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Description  |
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The invention relates to a radio transmission system.
The designation "integrated-services digital network" (ISDN) refers to a
network which makes digital connections available for a plurality of
services between standard, normalized user/network interfaces (Technische
Mitteilungen PTT, 1/1985, "ISDN-Dienstintegrierte Digital-Netze" by H. K.
Pfyffer, pages 2-8). In the journal article a survey of the essential
characteristics of ISDN is given and possible future developments are
indicated.
According to it, ISDN is characterized by the fact that the transmission of
voice and non-voice signals (e.g. data signals) occurs in the same network
and that a limited type range of normalized user/interfaces is provided
for. In the integrated-services digital network ISDN standard access by
subscribers to a plurality of services has to be provided, by means of a
unified standard interface being provided at the subscriber's end
throughout the network.
The interface which is standard to the network comprises so-called B
channels and one D channel which are transmitted in digital multiplex, a B
channel having a bit rate of 64 kbits/s and the D channel a bit rate of 16
kbits/s. At the interface the two B channels are independent of each
other, so that two separate connections can be set up simultaneously with
the interface circuit. The D channel is used on the one hand for
signalling and on the other hand also for the transmission of messages
with low bit rates (data transmission such as telemetry). The information
transmission of the individual services, e.g. telephony, facsimile,
high-speed data transmission, etc. is effected in the B channel with 64
kbits/s.
The OSI architecture model for the protocols is described in greater detail
and explained on page 5 of the journal article referred to above. The
integrated-services digital network ISDN then comprises layers 1 to 3 and,
in accordance with the requirement of a unified standard network interface
to be able to handle various services, the interface circuit in the first
(lowest) layer must also be standardized. This means that the first layer
has to be laid out for the service with the highest transmission demands
(e.g. bit rate) and is therefore not economical frequency-wise when used
in a radio transmission system.
A method for the setting-up of connections between subscriber stations with
different terminal equipment, i.e. with differing needs for transmission
channel capacities, is known from DE-OS No. 33 11 043. To establish the
connection a calling subscriber station sends out, together with selection
information, an information signal with which the required transmission
channel capacity and, if necessary, the terminal equipment of the called
subscriber station which has to be involved in the connection are
indicated. The transmission channel capacities which are required for the
particular connections are provided for by the switching station.
Additionally, the switching station, using the information signal and
particulars stored there concerning the calling subscriber station, tests
whether the desired connection can be set up or not. If the connection can
be set up, the required transmission-channel capacity is made available to
the called and calling subscriber stations and, if necessary, the called
subscriber station is informed what terminal equipment has to be brought
into the connection.
When applied in an integrated-services radio transmission system a process
of this kind with standard initial signalling has the disadvantage that it
demands different transmission/reception devices for signalling and for
the actual service.
In known similar radio transmission systems (cf., for example, TE KA DE
Technische Mitteilung 1980, pages 10-14) the subscriber sets (mobile radio
stations) are standard products as regards their function. The bodies
operating radio transmission systems lay down system constants, such as,
for example, the number of radio messages to be transmitted, audible
signals, indications, HF and jitter thresholds, number of repetitions or
guard times, in what are known as equipment specifications. The
manufacturers of the subscriber sets incorporate the prescribed system
constants somewhere in the particular programme developed, so that any
change in the system constants requires a change of the programmes stored
in the programme memory.
It has been recorded in P No. 34 31 302 how the body operating the radio
transmission system can alter the system constants during operation
without the manufacturer of the subscriber set having to alter the
programme stored in the programme memory. To that end system constants and
performance criteria are stored in a memory incorporated in the subscriber
set, so that the possibility is created to restrict certain services to
certain areas of the radio transmission system.
The problem to be solved by the invention is how to integrate various
services with differing transmission requirements in a radio transmission
system in such a way that little expenditure on the subscriber set is
necessary and that, as well, the total bandwidth of the individual radio
transmission channels provided for message transmission can be kept as
small as possible.
This problem is solved by a radio transmission system according to the
invention.
SUMMARY OF THE INVENTION
In the integrated-services radio tranmission system according to the
invention, and in contrast to the subscriber sets in the ISDN, the maximum
transmission rate is not made available to every possible user. The
reasons for this are: (1) the limited system bandwidth available; (2) the
fact that not all services are used at the same time by a participant; (3)
the fact that not all possible services have to be implemented in every
subscriber set; (4) the fact that only a few services have to be made
available throughout the integrated-services radio transmission system.
The total available bandwidth is divided between the subscriber sets with
regard to time and area (geographical reuse of frequency), in accordance
with the particular need at any given time.
With the integrated-services radio transmission according to the invention
the cost can be reduced in a simple manner even in the fixed radio station
if not every channel installation operates each of the many services. Thus
the stationary radio stations are only provided with such equipment for
various services for which a need exists on the part of subscribers.
In the integrated-services radio transmission system according to the
invention not every possible function is implemented in every subscriber
set since every user in any case makes use only of a few functions or
services (different user groups).
The costs of the integrated-services radio transmission system consist
essentially of the functions which are essential at least for access to
the integrated-services radio transmission system and the functions for
which a need exists on the part of the subscribers. The
integrated-services radio transmission system according to the invention
possesses the advantage for the manufacturers of the subscriber sets that
it is now possible for the first time to create different product lines
for the various market segments and to offer every subscriber a low-cost
set which is tailored to his needs. These various subscriber sets may, for
example, be: a simple paging receiver with alphanumeric indication; a
"vest-pocket telephone" with one receiving and one transmitting frequency
for use in a restricted geographical area and with a reduced service
quality; a "dispatch set" (in the "dispatch service" a common channel is
used in the direction from the fixed radio station to the vehicles of a
fleet, while in the reverse direction each vehicle can use its own radio
transmission channel) with one receiving and a few transmitting
frequencies for semi-duplex operation; a portable telephone with several
receiving and tramsitting frequencies for duplex operation and nation-wide
coverage, which can also be used in the vehicle; a mobile data terminal
with semi-duplex transmission in packet traffic with a low data rate; a
"mobile office" (e.g. for building sites, banks, etc.) with transparent
duplex transmission of 64 kbits/s, which is only operated from a fixed
site.
The integrated-services radio transmission system according to the
invention thus provides a frequency-economical solution for service
integration with low costs for the particular subscriber set, with
flexible and frequency-compatible introduction of future services and with
the possibility of introducing low-rate speech-digitalization processes.
Even if the services and the service spectrum cannot be provided for some
time, or not in all countries, by the wired public telecommunication
systems, the integrated-services radio transmission system according to
the invention already creates the basis on which it will be possible to
integrate these services in the future if these are offered by the wired
transmission and switching system or made possible by technological
progress (very large scale integration).
Furthermore, the integrated-services radio transmission system according to
the invention also permits the introduction of new services with various
bit rates and requirements, e.g. as regards error control, without
affecting already existing services or subscriber sets which are in
operation. In the direction from the subscriber set to the fixed radio
station a receiver whose frequency is switchable over the entire available
frequency band can be provided for every message-transmission channel. It
is thus possible, with a single receiver-and-channel unit specified for a
new service and its transfer into the metallic public network and
installed in the fixed radio station, to offer this new service. The
already existing receivers in the fixed radio station need be neither
replaced nor modified. The additional receiver may be designed for a
different modulation/demodulation process, a channel coding appropriate to
the service and also a different bandwidth. It is thus possible to
implement any combination of different services at low cost. The
receiver-and-channel units shown need only be installed in fixed stations
where they will in fact be used. This means that locally, regionally or
nationally different services can be offered.
The radio transmission system of one embodiment has the advantage that the
only switching element common to all subscriber sets is a similar receiver
for the transmission process from the fixed radio station to the
subscriber sets. This switching section can be provided at low cost by
means of a highly integrated circuit. For simple subscriber sets which are
only used locally (e.g. co-called "message pagers") it is not even
necessary for there to be a synthesizer. If a synthesizer is built into
the subscriber set, it can also be operated throughout the network.
"Pocket telephones" (with a fixed-frequency receiver and a fixed-frequency
transmitter) can also be used if every receiver in the fixed radio station
is switchable to all frequencies. The associated increased probability of
blocking if this frequency is engaged can be reduced if a limited number
of, for example, 40 transmitting frequencies can be selected in the
subscriber set.
The radio transmission system of one embodiment has the advantage that the
application of the multi-access process whereby subscriber sets have
access to the radio transmission channels located in the entire frequency
band ensures highly economical utilization of frequencies and quick access
of subscriber sets to the network, especially in cases of traffic
overload.
This embodiment includes a means for marking transmission channels. As is
known in radio transmission systems with numerous transmission channels,
it is necessary to separate the transmission channels from each other. In
this context, radio transmission channels are duplex channels separated by
different frequencies or different time slots, depending on whether
frequency division or time division multiplex techniques are used. In this
way, a special frequency or special time slot is marked.
The invention will now be described in greater detail and explained with
reference to the forms of embodiment shown in the diagrams.
DESCRIPTION OF THE FIGURES
FIG. 1 the structure of the integrated-services radio transmission system
according to the invention,
FIG. 2 in tabular form, the allocation of various services to various
susbscriber sets,
FIG. 3 various messages transmitted on a radio transmission channel,
FIG. 4 the block diagram of the transmitting section in the fixed radio
station, and
FIG. 5 the block diagram of the receiving section in the subscriber set.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 shows the structure of the integrated-services radio transmission
system according to the invention. In the radio transmission system there
are fixed radio stations BS arranged geographically in accordance with a
cellular system. Each fixed radio station BS is allocated a number of
radio transmission channels buy which messages are transmitted to mobile
subscriber sets TG. One of these radio transmission channels is used for
signalling for control of the system functions. This includes, for
example, the transmission of dialling information in the establishment of
calls to the subscriber sets TG. Various sorts of subscriber sets and
various services can be operated in the radio transmission system.
In FIG. 1 the various kinds of subscriber sets are characterized by the
reference numbers TGA 1, TGA 2, . . . Every subscriber set TG incorporates
devices for the reception and evaluation of an organizational data flow
conveyed by the radio transmission channel, which flow messages sent out
by the fixed radio station (see FIG. 5). The various symbols indicate the
service operable by the particular channel device. In the messages in the
organizational data flow the subscriber sets TG are informed by the fixed
radio station BS of the operable kinds of subscriber sets TGAx and/or the
services by offered by the station.
The table in FIG. 2 shows the allocation of the various services to the
various subscriber sets TG. As can be seen from the table, not all
possible services Dy are implemented in every subscriber set TG. Service
DA can, for example, be a voice service, service DB a data service with a
low bit rate and service DC a data service with a high bit rate.
FIG. 3 shows various messages transmitted on the radio transmission
channel. In the organizational data flow the subscriber sets TG are
informed by the fixed radio stations BS, by means of the message Kz, of
the radio transmission channels, e.g. K.sub.5, K.sub.17 and K.sub.31, on
which the fixed radio station BS is prepared for reception at that moment.
Moreover, the subscriber sets TG are informed of a service Dy and/or a
type of subscriber set TGAx in addition to the transmission channels Kz
which can be received by the fixed radio BS. To characterize the aforesaid
messages as organizational messages, a word designated as OM is placed at
the beginning of the data block. The generation of the organizational data
flow in the fixed radio station BS and the evaluation of the
organizational data flow received in the subscriber set TG will now be
described in greater detail and explained with reference to FIG. 4 and
FIG. 5.
FIG. 4 shows the block diagram of the transmitting section of the fixed
radio station BS for a fully built-out network. The data/speech flow
transmitted in the baseband is made up as follows. The digitalized speech
of each individual channel is first recoded in a transcoder 1 from PCM to
the transmission form, with a correspondingly lower bit rate, required for
radio transmission. A data source can be connected at the interface B--B.
In a channel coder 2 connected to the data source or transcoder 1 a
special coding for the protection of significant bits against transmission
errors on the transmission channel is added. This channel coding differs
according to the service to be transmitted. In a multiplexer 3 connected
to channel coder 2 the signalling accompanying the connection and the
synchronization information derived from a synchronizing circuit 4 are
added to the data flow. The TDM signal (Time Division Multiplex signal) at
the output of the multiplexer 3 thus contains, in the form of embodiment
shown in FIG. 4, four speech/data channels, a connection-accompanying
signalling channel (for a TDM channel bundle) and also the synchronization
bits required for synchronization in the subscriber sets TG. The
synchronization bits are mixed in code planes or frequency planes, as
proposed in P No. 35 11 430 4.
The TDM signal at the output of multiplexer 3 is multiplied by the code
words generated by code generator 5 at any particular time, in which
process two bits are combined to form a symbol and this symbol is spread
with the desired code. The resulting spread signal is then used to shift
the phase of the carrier signal derived from an oscillator 6, so that a
BPSK (Binary Phase Shift Keying) signal, a combination of the information
and the code word being used to modulate a low carrier frequency, is
obtained. The modulated code division multiplex CDM signal is taken to a
summator, whose output is connected to a bandpass filter 8. Eight of these
modulated CDM signals form after addition and bandpass filtering a joint
signal with several steps of amplitude which is finally converted to the
finally transmitted carrier frequency.
To that end a mixing oscillator in the form of a synthesizer 9 is provided
which can be switched with corresponding steps within the frequency range
of the integrated-services radio transmission system. The synthesizer 9 is
designed only for the few possible frequencies of the FDMA (Frequency
Division Multiplex Access) stage. The mixing of the CDM signal with the
corresponding frequency delivered by the synthesizer 9 is performed in a
device 10 which is connected to bandpass filter 11. The output of the
bandpass filter 11 is connected to a power amplifier and the filtered and
amplified transmission signal reaches aerial 14 through a transmitter
coupler. In smaller fixed radio stations BS with up to 32 message
transmission channels the transmitter coupler 13 is omitted completely.
The channel and code generator setting, the correct choice of the channel
coding and the insertions of messages Kz, TGA and Dy into the
organizational data flow are performed by means of a controlling device 15
incorporated in the fixed radio station BS. The selected radio
transmission channel can then be a TDM channel in a CDM plane (cf. P 35 11
530.4).
FIG. 5 shows the block diagram of the receiving section of subscriber sets
TG. The signal received from a common transmitting/receiving aerial 16
arrives through a receiving filter of a duplexer 17 at the input stage 18
of the receiver. The requirements to be met by the receiving filter of
duplexer 17 are relatively unexacting, so that a low-cost solution is
obtained especially for subscriber sets TG with a low demand for services,
e.g. a simple data radio set. The signal is amplified in the input stage
18 and then mixed with a synthesizer frequency derived from a synthesizer
19 to produce an intermediate frequency.
The intermediate frequency signal is taken to an IF section 20 in which
further amplification and filtering of the signals are performed. A simple
synthesizer which can be produced cheaply can also be used for the
synthesizer 19, as is the case for synthesizer 9, in the fixed radio
station BS. In the IF section 20 filters are arranged which perform the
adjacent-channel selection for rejection of neighbouring wideband channels
or for the suppression of mixing products. The actual noise filtering
takes place in correlators 23 to 25. Connected to the IF section 20 is an
amplitude control circuit 21 which raises the output signal from the IF
section 20 to a level sufficient to match up with the subsequent circuits
and to prevent a possible overloading of these circuits. The amplitude
control circuit 21 evens out different radio propagation attenuations and
level fluctuations caused by shadow effects. The control time constant of
the amplitude-control circuit 21 is basically determined by these shadow
effects.
The power-regulated IF signal at the output of the amplitude control
circuit 21 is transposed to the baseband in a demodulator 22 connected
with circuit 21. This can be effected, for example, on the principle of a
Costas loop, so that frequency and phase can also be taken into account.
Ambiguities about whole-number multiples of 180.degree. can be recognized
with the aid of the polarity of the synchronization words received (cf. P
No. 35 11 430.4) and correspondingly corrected.
Connected to demodulator 22 are three correlators 23, 24 and 25 which are
set by a control device 26 to the particular valid codes 1 and 2 and to a
synchronization code which is valid for the entire channel bundle in the
radio zone. By means of the control device 26 the received organizational
data flow is evaluated by reading out the data relating to the service(s)
desired by the subscriber and the radio transmission channels provided for
the type of subscriber set TGAx, by selecting a radio transmission channel
which has been indicated as free in the orgianizational data flow and is
also switchable in the subscriber set TG and then sending an access signal
to the fixed radio station BS on this selected radio transmission channel.
The output signal from correlators 23 to 25 is used on the one hand to
derive the symbol clock, the frame clock and also the bit clock and, on
the other, to evaluate the multi-path profile applicable at the moment.
Since a standard synchronization code with a correspondingly larger level
in the total channel bundle is radiated at the same moment (cf. P No. 35
11 430.4), a reliable recognition of synchronization and evaluation of the
multi-path profile result.
The outputs of correlators 23 to 25 are connected to sampling circuits 27
and 28 which sample the output signals from correlators 23 to 25 and apply
the particular result to a decision stage 29. In this process the results
of the samples occurring synchronously with the echoes of the multi-path
propagation in the decision stage 27 are weighted proportionally to the
amplitude of the echoes (by means of a device 30). The decision stage 29
has the task of assessing the transmitted code and the polarity of the
code. The assessment consequently permits the selection of the symbol
transmitted with the greatest probability. After the symbolto
transformation in the decision stage 29, the output signal is taken to a
TDM demultiplexer 31 connected to decision stage 29. The demultiplexer 31
is connected to a channel decoder 32 at whose output the transmitted data
flow is again available. In the case of digital speech transmission the
digital speech signal is decoded in a speech decoder 33, and taken to a
D/A converter and to a loudspeaker connected to the latter.
If, for example, data service is the type of service implemented in the
subscriber set TG, the data occurring at the output of the channel decoder
32 can be immediately, for example, displayed or printed out.
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