RADIO PAGING TECHNOLOGY
900 MHz FLEX™ Compared To UHF POCSAG
Okamura and others have demonstrated that frequency is a factor in path loss at VHF/UHF. At distances of from 1 km to 10 km or so, the increased path loss at around 930 MHz relative to that at 454 MHz would be between 2-3 dB on average. (See for example: W.C. Jakes, ed., Microwave Mobile Communications (IEEE Press, 1974) pp. 101-102.
of data at the receiver is a strong function of the "contrast
ratio" or the ratio of Signal Energy per Bit to
the Noise Energy per Bit, written
This quick calculation gives POCSAG 1200 about a 10 dB advantage over FLEX 6400.
The basic equation for received signal strength at VHF/UHF is:
That is, received signal strength is proportional to the Power transmitted, the antenna gain at the transmitter, the antenna gain at the receiver, the squared height of the transmit antenna, the squared height of the receive antenna and inversely proportional to the distance between transmit and receive antennas to the 4th power. Assuming all other factors in this equation remained constant, increasing the RSS by 10 dB would be equivalent to multiplying it by a factor of 10. That would yield a relationship like the following:
So, or approximately 2.
That is, the distance at which coverage would might a specified reception criterion would be about doubled for operation with the same infrastructure running POCSAG 1200 versus FLEX 6400.
This measure is quite consistent with the practical reductions in the coverage maps of carriers who have gone from POCSAG to high speed FLEX.
Non-linear factors and Interference Suppression
POCSAG is a binary Frequency Shift Keying (FSK) system. FLEX 6400 is a 4-level FSK system. That is, in POCSAG, to send a binary "0", the carrier frequency is offset to -4800 Hz relative to the carrier center frequency; and to send a binary "1", the carrier frequency is offset to +4800 Hz relative to the carrier center. In FLEX, the set of 4 signaling frequencies is ±4800 Hz and ±1600 Hz. Hence, there is less frequency separation between the distinct symbols in high-speed FLEX (3200 Hz) relative to POCSAG (9600 Hz) by a factor of 3. This parameter is called the modulation index in engineering literature on Frequency Modulation. A larger modulation index corresponds to an improved Signal to Noise Ratio at the receiver. This factor is worth about another 4-5 dB in the path loss budget.
To keep pager receiver designs simple, most device manufacturers construct receivers that map these received signal frequencies onto electrical signal voltage levels. For example, +4800 Hz might map to +1 Volt and -4800 Hz will map to -1 Volt. So POCSAG receiver logic will assume that it has received a binary "1" if it tests the voltage to be greater than 0 Volts, or assume a binary "0" if it test the voltage to be less than 0 Volts. A POCSAG receiver, or any binary receiver for that matter, can achieve this aspect of design quite easily by applying so much gain to the incoming signal that any small value greater than 0 Volts is "driven to the positive rail" while any small value less than 0 Volts is "driven to the negative rail", where the positive and negative "rails" are the highest and lowest available signal voltages in the receiver, usually the power supply or battery levels. This design is often called a "limiter" and it has the advantage of being able to effectively erase noise and interference. This is sometimes called the "capture effect" and for a simple POCSAG receiver, "capture" may occur when the desired signal is as little as 2 dB above an interferer.
The internal design of a 4-level FLEX decoder is much more complex, since it must be capable of distinguishing between 4 distinct signal values. For example, if ±4800 Hz map to ±1 Volt, then ±1600 Hz will map to ±0.333 Volt. The design must now support a linear one-to-one relationship between received radio frequency and signal voltage in the detector. Any small noise variations or interference values that appear at the antenna now remain as signal voltage fluctuations in the detector. As a result, high-speed FLEX pagers have a much reduced capture effect relative to interference than do POCSAG pagers.
There are many kinds of interference that can limit proper signal recovery in a pager; we'll limit our attention to two; namely, inter-modulation distortion and simulcast delay spread.
Inter-modulation Rejection Ratio Inter-modulation occurs when two powerful transmitter signals combine in the receiver to produce an interfering signal at just the right carrier frequency to confuse the receiver. This commonly occurs when other transmitters are at the same offsets from the desired carrier; for example, if we wish to receive a signal at 454 MHz and there are two other transmissions at 454 MHz + 50 kHz and 454 MHz – 50 kHz. In this case, even though there is no interference being transmitted at exactly the desired carrier frequency, limitations in the receiver itself can produce an interfering signal by combining the two nearby carriers to generate one. The good capture effect of a binary POCSAG receiver can significantly limit the impact of inter-modulation on the performance of the pager. The measure of this effect is called Inter-modulation Rejection Ratio (IRR) and will generally be better for a POCSAG pager than for a FLEX pager at high speed. The impact upon the user of this parameter will be that the POCSAG pager will continue to function properly in areas physically nearby to the inter-modulating transmitter sites, wherever they may be.
Simulcast Delay Spread
All paging systems, whether FLEX or POCSAG, transmit precisely the same signal from multiple transmitters in a highly synchronized manner. This is called simulcasting. The benefit at the pager is that the average signal power received will be the sum of all of the received power from all of the simulcasting transmitters. In the case of a simple POCSAG receiver, with good capture effect, it might be more accurate to say the received signal will be that of the best transmitter—the one that is at least 2-3 dB hotter than all the others at the pager's exact location.
Now, at any given location for the pager, there will be a variety of distances between it and the simulcasting transmitters that it can receive signal from. This variety of distances will correspond to a variety of delays in the arrival time of signals from each transmitter at the receiver. Anyone who has watched a TV channel that was received over the air is likely to have seen this delay effect on the screen, where it shows up as "ghosts" in the image; each "ghost" being a slightly delayed copy of the TV transmitter's signal. Of course, in TV, the cause is of ghosting is a delay due to the signal bouncing off objects such as buildings, hills, or even airplanes passing overhead. This same "ghosting" effect in paging is due to simulcast and it is commonly called simulcast delay spread or SDS.
SDS can be a problem at the pager if the relative delay between the nearest strong signal and the furthest strong signal is more than about 25% of a symbol time. For POCSAG 1200, this time is 25% of 833 microseconds or 208 microseconds; and for FLEX 6400, it's 78 microseconds. For radio signals traveling at the speed of light, these times correspond to distances of 62 km and 23 km, respectively. It is much more likely for SDS effects to occur in FLEX 6400 therefore, than is POCSAG 1200 simply because it's much more likely for powerful transmitters to be within a range of 23 km (15 miles) than within 62 km (39 miles) of one another. When SDS does present itself within the pager, it shows up in a very similar way as "ghosting" on a TV screen; that is, it mangles the sharp edges of the received "1" or "0" bit. In a simple POCSAG receiver, this effect is significantly mitigated by the same limiter-detector capture effect already presented: so long as the "mangling" doesn't cause the signal to cross the 0 Volt level, the limiter erases it. In a FLEX 4-level receiver, this erasing is impossible; and SDS edge effects can cause serious degradations in receiver performance.
To compensate for the effect of SDS on high-speed FLEX pagers, the carriers that have deployed FLEX have been forced to modify the designs of their site antenna systems and reduce their contributions to the simulcast power budget. Another way to say this is that high-speed FLEX networks enjoy less benefit from simulcasting relative to similar POCSAG networks.
To the end user, SDS effects will show themselves as rather mysterious and sharp reductions in reception near the edges of the coverage region at ground level and within the coverage region (often within office towers where it is easier to "see" distant sites).
POCSAG 1200 at 454 MHz will enjoy a 10 dB gain relative to FLEX 6400 at 930 MHz from simple factors pertaining to path loss at the different frequencies at due to its longer bit times. It will also enjoy about a 4-5 dB benefit due to its higher modulation index. Because of the typical binary limiter-detector design of a POCSAG pager receiver, non-linear effects such as Inter-modulation Distortion and Simulcast Delay Spread will also be mitigated relative to their impact on high-speed FLEX systems.
Source: Wikipedia contributors, "POCSAG," Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=POCSAG&oldid=347720900 (accessed April 13, 2010).
FLEX, ReFLEX, FLEXsuite, and InFLEXion, are trademarks or registered trademarks of Motorola, Inc.
* CCIR Comité Consultatif International des Radio Communications (French: International Radio Consultative Committee)
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