Note: Descriptions are shown in the official language in which they were submitted.
t~ - 2 l~C 9 4 7 CE30046P
CELLULAR RADIO SYSTEM WITH HOPPING
Field of the Invention
This invention relates to a cellular radio system comprising a
plurality of base stations, each base station having means for hopping
from frequency to frequency. Separately and in addition, the invention
relates to a base station.
Background of the Invention
Current GSM (Groupe Speciale Mobile) base station transceiver
systems (BTS's) are typically multi-carrier units which can implement ~
base - band hopping of channel data. Base-band hopping is the ability
for data associated with a logical air interface channel to be hopped
across all or some of the physical channels (the radio frequency carriers) ~ :
to improve system performance.
There are two ways of frequency hopping channel-coded data. In
the i;rst method channel-coded data associated with one channel is
always transmitted from the same radio, but is transmitted at a ~ ~ -
different frequency on a per GSM-timeslot basis. This is achieved by
retuning the radio for each timeslot. The other method is to not retune
the radio, but instead switch the channel-coded data between radios on a
per GSM-timeslot basis so that it is still transmitted at a different
frequency on a per GSM-timeslot basis. Both of these implementations, `
or a combination of the two, can take place within one BTS The first
method is called Fast Synthesiser Hopping and the second called Base-
band Hopping.
There are ~ culties associated with hopping at the radio
frequency band. Hybrid combiners (passive devices~ are used to combine
two carriers together to the same antenna, but each incurs a 3dB loss.
When more than three carriers are combined then the loss seriously
affects the output power at the antenna. When more than three carriers
are combined then a remote tuned combiner (RTC) is used reducing the
loss per carrier to less than 3dB. However the RTC cannot re-tune on a
per timeslot basis. For this reason base band hopping is used, so that
the RF combining is simpler and therefore not as much loss is incurred,
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such that the power at the antenna is closer to the actual PA output
power.
Accordingly, radio transceiver modules are generally provided
within the same base station transceiver system, each radio transceiver
module having a transmitter tuned to a different carrier.
Baseband hopping involves switching channel coded data fEom a
channel coder in a base station transceiver system to any one of the
radio transceiver modules within the same base station transce*er
system.
A transceiver architecture which provides a predefined number of
radio transceiver modules within a single base station transceiver
system, to enable base-band hopping between those modules, is
inflexible. Currently four or five radio transceiver modules are generally
provided in a single base station transceiver system in a single cabinet.
To provide more than five such modules would require a larger cabinet.
To provide fewer than four modules would be uneconomic in a single
cabinet designed for four or five modules.
Reference will be made herein to CEPT which stands for "Comité
Europ~éen des Postes et Télécommunications" and is a standardizing
authority.
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Summarv of the Invention
According to the invention, there is provided a base station of a
cellular communication system comprising serial interface means for
direct connection over a time divided serial bus (e.g. a CEPT link) to a
base station controller, means for receiving traffic data from the base
station controller through said serial interface means and a channel
coder for channel coding the traffic data and selectively outputting it
onto the serial bus for retrieval by another base station.
In this manner, the serial link that connects the base station to
the base station controller is used for carrying channel coded traffic data
between base stations. This arrangement enables a single carrier base ~ ~ `
station to be connected to a CEPT 2 Mbps link and for other base ~ ~-
stations to be connected to the link, with base-band hopping data being
transferred between the base stations, thereby allowing the base
stations to hop as a group, even though not physically coupled by any
connection other than the CEPT link.
It is through the time divided serial bus and the base station
controller that the base stations communicate with the outside world, in
the form of a connection from the base station controller to a switch of a
public switched telephone network.
The base station alternatively transmits the coded data as a radio
signal, depending upon a hopping sequence.
The invention also provides a cellular radio system comprising a
plurality of base stations, a base station controller for controlling the base
stations and a data communications link communicating between the base
station controller and the base stations, wherein each base station comprises
means for hopping from frequency to frequency through a hopping sequence,
characterised by means for outputting hopping information to the link for
informing the base stations of the hopping sequence of each base station and
means in each base station for receiving this information and controlling its
hopping sequence accordingly.
A detailed description of a preferred embodiment of the invention
will be given, by way of example only, with reference to the drawings.
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B~ief Descril?tion of the Drawings
Fig. 1 shows a prior art BTS implement.ing base-band hopping.
Fig. 2 shows a number of base stations in a system according to
the preferred embodiment of the invention.
Fig. 3 shows part of a time division multiplex slot allocation on
the air interface and the serial bus interface of Fig. 2.
Fig. 4 shows a table of time slot allocations as recorded by a base
station controller of Fig. 2.
De~Description of the Preferred Embodiment
In the prior art base station transceiver system of Fig. 1, there are
provided a number of radio channel units (RCU's), 10, 11 connected
through digital radio interfaces (DRI's) 12 and 13 to a pair of 64 Mbps
TDM buses 15. Also connected to these buses are a kiloport switch 16
and a Megastream (trademark) interface 17. The ~egastream interface
(MSI) 17 is connected to a CEPT link 18 for communication with a public
switched telephone network (PSTN).
The entire apparatus of Fig. 1 is enclosed in a single cabinet. In
addition to the RCU's 10 and 11, there are two or three further RCU's
and DRI's.
Each DRI has a channel coder and decoder (not shown). As
shown, one of the buses 15 is an output bus and the other is an input
bus. To achieve baseband hopping, traffic data is received by the DRI 12
from the TDM bus 15. It is channel coded and re-transmitted to the
TDM bus 15. After routing through the KSW 1~ it is then received by
another DRI 13 for transmission by the RCU 11. In this manner a given
channel coder always encodes the traffic for a given logical channel. The
coded data is routed to one of a number of RCU's each of which is
transmitting on a fixed frequency. Hence the channel coded data is
hopped through a sequence of frequencies by routing to the appropriate
RCU's for transmission.
The channel coder also deals with the other half of the same
conversation by virtue of the fact that the RCU 10 connected to the DRI
12 re-tunes it's receiver on a per timeslot basis in the same sequence as
the switching of the channel coded data between RCU's. In this manner
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the channel coder on the DRI 12 always deals with both halves of the
same conversation for a given logical channel.
The above arrangement is inconvenient because the need for the
dual buses 15, the megastream interface 17 and the kilo port switch 16
makes the architecture inflexible.
Referring to Fig. 2, a preferred embodiment of the invention
shown in which a number of radio channel units (RCU's) 20, 21 and 22
are coupled to a single CEPT serial time divided link 23 via an interface
40, 41, 42 respectively. Each of these RCU's is effectively a self-standing
base station. Each has an RF transceiver 24 (comprising a receiver 24~
and a transmitter 24~), a front end processor (FEP) 25 and eight channel
coders and decoders, of which two are shown 2~ and 27. .
It will be appreciated that a number of base stations 20 to 22 can
be connected to the CEPT link 23, three being shown as a mere example.
Also connected to the CEPT link 23, is a base station controller (BSC) 30
which includes means for transcoding between coded speech and
digitized or analog speech and also includes a connection to a switch of a
public switched telephone network. It will be appreciated that the BSC
30 is generally located remote from the base stations 20, 21 and 22. It
should also be noted that the base stations 20 to 22 themselves may be
dispersed geographically. The transcoding means may be remote from
the BSC, located at or near the connection into the PSTN (the "switch").
In the diagram of Fig 2, there are shown dotted arrows indicating
the flow of data. There are eight logical channels numbered 0 to 7.
Codec 2~ handles logical channel 0. Codec 27 handles logical channel 1
etc.
The example is given of logical channel 0.
Outbound traffic data arrives from the transcoder at the BSC 30
at a data rate of approximately 13 kbps. This data, speech coded but not
channel coded, resides on the CEPT bus 23 in certain predetermined
time slots described below. One of the base stations, in this case, base
station 21, identifies itself as having responsibility for channel coding of
that channel. This responsibility is indicated to base station 21 from
BSC 30 which acts as a "master" for control of the base stations.
Channel coder 31 codes the traffic data by adding error correction coding
and by interleaving the data over time, and it outputs the channel coded
data at approximately 22 kbps onto the CEPT link 23. This channel
coded data is referred to as base-band hopping data (BBH data). The
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BBH data is now presented on the link 23 for retrieval by any one of the
base stations connected to the link. Base station 20 identifies itself as
currently having responsibility for transmission of logical channel 0 and
accordingly base station 20 retrieves the BBH data and this is input to
front end processor (FEP) 25. FEP 25 merely controls the passing of the
data to RF unit 24 for direct transmission on the appropriate GSM slot.
Meanwhile, RF transceiver 24 is tuned by FEP 25 to receive the
corresponding in bound physical channel for that particular logical
channel. This tuning of RF transceiver 24 is fast synthesizer hopping
and is relatively simple on the receive carrier.
The received inbound physical channel is passed to channel
decoder 2G responsible for logical channel 0 and channel decoder 2G
performs reverse interleaving and performs error correction to produce
traffic data at 13 kbps, which it outputs onto the CEPT link 23, for
retrieval by the base station controller 30.
On a frame by frame (or timeslot by timeslot? basis, FEP 25
causes transceiver 24 to retune to a difference receive frequency. The
incoming channel coded traffic data on the new frequency is again
passed to channel decoder 2G for decoding of logical channel 0 (on
different timeslots, to channel decoder 27 etc for channel decoding of the
different channels). At the same time, a different one of the base
stations 20 to 22 retrieves the BBH data for the corresponding outbound
channel from the link 23 and transmits the outbound traffic data. Thus
channel coder 26 continuously handles the downlink half of the logical
channel 0 conversation. The other half of the conversation is received
through the RF base station, by re-tuning the receiver, and hence
channel coder 2G always deals with both halves of the same conversation
The other half of the conversation is received through the RF part of
base station 21 and decoded via channel coder 31. Meanwhile, channel
coder 2G is continuously decoding the inbound half of a conversation
received through RF part 24 of base station 20 and continuously
encoding the other half of that conversation, for transmittal from the
various base stations on a per - timeslot or frame by frame basis.
A number of base sites 20 to 22 hop in synchronism under the
control of the BSC 30. In the case will be described where four base
stations (20 to 22 plus one additional base station not shown) hop
together as a set.
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For reasons explained below, in the case of a 2 Mbps CEPT link,
four base stations are the maximum number that can hop in a set on a
single CEPT link. The base station controller 30 defines the set of base
stations in the hopping sequence, defines the starting point for hopping
and defines the order of hopping. The frequencies on which the base
stations will hop are determined by the predefined frequencies of the
four base stations. Hopping is conducted in rotation by each base
station retrieving base-band hopping data from different timeslots on
the link 23 in rotation. Each base station transmits. in rotation,
different logical channels. The receiving part of the RF section of each
base station is retuned on each hopping step and a single logical channel
is received through that receiver section over a discontinuous frequency,
that is to say, a frequency that hops through four frequencies in
rotation. In the meantime, the other half of the conversation is being
transmitted by a different base station in rotation. That other half of
the conversation is, however, always coded by the same channel coder
that performs the decoding for the first half of the conversation.
Referring to Fig. 3, the time divided data on the air interface for
one of the base stations and for the CEPT link is shown. The air
interface 101 is divided into frames of 8 slots per frame, forming eight
logical channels. One GSM timeslot is equivalent to five or four CEPT
frames 103.
The CEPT link 102 supports two Mbps. The GSM interface 101
requires 22 kbps. In addition to the single logical channel shown in Fig.
3, there are an additional seven logical channels for one base station and
there are four base stations on a CEPT link.
A CEPT link is divided into thirty two timeslots per frame giving
thirty two 64 kbps channels, which are utilized for data and link
synchronization according to the CEPT format. Accordingly to this
format, thirty timeslots are available for traffic and two are reserved for
link synchronization and control. In the present system, these thirty
timeslots are used as follows. For each base station, five slots are used
for base-band hopping data and two for traffic data. This provides
64kbps full duplex data for traffic channels (at approximately 13kbps
per channel). It also provides 320kbps full duplex for transfer of :
baseband hopping data between BTS's. This extra bandwidth is
required for per timeslot RF control information such as power level, etc ~ ~
and also includes a cydical redundancy check so that the integ~ity of the ~ ~-
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information can be checked on receipt at the trallsmitting FEP/RCU.
Fig. 3 shows five slots shaded in the CEPT link 102. These five slots are
used for base-band hopping for one base station.
Fig. 3 also indicates timeslot 16 in the centre of one of the frames
103. Timeslot 16 is used for control information. In the preferred
embodiment of the present invention, this slot is also used for hopping
control information.
Fig. 4 shows a table stored at BSC 30. The table shows the
timeslots allocated to four base stations, A, B, C and D. Base station A
is allocated timeslots 1 to 7 of the CEPT link. Two of these are for traffic
data and five are for base-band hopping data. Base station B is
allocated slots 8 to 14, shown in shaded outline in Fig. 3. Base station C
is allocated slots 15 and 17 to 22. Slot 16 cannot be allocated because it
is a control slot. Base station D is allocated slots 23 to 29. Slots 30 and
31 are vacant. Slot 0 is required for data link synchronization.
At the start of a hopping sequence, the base station controller
transmits the table of Fig. 4 to each of the base stations 20 to 22 etc.
This information informs each base station that there are four base
stations in the hopping sequence and informs each base station as to
where on the CEPT link the necessary base-band hopping data can be
found. At the same time, the BSC initiates hopping by indicating the
start of the hopping sequence. All of this information is transmitted on
timeslot 16 of the CEPT link.-
With this information available, the four base stations cancommence hopping in rotation on four allocated frequency carriers.
Each receiver is tuned in rotation to the four received frequencies. Base-
band hopping data is retrieved by a given base station from one of the
four sets of timeslots shown in Fig. 4 in rotation. All channel coders at a
particular base station output their channel coded data (base band
hopping data) on the same slots on the CEPT link. The channel coders
utilise the ÇEPT slots in rotation in synchronism with the air
interference timeslot timing. This data is, however, retrieved by a
different base station at each point in the hopping sequence. Thus, a
different base station transmits that data on its particular frequency at
each point in the hopping sequence.
Each of the sets of slots shown in Fig. 4 supports eight logical
channels. All eight logical channels on a given physical channel hop
together between frequencies. All eight channel coders or any
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combination thereof can likewise hop together through independent
sequences. In the case of a logical channel that is not hopping the
baseband hopping data can be sent to the CEPT and always read by the
local FEP such that no hopping occurs. It is not necessary that a
hopping set comprises four base stations. The architecture has the great
advantage that any number of base stations can be added to the li,nk up
to a maximum of four. If a link of greater capacity is available, more
than four base stations can be supported. Consider the case where there
are three base stations in a hopping sequence, coupled to a single link
and a fourth base station is added to the link, for example due to
expansion of the network or filling of a gap in the network. In this case,
the new base station identifies that slots 23 to 29 are vacant and
transmits a signal to the BSC 30 over the control channel indicating
that the new base stations wishes to use those slots and wishes to join
the hopping set. The BSC sends its signal on the control channel to all
the base stations informing the base stations that the hopping set now
comprises four base stations and indicating the channel numbers of
those four base stations. The BSC also indicates which timeslots are
allocated to which base station. The BSC also instructs the base
stations as to their respective starting channels. From this starting
point for hopping, the base stations commence hopping in rotation.
There has now been described a novel use of a CEPT interface to
carry channel encoded data from one base station to a remote base
station in order to perform base-band hopping. The dynamic allocation
of CEPT timeslots has been described for carrying base-band channel
coded data in order to minimise the total delay of voice data through the
local and remote base stations. Base-band hopping data is transmitted
over the GSM A - bis interface. This feature allows the CEPT link to be
used to switch the base-band hopping data between remote sites.
In order to keep the delay through the system at a minimum, the
timeslots used for base-band hopping are allocated to each base station in a
manner such that when base-band data becomes available, it is placed onto
the CEPT line in the next available timeslot.
