NZ615954B2 - Improvements relating to integration of mobile radio and paging systems - Google Patents
Improvements relating to integration of mobile radio and paging systems Download PDFInfo
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Abstract
Disclosed are methods of integrating land mobile radio (LMR) systems with paging systems. Paging messages are identified by way of predetermined symbols in a symbol stream which is modulated into an RF signal. The symbols may also enable synchronisation of the receiving station to the signal and/or convey individual status messages. Symbols are preferably derived from existing symbols in the mobile radio protocol. Multiple symbols may be used to represent individual bits in the message and thereby extend the range of transmissions in the system. convey individual status messages. Symbols are preferably derived from existing symbols in the mobile radio protocol. Multiple symbols may be used to represent individual bits in the message and thereby extend the range of transmissions in the system.
Description
IMPROVEMENTS RELATING TO INTEGRATION OF
MOBILE RADIO AND PAGING SYSTEMS
FIELD OF THE INVENTION
This invention relates to integration of land mobile radio (LMR) and paging, and
particularly but not only, to use of correlation sequences in a combined radio and pager.
BACKGROUND TO THE INVENTION
Conventional mobile radio protocols such as P25 and DMR, and paging protocols such as
POCSAG and Flex are used widely for communications, particularly in public services.
Paging remains a key technology for emergency service operations where it is commonly
used for calling out volunteers to react to emergency scenarios.
Mobile radio terminals use a wide range of communication protocols. They may be
narrowband RF transceivers for example, operating within a 12.5 kHz channel such as
provided in P25 APCO, DMR or analogue radio, or operating within a 6.25 kHz channel
as provided in dPMR. Alternatively they may be relatively wideband transceivers using
one or more 180 kHz resource blocks as provided in LTE.
P25 radio systems typically operate in 12.5 kHz analog, digital or mixed mode
using FDMA. Phase 1 radios use continuous 4 level FM (C4FM) modulation for digital
transmissions at 4800 symbols per second (ie. 4800 baud) and 2 bits per symbol, yielding
9600 bits per second for overall throughput on each channel. Phase 2 radios are being
developed using a 2 slot TDMA scheme.
A symbol is a waveform, a state or some other intended condition of a radio channel that
persists for a fixed period of time. A sending device places symbols on the channel at a
fixed and known symbol rate, and receiving devices have the job of detecting the
sequence of symbols in order to reconstruct the transmitted data. Symbols are typically
1/17
modulated onto an RF carrier to form and RF signal. There may be a direct
correspondence between a symbol and one or more small units of data ( eg. bits 1, 0) or
the data may be represented by the transitions between symbols or by a sequence of many
symbols. Existing paging technology sends information down to a recipient over a broad
geographical range using a low symbol rate. The family of POCSAG protocols can
operate at three speeds: 512, 1200 or 2400 bits per second. There are two message coding
formats defined for the information content part of paging messages. Numeric messages
are sent as 4 bit BCD symbols and alphanumeric messages are sent as 7 bit ASCII
symbols. The related Flex synchronous protocol achieves speeds of 1600, 3200 and 6400
bits per second
A paging recipient typically has to use an alternative method of responding to the page
such as a cell phone or landline. Some paging devices are capable of responding,
examples of the technology includes Reflex which includes a responding path or
alternatively, the use of GSM. Combined pager and cell-phone devices are described in
US 5701337 and US 8254970, for example.
Paging solutions provide an effective means of communicating small amounts of data to a
user or group of users. Traditional forms of paging technology offer very wide area
coverage often being sent via sites located at high points and using relatively high
powers. Such paging solutions may be private or commercial. Today, paging can be
accomplished using cellular technology. One example would be the use of 3G or 4G
possibly using a terminal end point that is a smart phone. The use of commercial cellular
is limited by its available coverage which is generally rolled out to cover populated areas.
The traditional form of paging however covers huge areas well outside commercial
cellular coverage. As a result, the traditional paging systems continue to be used in
specialised applications.
One such specialised application exists within the domain of emergency services. The use
of traditional pagers remains common for calling out emergency responders, such as fire
fighters, particularly in rural areas or in areas where the use of commercial cellular is
2/17
considered an undesirable operational expense. These pagers are used to alert the users to
pending jobs. Similar technology is also used in task based activity such as utilities where
pagers may be used to alert suitably qualified staff to attend specific jobs.
Generally, these response scenarios require both high reliability and are time sensitive.
The cellular network can become congested resulting in paged information taking a long
time to arrive. Alternatively in a disaster scenario, the cellular network may simply not be
available. Traditional paging networks can remain technologically favourable, given the
need for wide area coverage with time sensitive and reliability requirements.
Unfortunately, traditional paging systems are becoming less attractive from a business
case perspective for operators. In some cases, the specific user groups have deployed and
run their own networks. In other cases, commercial operators have either shut down or
are planning to shutdown traditional paging networks. LMR networks are also wide area
and cover similar geographic areas as one would expect to experience from a traditional
paging network. However, there has not yet been a successful integration of paging
services with LMR.
Traditional paging solutions offer a means of transferring small amounts of information
to a user on a downlink only. There is value however in enabling either an automatic or
user initiated response. Various methods which enable a communication back into the
paging network are known. A paging device is generally quite small leading to
limitations in both transmitter power and antenna efficiency. These limitations result in a
range issue. Specifically, the uplink side of such a paging system is the weak link.
SUMMARY OF THE INVENTION
It is an object of the invention to provide for improved paging techniques which integrate
into an LMR network such as P25 or DMR.
3/17
In one aspect the invention resides in a method of receiving a paging message at a station
in an LMR mobile radio system, including: receiving an RF signal from the system,
converting the RF signal into a stream of symbols, using one or more of the symbols to
synchronise the station with the RF signal, using one or more of the symbols to identify a
paging message in the RF signal, and converting the stream of symbols into a stream of
bits which represents the paging message.
Preferably the same symbols are used for both synchronisation of the station and
identification of the paging message. An initial part of the symbol stream may include a
correlation sequence which enables synchronisation of the terminal. A double length
sequence is preferably used for initial synchronisation.
Preferably the stream of symbols includes a correlation sequence which represents a user
status message. The symbol stream is also relatively long compared with the bit stream so
that each bit is represented by multiple symbols.
Preferably the symbol stream has a rate of 4800sps and the bit stream has a rate of
200bps. The symbol stream may have a rate of 480sps while the bit stream has a rate of
10bps. The symbols are preferably wave shapes in which the shape for bit 1 is inverted
relative to the shape for a bit 0.
In another aspect the invention resides in a method of transmitting a paging message in an
LMR mobile radio system, including: receiving a bit stream representing the paging
message, converting the bit stream into a symbol stream in which one or more symbols
identify a paging message, converting the symbol stream into an RF signal, and
transmitting the RF signal from a station in the radio system.
Preferably each bit in the bit stream is converted into multiple symbols in the symbol
stream. Preferably the symbols are wave shapes in which the shape for bit 1 is inverted
relative to the shape for a bit 0. The wave shapes may be derived from a mobile radio
protocol.
4/17
LIST OF FIGURES
Preferred embodiments of the invention will be described with respect to the
accompanying drawings, of which:
Figure 1 shows a typical traditional wireless paging system.
Figure 2 shows a traditional paging system existing in the same physical areas as
a standard LMR system.
Figure 3 shows a paging system with responder capability responding into a LMR
network.
Figure 4 shows a paging system based upon an LMR network.
Figure 5 illustrates use of correlation sequences to communicate data.
Figure 6 illustrates use of a correlation sequence for initial synchronisation and to
distinguish a standard P25 transmission from a paging transmission,
Figure 7 illustrates a paging transmit line up in relation to Figure 6,
Figures 8 illustrates a receiver needed to detect transmissions by the line-up in
Figure 7,
Figure 9 illustrates a correlation decoder.
Figure 10 compares waveforms in two correlator stages in the decoder.
Figure 11 represents a visualisation of the signals observed after correlation.
Figure 12 shows the modules that may exist in a responding pager.
Figure 13 is a flow diagram showing the operation of a transmitter.
Figure 14 illustrates the operation of a receiver to decode information
communicated according to Figure 13.
Figure 15 illustrates performance in line of sight conditions showing how the
paging algorithm operates compared to standard P25.
Figure 16 illustrates a performance in Rayleigh faded conditions showing how the
paging algorithm operates compared to standard P25.
Figure 17 illustrates a performance in delay spread conditions showing how the
paging algorithm operates compared to standard P25.
Figure 18 illustrates a performance in line of sight conditions for the message
error rate.
/17
Figure 19 illustrates message error rate in Rayleigh faded conditions.
Figure 20 illustrates message error rate performance in delay spread conditions.
DESCRIPTION OF PREFERRED EMBODIMENTS
Figure 1 illustrates a traditional paging architecture in which a message sourced from a
network component that may be a dispatch terminal in a Network Operations Centre is
transmitted via multiple sites to a traditional paging device 10. Typically, the message to
be communicated arrives at a Paging Encoder which then converts it into a form suitable
for transmission over the air. For example, in the case of POCSAG transmission, the user
message is packaged into a suitable structure, error correction added then converted into
an FFSK modulation which can then be converted to radio frequency transmission via an
analogue base station.
Figure 1 shows base sites 11, 12. In a paging system it is common to use the same
downlink frequency from each site which is known in the art as simulcast. This form of
transmission is made possible through tightly managed transmission time lock afforded
through the use of GPS as a timing reference. This timing reference is used to precisely
control the launch time of transmissions from each site such that signals arriving at a
paging receiver exist within acceptable timing boundaries.
Figure 2 offers a view showing how a typical P25 system may exist over the same
geographic area as a traditional paging system. Here, two sites 21, 22 support both a
traditional paging system transmitter 24 as well as a P25 transceiver 25. The other site 23
supports the P25 transceiver only. Implicitly the diagrams highlight how P25 may be
located at different sites to paging and the coverage areas may or may not overlap.
Further, in this particular case, the figure shows how both the paging system operating on
frequency f1 and the P25 system downlinking on frequency f2 exist in a simulcast mode.
The uplink frequency of the P25 system is f3. The use of P25 simulcast is shown here by
way of example whereas non simulcast may be used.
6/17
Figure 3 illustrates a modified paging device 30 which continues to receive traditional
paging transmissions (e.g via POCSAG) from base sites 31, 32, 33 but which is also
capable of transmitting either standard P25 or a modified version of P25 on the uplink.
Effectively this becomes a hybrid system using alternative systems on the downlink and
uplink. The uplink frequency used can be the same as a normal P25 channel (i.e f3)
whereupon the presence of a unique correlation sequence at the head of the transmission
can identify it as a paging signal. Alternatively, the transmission could occur on a
separate frequency (f4 not shown) in which case there may be no need at the base station
receiver to differentiate between this paging signal and any other type of transmission.
Figure 4 shows an embodiment in which the paging device 40 is capable of receiving and
transmitting from base sites 41, 42, 43 using the same method for both downlink and
uplink. In this illustration, standard P25 is used on the downlink and modified P25 on the
uplink. An alternative configuration may be the use of standard P25 on the downlink and
standard P25 on the uplink but only in the case where the coverage design is capable of
supporting the weaker uplink of standard P25 compared to the modified P25. The
illustration shows the use of frequency f2 for standard P25 on the downlink and f3 for the
transmission of the modified uplink. This is by way of illustration only as it is possible to
use any frequency pair or even to operate in a TDMA mode if such a structure exists in
the network. Although this implementation is based upon P25, more generally, the
approach could apply to other digital modulations such as DMR.
Traditional paging technology uses a low symbol rate. The use of a low symbol rate is
advantageous because the energy per symbol transmitted over the air is substantially
greater than using a high symbol rate. Generally the use of a low symbol rate results in
greater range. Simply lowering the symbol rate however exposes the receiver to the
effects of Doppler spread caused by mobility. A solution to this problem is to maintain
the underlying modulation rate of P25 whilst extending the energy of a single symbol
across a longer period of time. This can be achieved through the use of correlation
sequences. A correlation sequence may be relatively long compared to the underlying bit
sequence from which the symbols are formed.
7/17
Correlation procedures are used for a wide range of tasks in a mobile radio receiver when
the receiver is communicating with a base station. In a correlation process, the arriving
data is compared, in the form of sample values, with a sequence of symbols or other data
items, which are known in the receiver. If the arriving data matches the sequence of
known data, a correlation signal is emitted which indicates that the sequence of known
data items has been identified in the received signal. Correlator systems in a mobile radio
receiver are typically used for carrying out time-slot or frame synchronisation, and delay
estimation.
Figure 5 illustrates a correlator system in which the energy of a single symbol is extended
across a relatively long period of time. A bit stream is entering the system from a
transmitter 50 where a 1 is translated into a correlation waveform that is transmitted over
the air interface using a combination of shapes A and B. In this case the shape of the
wave form is transmitted representing a 1 by way of a unique correlation sequence shape
B, that happens to be the P25 sequence normally used for synchronisation. If the bit to be
transmitted is a zero, then shape A which is the inverse of the sequence for a 1 is used.
Conveniently, in this case, shape A represents an inverse sequence of the P25 sequence
normally used for synchronisation. At the receive side of the system, a correlator 51 is
searching for the waveforms used to represent either 1 or 0. Its output will peak
(correlate) when a wave form representing a 1 arrives as this represents the arrival of a bit
1. The same correlation filter structure can be used to detect the inverted waveform used
to represent a 0. Upon arrival of this shape, the correlation output will peak negatively
representing the arrival of a 0.
Figure 6 illustrates a further elaboration of the system in Figure 5, in which a unique
correlation sequence shape X is conveyed by transmitter 61 at the start of a stream of
symbols which form a transmission burst. This unique sequence is used at the receiver
side for two purposes, namely initial synchronisation and also the unique identification of
a paging signal as opposed to a standard P25 signal. A detector 62 signals either a paging
receiver 63 or a standard P25 receiver 63 according to the result of the correlation
8/17
process. Two independent sequences can be used for initial synchronisation and
identification, but it is convenient to use one signal for both. In an alternative form, a
unique correlation sequence could also be used to communicate a specific status message
such as “I am responding” or “I am available”.
Figure 7 shows a detailed signal processing structure needed to create the form of
transmission described in Figures 5 and 6, with a user message of 96 bits, by way of
example. This message is encoded using a ½ rate convolutional forward error correction
scheme 70 to produce a 196 bit encoded message. This error correction scheme is defined
in the APCO P25 standard. The encoded bit stream operates at a symbol rate of just
200bps and is translated 71 into a symbol stream, or unique correlation sequences or its
inverse. This is illustrated as Block A and Block B. In other words, a single bit is now
represented as a sequence of 24 symbols which forms the unique correlation sequence.
These blocks of symbols which operate at a rate of 4800 symbols per second are chained
and are passed through an encoding filter 72 which in the case of P25 is represented as a
raised cosine filer followed by an inverse sync. This filtered signal is them modulated 73
onto the air interface.
A standard P25 transmit line up operating at a base line rate of 4800sps thereby
communicates information at a rate of 200bps along with an associated boost in symbol
power and therefore range. The blocks containing symbols at 4800sps are passed through
an encoding Finite Impulse Response (FIR) filter whose coefficients produce a standard
P25 modulation. The stream of symbols are an analogue waveform which is then used to
frequency modulate a physical transmitter operating at an arbitrary carrier frequency
which is generally below 1GHz and commonly at very high frequency (VHF). In this
example the underlying modulation rate (M) is 4800sps and the bit rate for data
transmission (B) is 200bps, meaning the sequence used to represent a bit is 24 (i.e M/B).
Generally any value of M can be used and any value of B as long as the length M/B
yields a sequence that has a sufficiently strong autocorrelation property.
9/17
Figure 8 illustrates a receiver structure that could be used to decode the low rate
information being communicated by the transmitter shown in Figure 7. To begin, the
signal is down converted 80 from its carrier frequency to baseband In-Phase and
Quadrature. It is typical in practical implementations to have a residual carrier present.
This is typically removed 81 using a numerically controlled oscillator using a complex
multiplication process. The IQ signal is then translated into a frequency representation
through observing 82 the difference in phase over a known sample period. Undertaking
this process removes the amplitude dependency of the IQ signal. The signal at this point
now represents frequency and we convert this back 83 into a pseudo form of IQ with a
normalised magnitude. This signal can now enter a correlation algorithm 84 that operates
on normalised complex signals. The coefficients of a correlator are configured as the
known sequence representing the signal transmitted is the form of the standard P25
sequence while a parallel correlator is searching for the inverted sequence. The output of
the complex signal correlation is passed to a threshold algorithm 85 which detects either
positive going signals caused by a symbol correlation sequence or negative going signals
caused by the inverted symbol correlation sequence. This creates a bit stream that can be
passed to a forward error correction stage 86 where the original user message is decoded
and passed to the Paging application 87.
The coefficients of a complex FIR filter are the reversed version of the transmitted
correlation sequence representing a 1 - making the filter act as a correlator. The output of
the correlator is shown in equation (1) where subscript ‘c’ represents the coefficients
needed for initial synchronisation.
y (I jQ )(I jQ )
IN IN C C
(1)
(I I Q Q ) j(I Q Q I )
IN C IN C IN C IN C
The correlation filter can be visualised as two separate filter banks as shown in Figure 9,
one that produces the real part of the output, and one that produces the imaginary part.
The coefficients of the two banks are the same, except that they have been rotated by /2
radians.
/17
I Q I Q
C C d d
and (2)
Q I Q I
C C d d
Also shown in Figure 8 is an initial synchronisation detector 88. This represents another
correlation algorithm that is searching for the unique sequence that indicates the start of
the paging signal and is used to estimate where subsequent data carrying correlations can
be expected.
Also shown in Figure 8 is a paging signal detector 89. This represents another correlation
algorithm that is searching for a pattern (possibly the same as initial synchronisation) that
identifies the signal as paging. An affirmative indication of this can be used to switch
signal paths between a standard P25 receive line up 891 versus the paging path described
above.
Further, Figure 8 shows a paging message detector 892. This represents another optimal
(shown in dashed lines) correlating detector that is searching for specific waveform
shapes which are pre-defined as representing a known message. If detected the response
is passed to the paging application. One example is ‘I am responding’.
Figure 9 outlines the correlation process according to equation (1). Complex correlation
is indicated but real numbers only could also be used. A complex signal r , r ….. r
1 2 n
arrives at a differential phase process 90. Each complex signal is firstly converted 91 into
a phase angle (theta), the difference between two values of theta undertaken over a
known period represents frequency. This frequency representation is then converted back
into I and Q before the normalised IQ stream is passed into a complex correlation
process, one process 92 for detecting an initial sync/paging indicator and the other
process 93 for detecting data. Each correlation process contains two sets of coefficients
which represent the wanted signal where each set is pi/2 different to each other. The
reason for having a pair of correlation coefficients is to avoid a possible problem from the
impact of initial phase of the wanted signal.
11/17
Figure 10 shows how the output 100 of one bank of correlators in Figure 9 is dependent
upon the initial phase. The output reduces as the initial phase moves away from its
nominal position of zero pi. The output 101 from a second correlation bank whose
coefficients are pi/2 is compared to the first. Taking the square of the magnitude of each
bank then adding the result gives a constant correlation level irrespective of the initial
phase.
Figure 11 shows conceptually the combined output of the correlation process in Figure 9.
A strong initial synchronisation sequence results when the sequence used is twice the
length of the waveforms used to represent data. Once initial synchronisation is achieved,
a known period exists between this point and the point at which the first data correlation
is expected. The receiver uses a small time window to examine if a correlation appears at
the expected time. Following this, the expected time of data correlations is known and as
such the signal can be examined at each case to estimate what data was sent; either 1 or 0.
The example in Figure 11 represents the transmission of 1,1,0,1,0,1. Also shown is the
illustration is a false correlation. The process of using a window to examine expected data
minimises the chance of false correlations being interpreted as data.
Figure 12 shows a pager system that incorporates a paging receiver and a paging
responder and can be used to carry out the correlation processes described above.
Receiver hardware 120 filters and mixes an incoming RF signal from antenna 129 down
to signal frequencies that can be operated upon within the digital domain. The receiver
requires knowledge of the current RF frequency being received and this is supplied (for
the purpose of mixing) from the Frequency Generation system 121. The Frequency
generation system creates oscillations of a rate which allow practical mixing to take place
of the wanted RF signal. This mixing typically takes place to bring the signal frequency
down to a rate which can be quantized for signal processing operation. The Frequency
Generation system is typically implemented as a combination of hardware and signal
processing.Receiver signal processing 122 then filters, mixes, synchronises and decodes
the bit stream to a point where binary decisions can be made to allow the digital signal to
be passed to the control unit as bursts of data (or data blocks).
12/17
In Figure 12 a control unit 123 receives the decoded blocks of data and acts upon them
according to their content. In this example, the control unit may compare the decoded
bursts from slot 1 and slot 2 to establish that they are repeats of one another. Memory 124
contains the implementation code for the algorithms used to decode the wanted signal and
process the signal according to the required algorithms. The User interface 125 allows the
user to select a range of options typically available on a radio of this type such as
channel, user ID, signal strength, encryption keys, menu navigation. A Bluetooth or other
transmitter may also be provided.
When the base station in Figure 12 begins a transmission, a data block is created within
the control unit 123. The burst of data is then passed through a signal processing chain to
be processed into a waveform suitable for transmission on the air interface. In a DMR
system, a key element is an encoding filter which is described as a Route Raise Cosine
filter of alpha value 0.2. This shapes the waveform for transmission before it is mixed
back up to RF frequencies. Frequency generation subsystem 126 is used to supply the
relevant mixing components to pass the signal up to an intermediate stage ready for
processing by the transmitter hardware 127. .A Duplexer 128 is typically used at the front
end of the transceiver to allow dual reception and transmission. Received signal
operating on carrier frequency 1 is passed from the antenna 129 through to the receiver
line-up. Transmitted signal operating on carrier frequency 2 is passed out from the
transmitter line-up to the antenna 129.
Figure 13 outlines a process used to transmit a user message 130. It is assumed the bit
stream has already had error correction applied and this may represent the half rate
convolution code defined in P25 phase I. Several repeats of the same message may be
sent (defined here as N). Sending repeats increases the reliability of the communication.
To begin, an initial synchronisation sequence is transmitted 131. The next bit is then read
132 from a transmission buffer and tested 133. If the next bit is a 1 then a standard
correlation sequence is sent 134. If the next bit is a zero then the inverted correlation
sequence is sent 135. Here the standard correlation sequence we use is the
synchronisation pattern from P25 phase I. This process of selecting the next bit to be sent
13/17
continues until the transmission buffer is empty 136. To increase the reliability, the whole
message may be repeated 137 a number of times N. This represents a repeat of this flow
diagram which is not shown. The repeated transmissions are sent at a time that is symbol
synchronised with the first transmission to aid in diversity combining at the receiver. It
should be noted that if messages are repeated then the repeated message is sent in precise
symbol synchronisation compared to the preceding messages of the same information.
This is undertaken to enable diversity combining in a receiver if required.
Figure 14 outlines a process used to receive a message. To begin, the receiver is simply
waiting 140 for arrival of the first signal. If correlation occurs 141, the result is examined
to see if the arrival is a paging signal. Assuming it is a paging signal then the same signal
may represent initial synchronisation 142, otherwise the receiver continues to wait.
Following initial synchronisation, there is a specified period of time for arrival of data
correlations, determined by symbol counting 143. If after an expected time 144 a
correlation is detected 145 then result may be a 1 or a 0, otherwise a random result may
be allocated 146. A positive correlation is logged 147 as a 1. Alternatively a negative
correlation is logged 148 as a zero. This process continues until the end of an expected
data block has been reached.
Figure 15 illustrates Bit Error Rate performance based on the use of correlations
representing data. As a point of reference, the error rate performance is compared against
standard P25. Typically, P25 reaches a 1% BER at around -119dBm assuming the use of
a simple differential based receiver. Also shown in the figure is the performance observed
using an algorithm as described above. In this particular case, there is a 1% BER at
approximately -124dBm which represents a 5dB improvement. Greater improvements
can be acquired by slowing the paging symbol rate further or selecting enhanced forms of
receiver.
Figure 16 illustrates the performance in a faded environment. These results represent a
fade rate of around 80Hz which is substantially higher than expected if operating at VHF
at vehicle speeds. Nevertheless, the typical performance of P25 reception in the presence
14/17
of a differential receiver has a residual error rate even under good signal strength. With
the paging receiver however, there is a rapidly decreasing error rate with signal strength
such that under higher SNR the error rate becomes substantially low with a minimal
residual BER.
Figure17 shows the performance in the presence of delay spread representing two equal
power rays arriving at the candidate receiver whereupon each path is independently
Rayleigh faded. The typical performance of P25 is a 2% bit error rate at around 30uS.
The paging receiver matches this and offers a slight improvement.
Figure 18 shows the message error rate of the system described above. Here a 10%
message error rate occurs at -125dBm. Sending repeats of the same message yields an
increase in reliability where for example, the probability of a first copy of message. A
getting through is 90% whereas the probability of either the first or a second copy of
message. A getting through becomes 99% assuming the two messages experience
uncorrelated channel conditions.
Figure 19 shows the message error rate performance in Rayleigh faded conditions where
in this case, a speed of 100km/h at 800MHz operation was assumed. This represents an
extreme RF condition for the operation of the paging system where operation is likely to
occur in VHF.
Figure 20 shows the message error rate in delay spread conditions where again extreme
conditions were assumed with two independently Rayleigh Faded paths were assumed for
operation at 800MHz and 60km/h.
/17
Claims (8)
1. A method of receiving a paging message at a station in an LMR mobile radio system, including: 5 receiving an RF signal from the system, converting the RF signal into a stream of symbols, using one or more of the symbols to synchronise the station with the RF signal, using one or more of the symbols to identify a paging message in the RF signal, 10 converting the stream of symbols into a stream of bits which represents the paging message.
2. A method according to claim 1 wherein the same symbols are used for both synchronisation of the station and identification of the paging message. 15
3. A method according to claim 1 wherein an initial part of the symbol stream includes a correlation sequence which enables synchronisation of the terminal.
4. A method according to claim 1 wherein a double length sequence is preferably used for initial synchronisation.
5. A method according to claim 1 wherein the stream of symbols includes a correlation sequence which represents a user status message.
6. A method according to claim 1 wherein the symbol stream is relatively long 25 compared with the bit stream so that each bit is represented by multiple symbols.
7. A method according to claim 1 wherein the symbol stream has a rate of 4800sps and the bit stream has a rate of 200bps. 30
8. A method according to claim 1 wherein the symbol stream has a rate of 480sps and the bit stream has a rate of 10bps.
Publications (1)
Publication Number | Publication Date |
---|---|
NZ615954B2 true NZ615954B2 (en) | 2015-09-01 |
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