Note: Descriptions are shown in the official language in which they were submitted.
CA 02625425 2008-04-16
SUPPORT OF MULTIUSER DETECTION IN THE DOWNLINK
This application is a divisional of Canadian patent application Serial No.
2,399,078
filed internationally on February 2, 2002 and entered nationally on August 1,
2002.
BACKGROUND
The present invention relates generally to wireless time division duplex using
code division multiple access (TDD/CDMA) communication systems. In particular,
the invention relates to determining channelization codes for use in multiuser
detection in the downlink for TDD/CDMA systems.
= A TDD/CDMA communication system is illustrated in Figure 1. The system
10 has multiple base stations 12, to 125. Each base s-tation 12, has an
associated
operating area. User equipments (UEs) 14, to 143 in a base station's operating
area
communicate with that base station 12,. Communications transmitted from a base
station 12, to a UE 14, are referred to as downlink communications. and
communications transmitted from a UE 14i to a base station 12, are referred to
as
uplink communications.
In a wireless TDD/CDMA communication system, multiple communications
are sent in a shared frequency spectrum. One such system is proposed in a
third
generation wideband-CDMA (W-CDMA) standard. In CDMA systems, multiple
communications are sent in the shared spectrum and are distinguished by
channelization codes. In TDD/CDMA systems, the shared spectrum is also time
divided using repeating frames having a fixed number of time slots, such as
fifteen
(15) time slots. Each time slot is used to transmit either only uplink or
downlink
communications. As a result, the communications are distinguished by both
channelization codes and time slots. A single channelization code used in a
single
time slot is referred to as a resource unit. Based on a communications
bandwidth,
CA 02625425 2008-04-16
communication may require one or multiple resource units. Typical data
modulation
schemes used in TDD/CDMA systems are quadrature phase shift keying (QPSK),
binary
phase shift keying (BPSK) and N Quadrature Amplitude Modulation (QAM), such as
N=8, 8,16 or 6
Data is transmitted in such systems using communication bursts 16. A
communication burst 16 carries data in a single time slot using a single
channelization
code (a single resource unit). A typical communication burst 16 has a midamble
20, a
guard period 18 and two data bursts 22, 24, as shown in Figure 2. The midamble
20
separates the two data bursts 22, 24. The guard period 18 separates the
communication
bursts 16 to allow for the difference in arrival times of bursts 16
transmitted from
different transmitters. The two data bursts 22,24 contain the communication
burst's data.
The midamble 20 contains a midamble code for use in estimating the channel
response
between the receiver and transmitter.
Since multiple communication bursts may be transmitted in a single time slot,
a
receiver must be able to distinguish data from the multiple bursts. One
approach to
recover the received data is multiuser detection (MUD).
In MUD, a receiver recovers all communication bursts' data in a tirrie slot,
including bursts transmitted to other UEs. To recover all the bursts' data,
the MUD
receiver needs to know all of the channelization codes used to transmit the
bursts. In the
proposed TDD mode of W-CDMA, each UE 141 to 143 only knows which
channelization
and midamble codes are used for carrying information intended for it. To
determine all
the channelization and midamble codes, a bank of matched filters is used to
detect all
possible channelization/midamble combinations. The output power from each
matched
filter is compared to a threshold to determine whether a particular
channelization/midamble combination was used. Due to the number of required
matched
filters, this approach has a high complexity. Additionally, if there is a high
correlation
between channelization codes, this approach may have poor performance.
GB2351422 discloses an association of training codes and channelization codes.
The training and channelization codes are associated so that for a given
spreading factor
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each training code is only associated with one channelization code. To
facilitate the
association, each training sequence is associated with a branch of the DVSF
tree.
WO 99/40698 discloses using channel estimates derived from tracking sequences
to
estimate soft symbols of received communications.
Accordingly, it is desirable to have alternate approaches for UEs 14i to 143
to be
able to determine the active channelization codes.
SUMMARY
A wireless time division duplex communication system using code division
multiple access has a base station and user equipments. The system
communicates using
communication bursts. Each communication burst has a unique channelization
code and a
midamble code. Each midamble code is mapped to a set of at least one
channelization
code. For each communication burst to be transmitted in a time slot from the
base station,
the midamble code mapped to that burst's channelization code is determined.
Communication bursts are generated and transmitted in the time slot. Each
burst has the
determined midamble code for its channelization code. The user equipment
receives the
bursts and determines each received midamble code. The user equipment
determines the
channelization codes of the transmitted communication bursts based on in part
a result of
the determining of each received midamble code.
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The invention thus provides according to an aspect, for a method for a time
division
duplex/code division multiple access base station to transmit communication
bursts, each
communication burst having a unique channelization code and a midamble code,
the
method characterized by: providing a mapping between channelization codes and
midamble codes, each midamble is mapped to at least one channelization code
and at least
one midamble code is mapped to at least two channelization codes which may be
transmitted in communication bursts simultaneously; and transmitting
communication
bursts from the base station, each transmitted communication burst having a
channelization
code and the midamble mapped to that channelization code.
According to another aspect, the invention provides for a time division
duplex/code
division multiple access base station, the base station transmitting
communication bursts,
each communication burst having a unique chamielization code and a midamble
code, a
mapping exists between channelization codes and midamble codes, the base
station
characterized by: the mapping has each midamble mapped to at least one
channelization
code and at least one midamble code is mapped to at least two channelization
codes which
may be transmitted together in communication bursts simultaneously; and means
for
transmitting communication bursts, each transmitted communication burst having
a
channelization code and the midamble mapped to that channelization code.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is an illustration of a time division duplex/code division multiple
access
communication system.
Figure 2 is an illustration of a communication burst.
Figure 3 is an illustration of a simplified base station transmitter and a
user
equipment receiver.
Figure 4 is a flow chart of downlink channelization code identification.
Figure 5 is an illustration of midamble sequence to channelization code
mapping.
Figure 6 is a channelization code detection device.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
Figure 3 illustrates a siniplified base station transmitter 26 and a UE
receiver
28 using multiuser detection (MUD). Data to be communicated to the active UEs
14, to 143 is produced by data generators 32, to 32K. Each generator 321 to
32K
produces data to be sent in a particular communication burst. Each
communication
burst's generated data is subsequently formatted into a communication burst by
a
spreading and modulation device 34, to 34K. The spreading and modulation
device
34, to 34K adds the midamble and spreads the generated data with a
channelization
code associated with that communication burst. Additionally, the spread data
is time
multiplexed into the appropriate time slot. All of the communication bursts
are
combined by a combiner 52. The combined communication bursts are modulated up
to radio frequency, such as by a mixer 36, and the radio frequency signal is
radiated
by an antenna 38 through a wireless radio channel 30. If transmit diversity is
utilized
by the base station 14,, the radio frequency signal will be transmitted by
multiple
antennas.
At a UE receiver 28, radio frequency signals are received by an antenna 40.
The received signals are demodulated to a baseband signal, such as by a mixer
42.
A channel estimation device 44 is used to estimate the channel that the
communication bursts were transmitted in using the transrnitted midamble
codes. A
multiuser detection (MUD) device 46 processes the baseband signal using the
estimated channel information and the active channelization codes to produce
hard
symbols.
Identifying active channelization codes is shown in the flow chart of Figure
4. One approach to aid in identifying active channelization codes at the UE
14, is
to provide a mapping between midamble codes (midamble sequences) 54, to 54N
and
channelization codes 56õ to 56NM, 58. Each midamble sequence 54, to 54N is
associated with a set of channelization codes 56õ to 56NM, as illustrated in
Figure 5.
The sets may contain only a single channelization code, which is a one to one
mapping of midambles to channelization codes. A burst transmitted by the base
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station 12, with a channelization code of a midamble's set is formatted with
that
midamble sequence, 60, 62. To illustrate, if a burst with channelization code
21 was
sent, midamble sequence 2 is used for that burst.
At the UE receiver 28, after channel estimation, the transmitted midamble
sequences are detected by a midamble sequence detection device 48, 64. Based
on
the detected midambles, a logic block 45, utilizing the midamble to
channelization
code mapping 49, determines the set of possible channelization codes. A
channelization code detection device 50 determines the received channelization
codes based on the determination, 66. If a one midamble code to one
channelization
code mapping is used, the logic block 45 determines the channelization codes.
As
a result, for a one to one mapping, the channelization code detection device
50 is not
used. The MUD device 46 uses the determined channelization codes and the
channel
response for the midamble sequences associated with the channelization codes
to
detect the data from all the bursts, 68.
One channelization code detection device 50 is shown in Figure 6. Matched
filters 82, to 82M are matched to the possible channelization codes and
associated
channel responses as determined by the logic block 45. Since only the possible
channelization codes need to be checked, the number of matched filters 82, to
82M
is greatly reduced, reducing the complexity and improving the performance of
the
receiver 28. The power of the soft symbols produced by each matched filter 82,
to
82M is measured by corresponding power measurement devices 84, to 84M. The
comparitor 80 determines the received channelization codes based on die power
measurement for each channel. If the number of transmitted channelization
codes
is known, the comparitor 80 selects that number of channels with the highest
measured power. Otherwise, the comparitor 80 compares each channel's power
level
to a threshold to determine the transmitted channelization codes.
To aid in identifying channelization codes, channelization code information,
such as transmitted channelization codes or a number of transmitted
channelization
codes, may be signaled to the UE 14,. The signaled information can be used in
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conjunction with channelization/midamble code mapping or when mapping is not
used. The additional channelization code information will increase the
accuracy in.
determining the active channelization codes at the UE receiver 28. One such
signal
would be a layer one signal, where the midamble code or midamble code shift is
associated with the information. The midamble detection device 48 determines
the
received midamble code(s) and the logic block 45 recovers the channelization
code
information using the determined midamble codes. Using the recovered
information,
the channelization code detection device 50 uses the recovered information to
aid in
the channelization code determination. Another approach signals channelization
code information using a layer 2/3 signal. The signal is generated by the
network
circuitry. The layer 2/3 signal can be used in conjunction with layer one
signals or
with the midamble/channelization code mapping.
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