Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
" CA 02300895 2000-02-18
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FIELD OF THE INVENTION
This invention relates to a radio communication
system and in particular to such a system for use with a
s local area network having at least one receiver
transmitter unit connected thereto for communication with
one or more wireless mobile units.
BACKGROUND TO THE INVENTION
The invention has been developed in the context of an
io Asynchronous Transfer Mode (ATM) networking infrastructure
using radio communication which is capable of supporting
multi-media data traffic at rates from 25 Mb/s to 2.4 Gb/s
in local and wide areas. However, the invention could
also be used with other networking infrastructures.
is In a standard ATM network, all units operating on the
network are connected thereto by cable. There has,
however, been an increasing demand for mobile units to be
able to communicate with a network. These can be articles
such as laptop and notebook hand-held computers, cameras,
2o etc. A basic wireless network. systEm._is desci ~:~~d in our
paper entitled "Tile ORL Ra3io ATr' System, Architecture and
Implementation" dated 16 Januar~~ 1996 and publisher on our
Website at www.orl.co.uk.
In the system we have proposed, the network has a
2s number of base stations or Access Points (AP) connected to
a standard wired network and a plurality of mobile
stations or wireless Terminals (WT) connected to e.g.,
notebook and laptop computers which may be portable.
Figure 1 shows schematically a set of Access Points
3o and Wireless Terminals operating in the network. The
network 2 has the Access Points 4 connected to it. The
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Access Points themselves are physically separated and each
is able to transmit and recoi~:e messages over a limited
range e.g., up to ~o metres. The area covered by
transmission to and from a single Access Point is called a
s pico-cell 6. The pico-cells overlap to ensure that all
desired areas are covered by the radio system. The
channels with which the Access Points 4 transmit and
receive messages are selected so that adjacent Access
Points use different radio channels and thus do not
io interfere with each.
wireless Terminals a operate within the pico-cells 6
and are able to move from pico cell to pico cell. They
transmit and receive data from the Access Point 4
controlling the pico cell they are in. When they approach
is a boundary with an adjacent pico cell, a handshaking and
cross-over operation takes place as they move to the
adjacent pico cell and start to transmit and receive in
the radio channel of that pico cell.
A number of Access Points are located in a building
zo to provide full coverage.
Each Access Point communicates with the Wireless
Terminals in its pico-cell..usiag s frame transmission
structure. The frame st:.ucture provides time division
multiplexed access to the radio channel. The frame is
zs divided into two sections: during the first the Access
Point transmits, during the second the Wireless Terminals
may transmit.
The Access Points transmits firstly a preamtzle
comprising a frame description message followed by a
3o number of data and control messages. During the period
reserved for wireless Terminals, a number of different
Wireless Terminals may transmit. These transmissions
comprise data and control messages. Wireless Terminals
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are assigned time slots in which to transmit data
messages. Wireless Terminals can be assigned specific
periodic time slots in which to transmit. The Access
Points and Wireless Terminals exchange request and
s allocation messages to assign future time slots to a WT.
In addition to assigning time slots to particular
transmissions, the Access Point must assign priorities to
different data types and must manage the allocation of
time slots in accordance with this. Some data types such
io as voice data require a high_pr.iority to ensure that there
is no break in a real-time transmission being sent over
the radio link. Managing such a scheme presents various
problems in a radio communication scheme.
There is a further problem in such systems in that it
is is difficult to predict the split between upstream traffic
(WT to AP) and downstream traffic (AP to WT;. This will
depend on the requirements of users at any one time. With
a fixed frame structure this prevents full use of the
available bandwidth, particularly when there are a small
~o number of WT's within the range of an AP.
This can be overcome by using a variable frame
length, dependent on the total amount of traff.ic,. and a
variable split berween upstream and downstzE:am data
traffic. However, because adjacent AP's rave overlapping
2s areas of coverage this will lead to problems with
crosstalk between units in the system.
A further synchronisation problem arises because some
wT's are further away from the AP than others. This means
that nearby WT's receive data from the AP first. If these
3o assume the same delay between AP to WT and wT to AP
transmissions as remote WT's, this will result in lack of
synchronisation in WT to AP transmissions.
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SU1~SARY OF T8E INVENTION
A preferred embodiment provides a scheme for
assigning time slots to data transmissions in accordance
with the priority given to the data type for transmissions
s from AP's to WT's and from WT's to AP's. This scheme can
be implemented using Field Programmable Gate Arrays
(FPGA's) and memory. It assigns slots WT by WT and
prioritises transmissions for each WT.
A further embodiment provides a scheme for
io synchronising the frame structures used by adjacent Access
Points, i.e., giving each AP the same frame length and
split between upstream and downstream traffic and
synchronising transmissions. However, the frame structure
ilength and split) can be changed as the requirements of
is users in terms of data transmissions change, thereby
getting optimum usage of the available ba.~dwidth.
Preferably, WT's which are close to an AP insert a
variable delay prior to transmitting data to the AP to
synchronise that transmission with data transmitted by
2o wT's more distant from the AP.
The invention is defined in its various aspects with
more precision in the appended claims to which reference
should now be made.
BRIEF DESCRIPTION OF THE DRAWINGS
is The invention will now be described in detail, by way
of example, with reference to the drawings in which:
Figure 1 is a schematic diagram of network Access
Points and Wireless Terminals for use in an embodiment of
the invention and as described above;
3o Figure 2 shows a timing diagram for a series of
frames received and transmitted by a WT and AP of the
scheme of Figure 1;
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Figure 3 shows a table stored in memory for
prioritising transmissions;
Figure 4 shows a register bank used in conjunction
with the table of Figure 3 for scheduling transmissions;
s and
Figure 5 shows a set of synchronised frames
transmitted in adjacent pico-cells.
DETAILED DESCRIPTION OF PREFERRED EI~ODIMENTS
Data Transmissions
io The form of data transmissions between the Access
Point 4 and the Wireless Terminals 8 is best understood
with reference to Figure 2. The transmissions comprise a
series of frames each having a a downstream portion in
which an upstream Access Point 4 broadcasts to the wT~s in
is its pico-cell followed by a period during which Wireless
Textninals can transmit to the Access Point. Null periods
(TTn) are provided during which the Access Point and the
wireless Terminals switch between receive and transmit
modes. In Figure 2, the left-hand column shows what is
zo happening at a WT and the right-hand column shows what is
happening at an AP. The lighter shaded portions are AP to
WT transmissions and the darker shaded portions are WT to
AP transmissions.
The identifiers used in the example of Figure 3 are
2s as follows:
TTO: WT to AP turnround time
PRE: Access Point Preamble (not illustrated)
FD: Frame Descriptor Map
RG: Reservation Grant
so DACK: Downstream Acknowledgement
DCELL: Downstream Data cell Transmission
TT1: AP to WT turnround time
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RR: Upstream Reservation Request
UACK: Upstream Acknowledgement
UCELL: Ugstream Data, cell (start of burst)
In this example, the first portion of a frame
s comprises a Preamble (PRE - not shown) followed by a Field
Descriptor (FD) which is broadcast to all the mobile units
within the pico-cell 6 controlled by a particular Access
Point 4. This frame descriptor advises all the Wireless
Terminals a of the subsequent format and duration of the
io frame .
Following the frame descriptor, there is a
Reservation Grant slot (RG) which is used to advise WT's
which ones have contended for and been granted a future
transmission slot in a frame for transmission of data to
is the Access Point. This is an acknowledgement of a
Reservation Request and advises the WT of the number of
data cell transmission opportunities it currently has
allocated. This is the basic information contained in
every transmission from an Access Point to the Wireless
2o Terminals. This may be followed by a Downstream
Acknowledgement (DACK) of an Upstream Data cell
transmission from a mobile and a Downstream Data Cell
transmission (DCELL).
There then fellows a null period (TT7.) daring :~hich
zs the Access Point switches from transmit to receive mode
and the Wireless Terminals switch from receive to transmit
mode. Wireless Terminals 8 can then transmit Upstream
Reservation Requests (RR) in the next slot of the frame.
If more than one terminal sends an Upstream Reservation
so Request at the same time, there will be contention and the
Reservation Requests may be lost. This may then be
followed by an Upstream Acknowledgement of downstream data
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cells and by any Upstream Data Cells (UCELh) to be transmitted.
There is then a further null period whilst the Access
Point switches back to transmit mode and the Wireless
Terminals switch back to receive mode before commencement
s of the next frame. The above is the basic frame
s tructure .
As can be seen, the minimum frame is:
TTO PRE FD RG TT1 TTO.
A reservation request looks like this:
io . TTO PRE FD RG TT1 RR TTO PRE FD RG TT1.
A burst of 4 data cells from one mobile looks like:
RG TT1 rr UCELL(x4) TTO PRE FD RG DACK TT1
The process continues indefinitely.
Zs Immediately preceding the Field Descriptor is a
preamble portion lnot illustrated). This comprises some
basic housekeeping information such as synchronisation
data. The Access Point and Wireless Terminals use the
preamble to synchronise reception and adjust automatic
zo gain control for reception.
When an AP receives a Reservation Request it responds
with a Peservataon Grant transmission as described above.
This means that a time slot has been allocated to the
mobile. The WT is subsequently advised of this in a
~s permission to transmit message included in the FD of a
subsequent frame and then transmits a predetermined number
of data cells in that frame. If the WT has more data
cells than can be accommodated in that particular frame, a
further permission to transmit message will be included in
3o a subsequent frame. This will continue until the WT has
sent all its data.
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Prioritising Traasmi.ssions
The manner in which an AP deals with data
transmissions having different priority is illustrated
with reference to Figures 3 and 4.
s The AP stores in its local memory a table of the type
shown in Figure 3. This comprises for WT to AP and for AP
to WT transmissions records of the data types which can be
transmitted (DTN). A Flag F indicates whether or not a
particular WT has requested or requires a transmission for
io each data type in an upstream or downstream transmission,
and N is the number of data cells required for that
particular transmission.
The data types have different priorities. This
example shows only 3 data types but more could be used.
is DTI has highest priority and DT3 lowest priority. Entries
are made in this stored table for every forthcoming
transmission. Entries in the table correspond to a
reservation and this may correspond to any number of
transmissions. The reservation is maintained and
Zo continues to generate transmission opportunities until it
is removed by the WT.
Every time a new entry is made in the table of Figure
3; a corresponding entry is made in a register bank
illustrated in Figure 4. This comprises a shift registe~
2s for each data type for both upstream and downstream
transmissions. Thus, the columns representing the shift
registers in figure 4 correspond to the columns of the
table of figure3. These registers are used to form a queue
of forthcoming transmissions in the order in which they
ao were requested.
Counters are provided with the register bank to point
to the start and end points of entries in the register for
each data type. This is to ensure that new entries are
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entered in the correct place. The register operates as a
first in first out (FIFO) register.
when allocating data cells in a frame, the Access
Point will first allocate the earliest requested highest
s priority transmission from a WT. In the example given
this is a request from WT4 for an upstream transmission
and an allocation to WT5 on the downstream transmission.
Data cells will be allocated in the Field Descriptor on
the next frame and data transmitted. On the following
io frame the highest priority D~1 requests are both by wT3
and data cells will be allocated accordingly. The number
of data cells is taken from the table of Figure 3 which is
stored in memory. There will be no allocations of data
cells to DT2 and DT3 type data on either upstream or
is downstream transmissions until all the DT1 allocations
have been dealt with respectively. After these have been
dealt with, DT2 data is then transmitted. when there is
none of this outstanding, DT3 type data will be
transmitted. Thus, data is prioritised and scheduled to
~o ensure optimum transmission.
If.the number of data cells a particular WT requires
to transmit exceeds that available in a single frame,
excess. data cells will be held over until the following
game. If there are repeated requests for DT1
2s transmissions on upstream or downstream to the extent that
DT2 and DT3 will never be transmitted, the system can be
modified to ensure that there will be periodic
transmissions of DT2 and DT3 type data to ensure that
these do eventually get through.
30 The scheme can be implemented in a straightforward
manner using Random Access Memory, a bank of registers and
FPGA's and suitable control circuits or could be
implemented using software control.
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An additional entry shown in the table of Figure 3 is
for the antenna to be used on transmissions between the AP
and each WT. Preferably, the AP is provided with at least
two antennas and periodically tests the quality of
s transmissions to each WT in its particular pico-cell. A
value representing the antenna which gives best
communication with each WT is then stored. This
,,_" information is.then used for upstream and downstream
transmissions between the AP and each WT.
io This entry can be changed. if any of the WT's move.
Some WT's will generate data at fixed rate, e.g.,
audio data. In order to accommodate such data the scheme
can be modified such that the AP pre-allocates time slots
for that type of data transmission and uses this in
is addition to the queueing of data types shown in Figure 4.
The table of Figure 3 can be extended to include entries
for WT's which require constant bit-rate transmissions on
the upstream or downstream side. An entry for such a
transmission can then be made in every frame to ensure
2o that there is no loss of data.
Variable Frame Structures
As explained above, problems can occur with
irr_erferen;:e between transmissions in adjacent ~.~ico-cells,.
particularly when variable frame lengths are used and
2s transmissions in adjacent pico cells are not synchronised.
This can occur even when the frequency used in adjacent
pico cells are different and the isolation between
channels is limited.
We have appreciated that this problem could be
30 overcome by providing synchronised frame structures for
adjacent pico-cells, i.e., transmissions in each pico cell
that start and end at the same time and the split between
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upstream and downstream traffic is the same in each
adjacent pico cell. This idea is illustrated
schematically in Figure 5. This shows a series of
transmissions for two adjacent pico-cells named PC1 and
s PC2. Downstream transmissions from AP to WT are
represented by A and upstream transmissions from WT to AP
are illustrated by B. Thus, each frame comprises portion
.. A plus portion B.
As can be seen from the figure, portion A and portion
io B for each frame start and end at exactly the same time in
PC1 and PC2. Because of this, interference between the
two frames will be eliminated.
It will be seen, however, that the overall length of
the frames changes with time and, that the proportion of
is the frame allocated to upstream and downstream
transmissions also varies with time. This variation in
frame length is achieved by monitoring the upstream and
downstream traffic requirements of adjacent pico-cells at
a central controller connected to the Access Points over a
zo predetermined period of time and using this to determine
the optimum frame length and split between upstream and
downstream transmissions..
Delay Compensation
As explained above, WT's remote from the AP with
zs which they are communicating will receive downstream data
at a later time than WT's close to the AP. Thus, they
will start to compute the Turnaround Time (TT1 in Figure
2) at a later time and thus will send their upstream
transmissions at a later time. This will cause contention
3o at the AP with transmissions from other wT's.
In order to overcome this, each WT is provided with a
variable delay to insert between receipt of the downstream
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transmission and the AP. The system is organised to set
delays such that WT's close to the AP will have a delay
inserted to simulate the effect of that WT being at the
maximum possible distance from the AP.
s Initially, all mobiles have no delay information.
The first transmission that a mobile can make is in the
Reservation Request slot and the default transmission
,., delay is 0. This means that the Reservation Request may
appear early to the AP, but can only collide with other
io Reservation Requests from other. mobiles in the normal way.
The acknowledgement of Reservation Requests and data cells
by the AP includes delay correction information so that
the mobile can then synchronise to the appropriate delay.
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