematical error detection/correction codes used in wireless data communications.

Another reader had this to say:

I thought your tutorial on ReFLEX was great. I would like to recommend a follow-up article to cover the aspects of ReFLEX 2.7 especially that ReFLEX also supports 9600 [bps] on the reverse path, and 4 sub-channels @ 10kHz spacing in a 50kHz forward path channel. We have moved past ReFLEX ⁵⁰ and ReFLEX²⁵ to ReFLEX 2.7 which merged the two versions. Maybe this should be noted in the next newsletter [so] the device manufactures [can understand that] they must support ReFLEX 2.7 with the different channel spacing and speeds.

Maybe someone would like to volunteer to write a summary of the features of ReFLEX 2.7.x that I could publish in a future issue of the newsletter?

Another:

Your ReFLEX charts were great. Don't forget the 10 kHz mode for ReFLEX 2.7, where we pack four 10 kHz subcarriers into one 50 kHz channel. This is the "Skytel" mode of operation.  All 2.7 devices, as well as the Advantra "Barran" and "Karli Lite" products, can seamlessly switch between 10 kHz and 12.5 kHz channels.

And another:

I enjoyed reading your ReFLEX for the non-engineer article. One point of clarification or amplification I would like to bring to your attention is that on the forward channel the modulation speed is not fixed at 6400 bps as your article tends to indicate. You can operate a ReFLEX²⁵ channel on the same speed increments as you can a FLEX channel. Therefore, your forward channel speeds could be 1600, 3200, or 6400 bps. Just a point of clarification, but I certainly believe you hit most of the high spots on ReFLEX for those folks that don't work with this day in and day out.

One other point, the reason that the FCC has dubbed the spectrum narrow band PCS is because the individual channel segments or what you call sub-channels are much narrower than a traditional paging channel. A traditional paging channel or any of the 2-way radio channels authorized in Part 90 of the FCC rules are allowed to operate on a 25 KHz channel spacing. This contrasts with either 12.5 or 10 KHz channel spacings for ReFLEX hence the connotation of narrow band. Since you are referring to ReFLEX²⁵ I will stick to that for now. Using ReFLEX 2.6 protocol the channel spacing on the forward channel is 12.5 KHz thereby giving you the ability to only have 3 carriers on the forward channel in a 50 KHz channel slice. However, ReFLEX 2.7 changes the bandwidth assignment to allow flexibility between ReFLEX²⁵ and ReFLEX⁵⁰ systems. The network operator can set his system up to either support 12.5 or 10 KHz channel spacing on the forward channels with all the reverse channels still operating on a 10 KHz basis. Essentially, a ReFLEX²⁵ network operator could get an additional channel or 25% additional channel capacity by utilizing ReFLEX 2.7 protocol and having 2.7 devices. If there are legacy 2.6 devices on the network, the 2.7 devices are backward compatible with the 2.6 protocol, but a 2.6 device cannot operate on a 2.7 system.

Again, I enjoyed the article keep of the good work.

Note: Baud, bits-per-second, and symbols-per-second are not all the same.