TSG-RAN WG1#46bis
`Seoul, Korea
`October 9th – 13th 2006
`
`Agenda Item: 6.12.1
`Source:
`NEC Group
`Title:
`Downlink ACK/NACK Mapping for E-UTRA
`Document for: Discussion and decision
`
` R1-062771
`
`
`Introduction
`
` 1
`
`
`
`Previous RAN1 meetings have focussed on the structure and contents of the downlink
`shared control channel while discussions on the exact mapping of the channel onto the
`time/frequency plane is still largely open. Also open for discussion is the coding for the
`control channel and an estimate of the benefits of joint coding as opposed to individual
`coding as in Release 5 WCDMA. In this document we analyse the control channel
`overhead and propose multiple options for the mapping of the control channel on the
`time/frequency resources.
`
`2 Downlink Control Channel Structure
`In the last RAN1 meeting, the basic information to be carried in the control channel has
`been agreed and incorporated in the TR.
`Given the different types of information that the control channel must carry, it is
`imperative that the size of the control channel will depend on the individual UE’s
`situation. Examples of situations which lead to different control channel sizes are given
`below –
`
`Case
`
`
`
`UE scheduled on UL and DL,
`and awaiting ACK/NACK
`UE scheduled on DL only,
`and awaiting ACK/NACK
`UE scheduled on UL only,
`and awaiting ACK/NACK
`UE not scheduled on UL or
`
`1
`
`2
`
`3
`
`4
`
`
`
`DL Scheduling
`Information
`Required
`
`UL Scheduling
`Information
`Required
`
`Required
`
`
`
`ACK/NACK
`
`Required
`
`Required
`
`
`
`
`
`Required
`
`Required
`
`
`
`Required
`
`1
`
`BlackBerry Exhibit 1004, pg. 1
`
`

`
`awaiting
`
`and
`DL,
`ACK/NACK
`UE scheduled on UL and DL,
`not awaiting ACK/NACK
`UE scheduled on DL only, not
`awaiting ACK/NACK
`UE scheduled on UL only, not
`awaiting ACK/NACK
`
`5
`
`6
`
`7
`
`Required
`
`Required
`
`Required
`
`
`
`
`
`Required
`
`
`
`
`
`
`
`
`At the very least, the control channel needs to contain information on the resource
`allocation and an identity for the scheduled UE. In order to reduce the number of options
`on the control channel size, it is beneficial to remove the ACK/NACK field from the
`control channel itself into a dedicated (semi-static) time/frequency resource. In addition,
`if a UE is scheduled on both UL and DL then the UL scheduling information can be
`contained within the allocated DL resource block. This leaves two cases for the DL
`control channel size:
`Type 1: DL Scheduling Information (used in cases 1, 2, 5 and 6 above)
`Type 2: UL Scheduling Information (used in cases 3 and 7 above)
`
`3 Proposed ACK/NACK Channel Mapping in Downlink
`It is proposed that one or more subcarriers in the downlink be reserved for carrying
`ACK/NACK information for UE’s expecting such information in the downlink. The
`number of resources reserved for such usage and their locations in the time/frequency
`plane can be intimated to the UE’s through common signalling. The UE knows when to
`expect the ACK/NACK, and it can work out (from knowledge of the UL chunks used for
`the UL transmission) on which sub-carriers the ACK/NACK will be transmitted.
`It is assumed that an ACK/NACK command is transmitted over M*N sub-carriers, where
`N is the number of UL chunks transmitted by the UE and M is the number of subcarriers
`allocated to each ACK/NACK channel. The transmitted power of each ACK/NACK
`command is inversely proportional to N, so that the total energy per ACK/NACK
`command is independent of the number of chunks being acknowledged. Figure 1 and
`Figure 2 show two examples of possible ACK/NACK resource multiplexing exploiting
`the maximum frequency diversity for the 5 MHz case. In the FDM multiplexing case, all
`the ACK/NACK’s are multiplexed within the second OFDM symbol. These resources
`will obviously reduce the number of subcarriers available in that symbol for the downlink
`
`
`
`2
`
`BlackBerry Exhibit 1004, pg. 2
`
`

`
`control channel. This structure also allows support of micro-sleep mode at the UE, since
`an UE expecting an ACK/NACK need monitor only the first two OFDM symbols.
`The structure in Figure 1 is designed to support a maximum of 12 simultaneous users
`within 5 MHz (each user with one chunk) with each chunk being acknowledged by a six
`subcarrier ACK/NACK channel.
`Assuming a chunk size of L subcarriers, N chunks within the allocated bandwidth, M
`subcarriers per ACK/NACK channel and an ACK/NACK subcarrier position offset
`within a chunk of ∆, the mapping between the uplink transmitted chunk number i and the
`corresponding downlink ACK/NACK is as below –
`Position[0] = L*(i div M) + (i mod M) + ∆
`where 0 <= ∆ < L
`For j > 0
`Position[j] = Position[j – 1] + L*N/M
`Figure 1 demonstrates the case for N = 12, L = 25, M = 6 and ∆ = 0. It can be noted that
`M needs to be a factor of N in order to exploit the full frequency diversity with an equally
`spaced ACK/NACK subcarrier distribution.
`Another mechanism of the TDM mapping scheme is to spread the N*M ACK/NACK
`subcarriers uniformly over the entire band within the second OFDM symbol. However, if
`M is not a factor of L, the ACK/NACK spacing will be non-uniform in this case.
`
`
`
`
`3
`
`BlackBerry Exhibit 1004, pg. 3
`
`

`
`DL 2nd OFDM Symbol
`
`6 subcarriers per ACK/NACK
`
`UL 5 MHz Bandwidth
`
`
`
`Figure 1 : FDM Multiplexing of ACK/NACK
`
`
`Alternatively in the scatter (i.e. FDM/TDM) multiplexing case, the ACK/NACK resource
`is scattered over the remaining (all but the first OFDM symbol which contains the pilot
`and control channels only) OFDM symbols as shown in Figure 2. In this case, the chunk
`bandwidth for user data is reduced by a single subcarrier and micro-sleep possibility is
`reduced.
`
`
`
`4
`
`BlackBerry Exhibit 1004, pg. 4
`
`

`
`DL 2nd OFDM Symbol
`
`DL 3rd OFDM Symbol
`
`DL 4th OFDM Symbol
`
`DL 5th OFDM Symbol
`
`DL 6th OFDM Symbol
`
`DL 7th OFDM Symbol
`
`UL 5 MHz Bandwidth
`
`
`
` Figure 2 : Scatter Multiplexing of ACK/NACK
`
`
`
`
`Assuming a chunk size of L subcarriers, N chunks within the allocated bandwidth, M
`subcarriers per ACK/NACK channel, an ACK/NACK subcarrier position offset within a
`chunk of ∆ and Nsym number of available OFDM symbols, the mapping between the
`uplink transmitted chunk number i and the corresponding downlink ACK/NACK is as
`below –
`Position[0] = L*i + ∆
`where 0 <= ∆ < L
`For j > 0 and j < M
`Position[j] = (( Position[j – 1] + L*N/M ) mod L*N) in symbol j*Nsym/M
`Figure 2 illustrates the case for N = 12, L = 25, M = 6, ∆ = 0 and Nsym = 6.
`It can be noted that M needs to be a factor of Nsym to enable a uniform spacing of the
`ACK/NACK commands in the time domain.
`Figure 3 shows another alternative structure for the downlink ACK/NACK channel.
`Similar structures have already been proposed in WG1 in [2] and [3]. In this structure,
`each uplink resource block is associated with an orthogonal Hadamard sequence of a
`length equal to the number of resource blocks within the entire bandwidth. The downlink
`
`
`
`5
`
`BlackBerry Exhibit 1004, pg. 5
`
`

`
`ACK/NACK for a specified resource block is spread with the corresponding Hadamard
`code and mapped to a set of reserved resources. In the figure, a set of subcarriers equal to
`the number of uplink resource blocks is reserved within the downlink resources for the
`ACK/NACK channel (One subcarrier in each resource block in this 5 MHz example).
`The number of ACK/NACK’s transmitted to an UE depends on the number of resources
`the UE used to send its uplink transmission. Each such ACK/NACK is spread with its
`own spreading code and combined at the receiver with other ACK/NACK’s carrying
`information for the same uplink transport block. The advantage of using multiple
`ACK/NACK codes is to reduce the power required per code in inverse proportion to the
`number of used codes.
`It can be noted that Figure 3 demonstrates one possible position for the ACK/NACK
`resources and that other alternative mappings are indeed possible keeping the total
`number of resources equal to the number of resource blocks. The CDM ACK/NACK
`follows the earlier TDM structure and also permits micro-sleep and power saving at the
`UE.
`Assuming a chunk size of L subcarriers, N chunks within the allocated bandwidth, M
`subcarriers per ACK/NACK channel and an ACK/NACK subcarrier position offset
`within a chunk of ∆, the mapping of the ACK/NACK subcarrier within chunk number j is
`as given below –
`Position[0] = ∆
`where 0 <= ∆ < L
`For j > 0
`Position[j] = Position[j – 1] + L
`Uplink chunk number i uses all the available ACK/NACK subcarriers spread with
`Hadamard code i where 0 <= i < N
`It needs to be kept in mind that the performance of the CDM ACK/NACK is sensitive to
`the distribution of the input ACK/NACK’s to the spreading operation. In the case when
`all the transmitted signals are ACK’s (or NACK’s) transmitted with the same power, the
`superposition of the Hadamard sequences at the transmitter will lead to the cancellation
`of all but the first term in the superimposed sequence. The performance in this case is
`thus strongly dependent on the fading experienced by the single subcarrier on which this
`non-zero element is mapped which is clearly undesirable. Hence, choosing a spreading
`sequence which ensures frequency diversity independent of ACK/NACK distribution is
`crucial to ensure good CDM performance.
`
`
`
`6
`
`BlackBerry Exhibit 1004, pg. 6
`
`

`
`Length 12
`Hadamard Sequence
`
`DL 2nd OFDM Symbol
`
`{1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1},
`{1, -1, 1, -1, 1, 1, 1, -1, -1, -1, 1, -1},
`{1, -1, -1, 1, -1, 1, 1, 1, -1, -1, -1, 1},
`{1, 1, -1, -1, 1, -1, 1, 1, 1, -1, -1, -1},
`{1, -1, 1, -1, -1, 1, -1, 1, 1, 1, -1, -1},
`{1, -1, -1, 1, -1, -1, 1, -1, 1, 1, 1, -1},
`{1, -1, -1, -1, 1, -1, -1, 1, -1, 1, 1, 1},
`{1, 1, -1, -1, -1, 1, -1, -1, 1, -1, 1, 1},
`{1, 1, 1, -1, -1, -1, 1, -1, -1, 1, -1, 1},
`{1, 1, 1, 1, -1, -1, -1, 1, -1, -1, 1, -1},
`{1, -1, 1, 1, 1, -1, -1, -1, 1, -1, -1, 1},
`{1, 1, -1, 1, 1, 1, -1, -1, -1, 1, -1, -1} }
`
`UL 5 MHz Bandwidth
`
`
`
`
`
` Figure 3: CDM Multiplexing of ACK/NACK
`
`
`
`All of the above three proposed schemes assume the number of ACK/NACK resources to
`be equal to the number of uplink resource blocks within the complete bandwidth.
`Alternatively, the number of ACK/NACK’s in any subframe can be made equal to the
`number of users who are expecting such information in that subframe. This will however
`require dynamically changing the number of ACK/NACK’s per subframe and additional
`signaling to the UE to inform it about the used resources for such information in that
`subframe.
`The performance of the two structures proposed is shown in Figure 4 and Figure 5.
`It can be seen that the performance of the different mappings is very similar for the two
`velocity cases simulated. The two TDM plots per figure illustrate the performance with
`and without time domain interpolation between successive subframes.
`
`
`
`7
`
`BlackBerry Exhibit 1004, pg. 7
`
`

`
`
`
`
`
` Figure 4 : ACK/NACK demodulation performance
`
`
`
` Figure 5 : ACK/NACK demodulation performance
`
`
`
`
`
`
`
`
`
`8
`
`BlackBerry Exhibit 1004, pg. 8
`
`

`
`The similarity in the performance of the alternative schemes implies that the TDM
`ACK/NACK (maybe with a CDM component if a suitable spreading code is identified)
`can be adopted without any performance penalty as long as all the ACK/NACK symbols
`are located within the second OFDM symbol, thus allowing micro sleep mode possibility
`at the UE.
`
`4 Conclusions
`In this document we have analysed the overhead needed for the downlink control channel
`and proposed a structure for its mapping onto the downlink time/frequency resources.
`The proposed scheme separates the scheduling information from the ACK/NACK
`information needed for uplink control which is essential in order to minimise the number
`of possible control channel sizes and allow an optimal design. Link level simulations
`indicate that a TDM structure of the ACK/NACK channel provides superior performance
`in addition to permitting micro sleep mode at the UE.
`
`5 References
`[1] R1-060830 – “Resource Allocation for E-UTRA” – NEC Group – TSG RAN
`WG1#44Bis
`[2] R1-061900 – “DL Acknowledgement and Group Transmit Indicator Channels” –
`Motorola – TSG RAN WG1 LTE Adhoc
`[3] R1-061697 – “Downlink ACK/NACK Signalling” – Samsung – TSG RAN WG1
`LTE Adhoc
`
`
`
`
`
`9
`
`BlackBerry Exhibit 1004, pg. 9