Patent Application
20100172330
When a plurality of terminals simultaneously use an acknowledgement/negative acknowledgement (ACK/NAK) channel in a wireless communication system, code division multiplexing (CDM) may be used to allow for the plurality of terminals. In CDM, each of the plurality of terminals transmits a signal multiplied by a spreading code allocated thereto.
The present invention relates to code hopping for efficiently mitigating interference among terminals in the same cell and between terminals of adjacent cells when each of a plurality of terminals uses a spreading code along a frequency axis and a spreading code along a time axis.
The present invention is derived from a research project partly supported by the Information Technology (IT) Research & Development (R&D) program of the Ministry of Information and Communication (MIC) and the Institute for Information Technology Advancement (IITA) [2005-S-404-13, Development of Radio Transmission Technology for 3G Evolution].
The present invention relates to a method of transmitting an acknowledgement/negative acknowledgement (ACK/NAK) signal by a terminal as a response to data received from a base station.
A receiver sends an ACK signal to a transmitter when the receiver is successful in demodulating received data, and sends a NAK signal to the transmitter when the receiver is unsuccessful in demodulating the received data. Each of the ACK/NACK signal is expressed as one bit per codeword. A plurality of terminals should be able to simultaneously transmit their ACK/NAK signals by using given time and frequency resources through multiplexing.
Multiplexing techniques are classified into frequency division multiplexing (FDM) and code division multiplexing (CDM). While FDM is a form of multiplexing where a plurality of terminals use different time and frequency resources, CDM is a form of multiplexing where a plurality of terminals use the same time/frequency resources but transmit signals multiplied by orthogonal codes so as for a receiver to distinguish the plurality of users.
In uplink, a Zadoff-Chu sequence is widely used because it has an ideal peak to average power ratio (PAPR). Such a Zadoff-Chu sequence achieves orthogonality between terminals through cyclic delay without multiplying a signal by a specific code in a frequency domain.
A terminal transmits an uplink ACK/NAK signal to a base station to signify successful or unsuccessful receipt of downlink data. The uplink ACK/NAK signal requires one bit per codeword used to transmit the downlink data.
In
In this case, in order to distinguish the plurality of terminals using the same control channel, a specific code sequence is allocated to each of the plurality of terminals. That is, each of the plurality of terminals forms and transmits a signal spread on a frequency axis and a time axis by using its own specific code.
When CDM is used in order to distinguish signals of a plurality of terminals, a symbol and a sequence may be mapped to each time/frequency resource as shown in
The base station estimates a channel by using a reference signal, and demodulates an ACK/NAK symbol transmitted by a control signal by using the estimated channel. Each time/frequency resource transmits a signal multiplied by two or three symbols.
That is, a time/frequency resource on which a reference signal is transmitted is obtained by multiplying a frequency-axis sequence symbol Cqm(k) a time-axis sequence symbol Ri(i=0,1,2). A time/frequency resource on which a control signal is transmitted is obtained by multiplying a frequency-axis sequence symbol Cqm(k), a time-axis sequence symbol Ci(i=0,1,2,3), and an ACK/NAK symbol Q.
In
where NZC is the length of a Zadoff-Chu sequence applied to a kth subcarrier on the frequency axis. The small letter m is a primary index, and q is a cyclic delay index
One sequence is applied to each of a reference signal and a control signal along the time axis. That is, a sequence applied to a control signal in
Currently, in 3GPP LTE, three reference signals per slot are used for an uplink ACK/NAK channel.
Also, in order to distinguish terminals, a Zadoff-Chu sequence along a frequency axis is used and a discrete Fourier transformation (DFT) vector, a Walsh-Hadamard sequence, or a Zadoff-Chu sequence along a time axis may be used.
In a wireless communication system, acknowledgement/negative acknowledgement (ACK/NAK) control information sent by a terminal to a base station in order to signify that the terminal is successful or unsuccessful in demodulating data is transmitted and received. In order to efficiently transmit and receive such ACK/NAK control information, the same ACK/NAK resources are used by a plurality of terminals, thereby causing interference between terminals in a cell or between terminals of adjacent cells. In order to mitigate the interference, there is a demand for efficient code hopping.
According to an aspect of the present invention, there is provided a method of forming a signal in a wireless communication system in which a plurality of terminals commonly use time and frequency resources, the method comprising: allocating the same frequency-axis sequence and different time-axis sequences to a plurality of terminals by is using a resource index according to a first slot in the first slot; and allocating different frequency-axis sequences and different time-axis sequences to the plurality of terminals by using a resource index according to a second slot in the second slot.
According to another aspect of the present invention, there is provided a method of forming a signal in a wireless communication system in which a plurality of terminals commonly use time and frequency resources, the method comprising: allocating the same frequency-axis sequence and different time-axis sequences to a plurality of terminals by using a resource index according to a first slot in the first slot; and allocating different frequency-axis sequences to the plurality of terminals by using a resource index according to a second slot in the second slot.
According to another aspect of the present invention, there is provided a method of forming a signal in a wireless communication system in which a plurality of terminals commonly use time and frequency resources, the method comprising: causing one or more terminals, which use the same frequency axis code and different time axis codes in a first slot, to constitute a terminal group for a reference signal symbol block; causing one or more terminals, which use the same frequency axis code and different time axis codes in a first slot, to constitute a terminal group for a control signal symbol block, which is independent from the terminal group for the reference signal symbol block; causing the one or more terminals belonging to the same terminal group for the reference signal symbol block in the first slot to belong to different terminal groups for a reference signal symbol block in a second slot; and causing the one or more terminals belonging to the same terminal group for the control signal symbol block in the first slot to belong to different terminal groups for a control signal symbol block in a second slot.
According to another aspect of the present invention, there is provided a method of forming a signal in a wireless communication system in which a plurality of terminals commonly use time and frequency resources, the method comprising: causing one or more terminals, which use the same frequency axis code and different time axis codes in a first symbol block, to constitute one terminal group on one frequency code; and causing the one terminal group to use a frequency axis code, which is different from the frequency axis code used in the first symbol bock, in a second symbol block different from the first symbol block in one slot.
According to another aspect of the present invention, there is provided a code hopping method for reducing interference between terminals in a wireless communication system in which a plurality of terminals commonly use time and frequency resources, the code hopping method comprising: causing one or more groups, which use the same frequency axis code and different time axis codes in a first slot, to constitute one terminal group; and, when a time axis code length is less than a slot length, changing a terminal group according to the time axis code length achieving orthogonality so that the one or more terminals belonging to the same terminal group in the first slot belong to different terminal groups in a second slot.
According to another aspect of the present invention, there is provided a terminal using code hopping in a wireless communication system, the terminal apparatus comprising: a resource index receiving unit receiving a resource index that is changed according to a change in a slot or a symbol block; a frequency-axis code sequence allocating unit determining a frequency-axis code sequence according to the received resource index and allocating the determined frequency-axis code sequence to a terminal; and a time-axis code sequence allocating unit determining a time-axis code sequence according to the received resource index and allocating the determined time-axis code sequence to the terminal.
According to another aspect of the present invention, there is provided a base station using code hopping in a wireless communication system, the base station comprising: a resource index transmitting unit transmitting a resource index that is changed according to a change in a slot or a symbol to terminals; a frequency-axis code sequence determining unit determining a frequency-axis code sequence according to the resource index; and a time-axis code sequence determining unit determining a time-axis code sequence according to the resource index.
As described above, when acknowledgement/negative acknowledgement (ACK/NAK) control information sent by a terminal to a base station in a wireless communication system is transmitted and received, the present invention can efficiently mitigate interference between terminals in a cell or between terminals of adjacent cells which is caused when a plurality of terminals use the same ACK/NAK resources.
A code hopping method and apparatus for mitigating interference between terminals in a wireless communication system according to the present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown.
Detailed explanation will not be given when it is determined that detailed explanation about well-known functions and configurations of the present invention may dilute the point of the present invention. Terms used hereinafter are used considering the functions in the present invention and may be changed according to a user's or operators intention or usual practice. Accordingly, the terms will be defined based on the entire content of the description of the present invention.
In particular, the term “frequency axis code” or “frequency axis code index” used hereinafter is interchangeable with “cyclic shift” or “cyclic shift index”, and the term “time axis code” or “time axis code index” used hereinafter is interchangeable with “orthogonal cover” or “orthogonal cover index”.
Referring to
When a plurality of terminals are used, a receiver should be able to receive and distinguish reference signals transmitted by the plurality of terminals, and also should receive and distinguish control signals transmitted by the plurality of terminals.
In order to distinguish signals, code division multiplexing (CDM) using both frequency and time resources may be used as described above.
In CDM, a time-axis sequence is an orthogonal sequence. When the number of continuous orthogonal frequency division multiplexing (OFDM) symbols along a time axis is Nt, a sequence length may be Nt and Nt sequences achieving orthogonality therebetween may be formed. When an ith sequence is expressed as a row vector Gi=[Ci,0, C1,1, . . . , Ci, N
Theoretically, since the total number of resources on a frequency axis is M and there are 3 reference signals in
Since the total number of resources on the frequency axis is M and there are 4 control signals in
However, since each terminal should transmit at least one reference signal in order for a base station to demodulate a control signal by using the reference signal, the total number of distinguishable terminals is M×3. In this case, an orthogonal sequence having a spreading factor (SF) of 3 is used for the reference signals, and an orthogonal sequence having an SF of 4 is used for the control signals.
Code hopping for efficiently mitigating interference according to the present invention will now be explained. Code hopping is performed on both or each of a frequency axis code and a time axis code.
(1) Code Hopping on Symbol Block-by-Symbol Block Basis
One frequency-axis code sequence and one time-axis code sequence are allocated to each terminal. It is assumed that a frequency axis code and a time axis code which are used by an ith terminal in a tth block of an sth slot are expressed as qi,t,s and ri,s, respectively. In a multiplexing structure, 3 or less terminals in one symbol block use the same frequency axis code and different time axis codes that are transmitted through a plurality of blocks.
The present invention is not limited to the slot structures of
A Zadoff-Chu sequence of Equation 1 may be applied to a frequency axis code. However, the frequency axis code is not limited to the Zadoff-Chu sequence, and may use any sequence that achieves orthogonality.
At this time, a frequency axis code, which is used by a plurality of terminals in the same cell, has the same primary index and different cyclic delay indices. However, terminals belonging to different cells have different primary indices.
A frequency axis code used by each terminal may be changed according to a block index “t”. It is assumed that a frequency axis code used by an ith terminal for blocks belonging to an sth slot is expressed as {qi,s,t=0, qi,s,t=1, qi,s,t=2, . . . , qi,s,t=7}. A time axis code may be expressed as ri,s. If frequency axis codes used for a first symbol block by a jth terminal and an ith terminal are the same, the same frequency axis code should be used for remaining symbol blocks. That is, when {qi,s,t=0=qj,s,t=0, then qi,s,t=k=qj,s,t=k (k=1,2,3, . . . , 6) should be satisfied. That is, frequency axis codes used by two terminals in all symbol blocks should be the same. Also, if qi,s,t=k≠qj,s,t=k, (then qi,s,t=k≠qj,s,t=k (k=1,2,3, . . . , 6) should be satisfied. That is, frequency axis codes used by two terminals in all symbol blocks in the same slot should be different from each other. The aforementioned code hopping may be implemented as follows.
For starters, a terminal group is defined as follows. One terminal group includes one or more terminals which use the same frequency axis and different orthogonal time axis codes. A plurality of terminals groups are generated on a frequency axis, and one frequency code hopping pattern corresponds to each of the plurality of terminal groups.
Since the one or more terminals belonging to the same terminal group use the same frequency axis code even when blocks are changed, orthogonality on a time axis between the terminals can be maintained.
If the terminals constituting the same terminal group are changed as a symbol block index is changed, orthogonality on a time axis between the terminals is broken, resulting in interference between the terminals.
Referring to
In other words, the terminals #0, #6, and #12 of the terminal group in a first symbol block use the same frequency axis code but each terminal uses different time axis code. In a second symbol block, the terminals #0, #6, and #12 of the terminal group use a new frequency axis code different from that in the first symbol block, thereby performing code hopping on a symbol block-by-symbol block basis.
(2) Code Hopping on Slot-by-Slot Basis
Code hopping performed on a slot-by-slot basis will now be explained.
An uplink ACK/NAK channel includes two slots. A plurality of terminal signals are introduced through an ACK/NAK channel in code division multiple access (CDMA). At this time, terminal speeds and powers are different from one another. A terminal group is formed for each slot, and terminals belonging to the same terminal group in a first slot belong to different terminal groups in a second slot in order to mitigate interference between the terminals, thereby improving receiving performance.
Terminals in a terminal group are distinguishable by using different time axis codes as described above. For example, a specific terminal in a terminal group may have a high speed or high power because of incomplete power control. Other terminals are interfered by this specific terminal. In particular, remaining terminals other than the specific terminal in the same terminal group are most severely interfered by the specific terminal.
If constituent terminals of a terminal group are changed according to a slot such that terminals belonging to the same terminal group in a first slot belong to different terminal groups in a second slot, interference due to the specific terminal may be mitigated.
Code hopping performed on a slot-by-slot basis will now be explained with reference to
Referring to
Referring to
Accordingly, terminals belonging to the same terminal group in the first slot may not belong to the same terminal group in the second slot, and may constitute a new terminal group together with other terminals.
The code hopping performed on the slot-by-slot basis can randomize and average interference between terminals. That is, since a terminal with high power in a first slot does not belong to the same terminal group in a second slot, interference can be mitigated.
The code hopping performed on the slot-by-slot basis may change a time axis code allocated to each terminal when a terminal group is changed according to slots. That is, a time axis code allocated to the terminal #0 in
In this case, code hopping is independently performed for a reference signal symbol block and a control signal symbol block. In
A terminal group defined in the control signal symbol blocks #0, #1, #5, and #6 in the first slot of
For example, while terminals #0, #6, and #12 constitute one terminal group in the first slot of
A terminal group defined in the reference signal symbol blocks #2, #3, and #4 in the first slot of
When a terminal group is changed, a time axis code allocated to each terminal may be changed. A time axis code allocated to the terminal #0 in
(3) Code Hopping Changing Terminal Group According to Time Axis Code Length Basis
When a time axis code length does not occupy one slot, a terminal group may be changed according to a time axis code length basis. A method of changing a terminal group according to a time axis code length basis in code hopping will now be explained.
In the frequency code hopping patterns of
In this case, a terminal group is changed every 2 control signal symbol blocks. When a time axis code having a length of 3 is applied to reference signal symbol blocks R0, R1, and R2, a terminal group is not changed in the reference signal symbol blocks R0, R1, and R2.
Whenever a terminal group is changed, a time axis code allocated to each terminal may be changed. When a time axis code length of 2 is applied to the control signal symbol blocks, C0, C1, C2, and C3, 4 time axis codes are transmitted for one terminal.
That is, a time axis code applied to the control signal symbol blocks C0 and C1 of a first slot, a time axis code applied to the control signal symbol blocks C2 and C3 of the first slot, a time axis code applied to the control symbol blocks C0 and C1 of a second slot and a time axis applied to the control signal symbol blocks C2 and C3 of the second slot are transmitted for one terminal. In this case, a terminal group is changed three times, and a time axis code used by each terminal is accordingly changed.
The terminal 1000 includes a resource index receiving unit 1010, a frequency-axis code sequence allocating unit 1020, and a time-axis code sequence allocating unit 1030. The resource index receiving unit 1010 receives a resource index according to a change in a slot or a symbol block. That is, a resource index is a basic value for determining a frequency-axis code sequence and a time-axis code sequence which are to be determined according to a change from a first slot to a second slot. A terminal receives a resource index from a base station. The terminal and the base station share a frequency-axis code sequence and a time-axis code sequence allocated to the terminal on the basis of the resource index.
The frequency-axis code sequence allocating unit 1020 determines a frequency axis code to be used by a current slot on the basis of the resource index. The determfined frequency axis code is allocated to a terminal. In code hopping performed on a symbol block-by-symbol block basis, a frequency axis code to be determined according to a symbol block index that is changed whenever a symbol block is changed is determined and allocated to a terminal.
A plurality of terminals having the same frequency axis code in a first slot receive a new resource index in a second slot, and accordingly have a new frequency axis code. Also, a plurality of terminals belonging to one terminal group have a time axis code in a second slot which is different from a time axis code in a first slot. Accordingly, terminals belonging to the same terminal group in a first slot do not belong to the same terminal group in a second slot, and constitute a new terminal group together with terminals belonging to different terminal groups in the first slot. A terminal group may be changed on a symbol block-by-symbol block basis in this way.
The time-axis code sequence allocating unit 1030 calculates a time-axis code sequence to be allocated to terminals in a current slot or symbol block on the basis of the resource index that is changed according to the slot or the symbol block, and allocates the calculated time-axis code sequence to the terminal.
In order to identify terminals, a base station should know the information of a code sequence allocated to each terminal. As described above, the base station transmits a resource index and shares a time-axis code sequence and a time-axis code sequence with each terminal by using the resource index. As a result, the base station knows a frequency axis code and a time axis code of each terminal in a slot or symbol block.
The base station knows from a resource index in a first slot that a plurality of terminals have the same frequency axis code. The base station also knows from a resource index in a second slot that terminals belonging to the same terminal group in the first slot do not belong to the same terminal group, but have different frequency axis codes and belong to different terminal groups in the second slot. The base station should also know that a time axis code may be changed according to a resource index in a slot.
As described above, a time axis code allocated to terminals in a first slot is different from a time axis code allocated to the terminals in a second slot.
A base station apparatus performing the aforesaid functions will now be explained.
Referring to
Code hopping according to the present invention may be embodied as computer-readable codes on a computer-readable recording medium. The computer-readable recording medium is any data storage device that can store data which can be thereafter read by a computer system. Examples of the computer-readable recording medium include read-only memories (ROMs), random-access memories (RAMs), CD-ROMs, magnetic tapes, floppy disks, optical data storage devices, and carrier waves (such as data transmission through the Internet).
The computer-readable recording medium can also be distributed over network coupled computer systems so that the compute readable code is stored and executed in a distributed fashion. Functional programs, codes, and code segments for embodying the present invention may be easily deducted by programmers in the art to which the present invention belongs.
While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by one of ordinary skill in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the present invention as defined by the following claims. The preferred embodiments should be considered in a descriptive sense only and not for purposes of limitation. Therefore, the scope of the invention is defined not by the detailed description of the invention but by the appended claims, and all differences within the scope will be construed as being included in the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2007-0058590 | Jun 2007 | KR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/KR08/03396 | 6/16/2008 | WO | 00 | 12/14/2009 |
