Note: Descriptions are shown in the official language in which they were submitted.
CA 02473828 2004-08-05
TITLE OF THE INVENTION
CHANNEL ASSIGNMENT IN A SPREAD
SPECTRUM CDMA COMMUNICATION SYSTEM
This application is a divisional of Canadian patent application Serial No.
2,371,482 filed internationally on May 19, 2000 and entered nationally on
November
16, 2001.
BACKGROUND OF THE INVENTION
The invention relates generally to resource allocation in a wireless code
division multiple access communication system. More specifically, the
invention
relates to assigning uplink and downlink channels in response to access
requests of
user equipment.
Figure 1 depicts a wireless spread spectrum Code Division Multiple Access
(CDMA) communication system lg. A base station 20 communicates with user
equipment (UE) 22- 26 in its operating area. In a spread spectrum CDMA system
18,
data signals are communicated between UEs 22-26 and the base si;ation 20 over
the
same spread bandwidth. Each data signal in the shared bandwidth is spread with
a
unique chip code sequence. Upon reception, using a replica of the chip code
sequence,
a particular data signal is recovered.
Since signals are distinguished by their chip code sequences (code), separate
dedicated communication channels are created using different codes. Signals
from the
base station 20 to the UEs 22-26 are sent on downlink channels and signals
from the
UEs 22-26 to the base station 20 are sent on uplink channels. For coherent
detection
CA 02473828 2004-08-05
of downlink transmissions by the UEs 22-26, pilot signals are transmitted to
all the
UEs 22-26 within the base station's operating range. The UEs 22-26 condition
their
receivers based on the pilot signals to enable data reception.
In many CI7MA systems, a common packet channel (CPCH) is used for uplink
transmissions. A CPCH is capable of carrying packets of data from different
UEs 22-
26. Each packet is distinguishable by its code. For detection by the base
station 20,
the packets have a preamble which also distinguishes it from other packets.
The CPCH
is typically used to carry infrequently communicated data at high rates.
Figure 2 depicts a CPCH access scheme 28. The CPCH access scheme 28 is
time divided into intervals having time slots 30-34, such as 8 time slots
proposed for
the Third Generation Mobile Telecommunications System (IMT-2000). A group of
predetermined signatures 36-40 are assigned to the time slots 30-34 to allow
more
than one UE 22-26 to use the same time slot 30-34. A particular signature used
within
a particular time slot is referred to as an access opportunity 66-82. For
instance, for
each of the 8 time slots in the proposal for IMT-2000, one out of 16
signatures is
available to be chosen, resulting in 128 access opportunities. Each signature
36-40 is
preassigned a virtual channel. A virtual channel uniquely defines operating
parameters
for both the uplink and downlink, i.e., an uplink spreading factor and a
unique code for
the downlink.
Broadcast from the base station 20 to each UE 22-26 is the availability of
each
virtual channel over an acknowledge indication channel (AICH). The UE 22-26
monitors the RICH to determine the availability of each virtual channel. Based
on the
operating parameters required by the UE 22-26 and the availability of the
virtual
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channels, the UE determines the access opportunity to select. Upon identifying
a
particular access opportunity,' the base station 20 sends out an
acknowledgment
message (ACK) if the corresponding downlink channel is still .available. In
the
proposal for IMT-2000, the ACK simply repeats the signatures 36-40 associated
with
the access attempt. If the downlink channel is not available, a negative
acknowledge
(NAK) is sent.
After receiving a corresponding acknowledgment, the UEs 22-26 determine the
proper code to recover communications on the downlink channel based on the
access
opportunity 66-82 used to send the UEs' packet. Either stored in the UEs 22-26
or
transmitted on a Base Station°s Broadcast Channel is a list of the code
assigned to each
access opportunity 66-82. This scheme severely increases the packet collision
probability and therefore the packet delay which is undesirable.
In some situations, monitoring the AICH is not desirable.. At a particular
moment, some UEs 22-26 will be operating in a "sleep" mode. In the "sleep"
mode, the
UE 22-26 only runs when there is a need to send data. Monitoring the AICH
during
"sleep" mode will both reduce battery life and introduce a delay in the
transmission of
the first packet. Additionally., when a UE 22-26 borders between two base
station's
operating areas, monitoring multiple AICHs further exacerbates these
drawbacks.
Monitoring creates other problems. It further complicates the UE's receiving
circuitry, making the UE 22-26 more expensive.
Monitoring results in a suboptimum use of the CPCH. RICH monitoring
provides information when a <;hannel becomes busy. The time at which the
channel
becomes free is deduced on a worst case maximum packet length. If a packet is
not
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maximum length, the channel 'will be idle while the UEs 22-26 are waiting to
transmit.
On the other hand, if monitoring is not performed in such a system, channel
availability
information is unavailable. The UE 22-26 may randomly choose a busy virtual
channel
increasing the packet delay by causing a collision. Accordingly, it would be
desirable
to allow the UEs 22-26 to wait a period shorter than the maximum packet length
and
provide for some other collision reducing mechanism.
One technique to reduce the possibility of collision is to raise the number of
codes, for instance to 128 different codes. In the proposal for IMT-2000 the
128
sequences represent approximately half of the sequences available at the base
station
20. Accordingly, this solution is undesirable. Additionally, since monitoring
the AICH
complicates the UE receiver circuitry increasing its cost, it is undesirable.
WO 9849857 discloses a system for transmitting random access packets from
mobile stations to a base statian. A preamble signature is added to each
packet to be
transmitted by the mobile station in a time slot of repeating frames. The
signature is
a short spreading code which is concatenated with a unique long spreading
code. The
packet including the preamble are received by the base station. The preamble
facilitates recovery of data from the packet.
TIA/EIA-95-B, "Mobile Station-Base Station Systems°' discloses an
approach
for channel assignment in a code division multiple access system. A channel
assignment message is sent to a mobile station. The message indicates
information
regarding a channel, such as its spreading code. Accordingly, an alternate
approach to
assign virtual channels is desirable.
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SUM1VIARY OF THE INVENTION
A selected user equipment transmits a signature in a selected one of the
common packet channel's time slots. The base station, in response to receiving
the
transmitted signature, selects a currently unused code, if available, out of a
plurality of
code associated with the access opportunity defined by the selected signature
and
selected time slot. The base station transmits an acknowledgment signal
comprising
an identifier of the selected code. The selected user equiprrient receives the
acknowledgment signal. The selected user equipment and the base station.
According to one aspect, the invention provides a method for sending data in a
wireless spread spectrum code division multiple access communication system
between a selected user equipment out of a plurality of user equiprnents and a
base
station, the system having a common packet channel defined by a prf;determined
set of
codes, the common packet channel having a plurality of access opportunities,
each
defined by a time slot and signature, the selected user equipment transmitting
a
signature in a selected one of the common packet channel time slots, the
method
comprising:
in response to receiving the transmitted signature at the base station,
selecting
a currently unused code, if available, based on a code association with the
access
opportunity defined by the selected signature and selected time slot;
transmitting from the base station an identifier of the selected code;
receiving the identifier at the selected user equipment; and
communicating data between the selected user equipment and the base station
using the selected code.
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According to another aspect, the invention provides a base station for a
wireless
spread spectrum code division multiple access communication system comprising
a
receiver for receiving access opportunities and packet data aver a common
packet
channel, the common packet channel defined by a predetermined. set of codes
and
having a plurality of access opportunities, each access opportunity defined by
a time
slot and a signature, the base station characterized by:
a code selection controller associated with said receiver for selecting a
currently unused code based on a code association with a received access
opportunity,
such that a code, if available, is selected in response to receiving a
signature in one of
said time slots from a user equipment, the access opportunity defined by the
received
signature and said one time slot; and
a transmitter associated with said controller which transmits to the user
equipment an identifier of the selected available code, whereby said
transmitter sends
communications to the user equipment encoded with said selected code.
According to yet another aspect, the invention provides a wireless spread
spectrum code division multiple access communication system having a base
station
and a plurality of user equiprnents, the system using a common packet channel
for
communication, the common packet channel defined by a predetermined set of
codes
and having a plurality of access opportunities, each access opporhmity defined
by a
time slot and signature, the system comprising the plurality of user
equipments, each
having means for transmitting a signature in a selected one of thf; common
packet
channel time slots; the system characterized by:
each of the plurality of user equipments further having:
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means for receiving an identifier, the identifier indicating a selected
code; and
means for comr;~unicating with the base station using the selected code;
and
the base station having:
means, in response to receiving the transmitted signature, for
determining the selected code by selecting a currently unused code, if
available, based on a code association with the access opportunity
defined by the selected signature and selected time slot;
means for transmitting the identifier of the selected code; and
means for communicating with the user equipment using the selected
code.
BRIEF DE~'CRIPTION OF THE DRAWING
Figure 1 is an illustration of a typical wireless spread spectrum CDMA
communication system.
Figure 2 is an illustration of a common packet channel access scheme.
Figure 3 is an illustration assigning virtual channels.
Figure 4 is a graph of the probability of a collision versus demand for the
prior
art and the virtual channel assignment.
Figure 5 illustrates a simplified base station and user equipment.
Figure 6 is an identifier transmitter circuit.
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Figure 7 is an identifier receiver circuit.
Figure 8 is a table of an assignment of Golay sequences.
Figure 9 is a circuit for detecting the Golay sequences of Figure 8.
Figure 10 is an assignment table for a system having physical channels with
two
time slots.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The preferred embodiments will be described with reference to the drawing
figures where like numerals represent like elements throughout. Figure 3
illustrates
a virtual channel assignment scheme. Each virtual channel 48-64 is defined by
its
operating parameters, such as uplink spreading factor and down link code.
Additionally, instead of assigning virtual channels 48-64, the same principles
may be
applied to assigning physical channels which are defined by their downlink
code.
To reduce the number of used physical channels and increase the power level
of each channel, each physical channel may be multiplexed, such as by using
two time
slots. Using two time slots will increase the channel's effective data rate,
such as from
8 Kbps to 16 Kbps. In such a system, the virtual channel 48-64 also defines
which
multiplexed signal is assigned to the UE 22-26.
Instead of assigning a single virtual charmel for each signature 36-40 as in
the
prior art, a set 42-46 of virtual channels are assigned for each grouping 116-
120 of
access opportunities. A grouping may contain all of the channels in one group
or as
few as 2 or 3 channels. One possible virtual channel grouping may group all
virtual
channels with the same data rate for the uplink. For groups having the same
uplink data
_g_
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rate, the UE 22-26 selects an access opportunity out of the groups having the
UE's
desired uplink data rate. Another grouping may be formed based on the access
opportunities' signature 36-40. Based on the selected access request and the
UE's
priority, one of the virtual channels 48-64 assigned to the group 116-124
associated
with the access attempt is used for the UE if available. Once the virtual
channel is
assigned, it will not be assigned again until the particular UE's transaction
is complete.
Additionally, a receiving circuit at the base station 20 with the proper data
rate is
assigned to the UE 22-26.
In the prior art system, the UEs 22-26 determine which channel is assigned to
the downlink based on the access opportunity 66-82. Virtual channel assignment
transmits a channel identifier 84-88, preferably along with the ACK,
indicating which
of the set 42-46 of channels assigned to the group 116-120 is selected. When
all of
the virtual channels are in the same group, the identifier 84-88 indicates the
selected
virtual channel. If no channel is available out of the set 42-46, a no channel
is available
(NAK) identifier is sent. Since more than one virtual channel is potentially
assigned
to a particular access attempt, the probability of UE collisions is reduced.
Figure 4 is a graph 91 depicting the probability of a collision (Collisions)
versus the number of UEs 22-26 requesting access (Demand). As shown, the
collisions using 2 or 3 virtual channels per group (2 states/AP or 3
states/AP) is lower
than the prior art (AICH monitor) regardless of demand.
Figure 5 illustrates a simplified base station 20 and a LJE 22 for use in
implementing channel assignment. The UE 22 has a controller 144 for
determining the
code of the uplink and downlink communications. A UE transmitter 140 sends
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communications, such as access opportunities and uplink packet signals, to the
base
station 20. A UE receiver 142 .receives communications, such as AC.K messages,
NAK
messages and downlink signals.
The base station 20 has a controller 134 for determining the code of uplink
and
downlink communications as well as determining channel availabiliity. A base
station
transmitter 136 sends communications, such as ACK messages, N.AK messages and
downlink signals, to the I:JE 22. The base station receiver 138 receives
communications, such as access opportunities and uplink packet signals
Techniques for sending the identifiers are to attach extra bits to the ACK or
to
I O change the phase of the ACK to indicate the selected identifier. For a
system using a
single group of virtual channels, the extra bits identify the selected virtual
channel.
Circuits for sending the identifiers by phase shifting the ACK are depicted in
Figures
6 and 7. The circuits are capable of sending up to four channel identifiers
without a
NAK identifier or three channel identifiers with a NAK identifier. In the
transmitter
circuit 92 of Figure 6, the ACK sequence is generated by a sequence generator
94.
The sequence itself is associated with the preamble access opportunity and is
unique
to the access attempt. Several such sequences may be transmitted to several
users at
the same time. The ACK sequence is passed through a mixer 96 which multiplies
the
signal with either +1 or -1. The mixed signal is subsequently passed through
another
mixer 98 where the signal is rnixed with an in-phase carrier (cos wt) or a
quadrature
carrier (sin wt). As a result of the two mixers 96, 98, the transmitted ACK is
at one of
four phases 0°, 90°, 180° or 270°. Each identifier
84-88 is preassigned to one of the
phases.
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The receiver circuit 14 of Figure 7 is used to determine the phase of the ACK
sent
by the transmitter circuit 92 of Figure 6. The ACK is mixed with both an in-
phase
carrier by mixer 100 and a quadrature carrier by mixer 102. Each of the mixed
signals
are correlated with a replica of the ACK's sequence by sequence correlators
104,106.
The in-phase and quadrature correlation signals are each negated by mixers
108, 110
by multiplying the correlation signals by -1. The two correlated signals and
the two
negated signals are supplied to an identifier circuit 112. The identifier
circuit 112
determines which of the four phased versions of the correlated signal has the
highest
magnitude. Since the downlink transmissions from the base station are
synchronized
and their phase is known; the identifier circuit 112 determines which
identifier 84-88
was sent based on the phase of the ACK. A list stored either in the UEs 22-26
or
transmitted on a Base Station's Broadcast Channel is used to determine the
virtual
channel 48-64 associated with the identifier 84-88 and the group 116-120 of
the UE's
access request. Using the determined virtual channel 48-64, transmissions sent
by the
base station 20 using the selected downlink channel's code are recovered at
the UE 22-
26.
Another technique for sending the identifier 84-88 is to use the ACK and a
collision resolution signal (CR). After a collision between UEs 22-26 is
detected at
a base station 20, in many spread spectrum systems the base station 20 sends a
CR
directed to the colliding UEs. The CR has a sequence which is associated with
a
specific UE 22 for detection by the UE 22. By inverting the ACK and CR; an
identifier
84-88 is sent to the specific UE 22. An inverted ACK indicates a NAK. By
inverting
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the CR, one virtual channel is assigned to +CR a second virtual channel is
assigned to
-CR. Accordingly, using the ACK and CR an identifier indicating either a NAK
or one
of two channels is sent. Additionally, using a CR with multiple states, such
as three,
one of multiple channels is assigned to the CR.
Alternatively, the identifier is sent with a signal using a Golay sequence. A
Golay sequence is constructed out of short sequences, such as X and Y. By
inverting
the shorter sequences and changing their order many unique longer sequences
may be
constructed as shown in table 122 of Figure 8. To reduce the sizf; of the
table 122,
only half of the possible sequences are shown. By negating each sequence;
another
unique Golay sequence results. As shown in Figure 8, each UE 22-26 is assigned
a
unique set of Golay sequences, such as 4. For instance, user 0 is assigned
four
sequences, the top two sequences and the negation of those sequences. By
assigning
each of the Golay sequences a virtual channel, upon reception, the receiving
UE 22-26
determines the code of the downlink transmission.
A Golay sequence detector is shown in Figure 9. The received signal' is
correlated with a Golay Correlator 123 and interleaved by an interleaver 124
to detect
the short codes. The arrangement of the short codes for two assigned.
sequences within
the long codes is shown as Signature 0 and 1. Using mixers 125, 126, the
signatures
are mixed with the detected short codes. Adders 127,128 are coupled to the
mixer and
also to delay devices 129, 130. The delay devices 129, 130 take the output of
the
adders 127, 128 and feed them back to those adders 127, 128 for correlation
with the
next short sequence. The output of each adder 127, 128 determines the Golay
sequence of the received signal.
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Figure 10 illustrates an assignment scheme for a system using two time slot
multiplexing for the physical channels. In table 132, each of the sixteen
different
signatures is assigned a downlink code and one of two time slots. The selected
time
slot is indicated by the transmitted identifier.
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