3GPP TSG RAN WG1 Meeting #47bis
`Sorrento, Italy, January 15 – 19, 2007
`
`R1-070394
`
`
`
`Source:
`
`Nokia
`
`Title:
`
`Agenda item:
`
`Document for:
`
`Multiplexing of L1/L2 Control Signals between UEs in the absence of UL data
`6.9.1
`Discussion and Decision
`
`
`
`1. Introduction
`This paper deals with L1/L2 control signalling transmitted in LTE UL. This signalling consists of potential UL data-
`associated control signalling and data-non-associated signalling, such as ACK/NACK and CQI caused by the DL
`transmission. We concentrate on case when the UE has only L1/L2 control signals to be transmitted but not the UL data
`(i.e., data-non-associated control). This contribution is a modified resubmission of [8].
`
`RAN1 meeting #46bis held in Seoul, some working assumptions related to the UL control signaling were agreed.
`Related to the data-non-associated transmission in absence of UL data, the working assumptions is to use a reserved
`frequency and time resource for control signaling.
`
`There are three basic ways to multiplex control signals between different UEs within the given time and frequency
`region, FDM, TDM and CDM (and hybrids of them). This paper discusses the merits and demerits related to the
`different multiplexing schemes. We are focusing on FDM and CDM based techniques. TDM is excluded from these
`comparisons due to the insufficient coverage properties. Some performance figures will be presented to show the
`feasibility of proposed schemes under practical assumptions.
`
`2. Comparison of FDM and CDM -based Approaches
`Frequency Division Multiplexing (FDM)
`[5] presents a FDM based approach for control signal multiplexing between different UEs having L1/L2 control
`signalling but no UL data. Similar type of multiplexing scheme is shown also in [2]. [5] assumes that 12 UEs are FDM-
`multiplexed into a frequency resource of 25 sub-carriers. Each UE occupies two consecutive sub-carriers in the LBs
`(localized FDMA) and one sub-carrier in the SBs. [2] and [5] also assume that the frequency hopping is applied inside
`TTI.
`
`It can be noted that FDM leads to the utilization of very narrow transmission bandwidths. This will increase power
`differences between UEs occupying larger bandwidths. Thus, there is a risk that intra-cell orthogonality will be lost to
`some extent under practical non-idealities such as frequency and timing errors. Another problem with FDM approach is
`related to the reference signals needed in coherent detection. It has been shown in various contributions [6, 7] that FDM
`type of multiplexing is very sensitive for inter-cell interference. In scheme presented in [5] there is only one pilot sub-
`carrier allocated for a single UE. Thus, having frequency hop inside the TTI there is no room for coherent averaging
`against inter-cell interference. It should also be noted that allocation of only two sub-carriers is not in line with the
`minimum physical resource block size which is 12 sub-carriers.
`Code Division Multiplexing (CDM)
`CDM can be used to multiplex different users orthogonally into a certain frequency and time resource. Two ways to
`perform of CDM type of multiplexing are presented in this section
`• CDM Option #1: Multiplexing using block-wise spreading (e.g., using Hadamard codes)
`• CDM Option #2: Multiplexing scheme using modulated CAZAC sequences
`
`BlackBerry Exhibit 1005, pg. 1
`
`

`
`In CDM Option #1 the orthgonality is Doppler-limited
`
`It should be noted that both CDM Options are configured to transmit at least 12 sub-carriers which is the minimum size
`of one physical resource block. The main issue with CDM type of multiplexing is the well-known near-far problem. A
`good thing with the proposed schemes is that they maintain the orthgonality also in frequency selective channels.
`However, it is noted that, the orthogonality properties of CDM techniques must be carefully studied. We note that
`•
`•
`Near-far problem of CDM based schemes is visible also with FDM-based techniques. As discussed, the FDM leads to
`very narrow bandwidth utilization and thereby large power differences between UEs occupying larger bandwidths. This
`means that intra-cell orthogonality is lost to some extent under practical non-idealities such as frequency and timing
`errors.
`
`In CDM Option #2 the orthogonality is Delay-spread –limited
`
`The principle of block-wise spreading is shown in Figure 1. With control channel application the block-wise spreading
`(e.g, Hadamard) can be applied e.g., for the 4 LBs of the slot. Spreading factor equals to four in this arrangement. The
`spreading is used on top of DFT-S-OFDMA transmission scheme [1]. As can be seen in Figure 1 the LB#1 and LB#7 of
`the slot are used for pilot transmission together with LB#4. This is due to the fact that the operation point in terms of
`SNR decreases when the spreading is applied. To optimize the link performance in the spread system the pilot energy
`has to be increased. Another point is to balance the number of pilot and data resources.
`
`The considered CDM type of allocation, with allocation of a single PRB provides 4 orthogonal resources in 180 kHz
`frequency band each having symbol rate of 24 ks/s. It should be noted that the orthogonality is maintained also in
`frequency selective channels. Perfect orthogonality between the UEs is achieved if the channel stays unchanged during
`the spreading period (five LBs).
`
`sN
`
`SN,1 SN,2 SN,3 SN,4
`
`……
`
`s 1
`
`s2
`
`S1,1 S1,2 S1,3 S1,4
`
`S2,1 S2,2 S2,3 S2,4
`
`Symbol
`Sequence
`
`Hadamard
`spreading
`(SF=4)
`
`Mapping
`
`S1,1 S2,1
`
`…
`
`SN,1
`
`S1,2 S2,2
`
`…
`
`SN,2
`
`S1,3 S2,3
`
`…
`
`SN,3
`
`S1,4 S2,4
`
`…
`
`SN,4
`
`FFT
`
`…
`
`…IFFT
`
`CP
`
`CP
`
`PILOT
`
`CP
`
`DATA
`
`CP
`
`DATA
`
`CP
`
`PILOT
`
`CP
`
`DATA
`
`CP
`
`DATA
`
`CP
`
`PILOT
`
`Figure 1. Block diagram of block-wise spreading.
`
`
`
`The block diagram related to another CDM approach, based on modulated CAZAC sequences is shown in Figure 2. In
`sequence modulator the CAZAC sequence of length 12 symbols is modulated by using BPSK, QPSK or 8PSK. Each
`sequence carries 1 bit, 2 bits, or 3 bits, depending on the modulation scheme. It is possible to multiplex different UEs
`into the given frequency and time resource by allocating different cyclic shifts of CAZAC sequence for them. The main
`advantage of sequence modulation scheme compared with conventional DFT-S-OFDM transmission is that increased
`coverage for control signalling can be provided due to the reduced OBO and improved Eb/No requirements.
`
`The considered CDM type of allocation, with allocation of a single PRB provides 6 orthogonal resources in 180 kHz
`frequency band each having symbol rate of 12 ks/s. This assumes that 6 cyclic shifts of CAZAC codes are used by
`different UEs. The requirement for perfect orthogonality between the UEs is that the delay spread of the radio channel
`does not exceed the length of the cyclic shifts.
`
`
`
`BlackBerry Exhibit 1005, pg. 2
`
`

`
`
`
`Control
`signalling
`bits
`
`CA ZAC
`CODES
`with U E
`specific
`
`cyclic
`shift
`
`Phase
`modulator
`
`Sub-
`carrier
`mapping
`
`IFFT
`
`
`
` CP
`
`Figure 2. Block diagram of CAZAC sequence modulator.
`
`
`
`
`
`4. Multiplexing Examples
`
`4.1 Multiplexing between different UE’s
`As mentioned, the working assumption in RAN1 is to use a reserved frequency and time resource for L1/L2 control
`signaling in the absence of UL data. Our view is that the size of separate time-frequency resource should be equal to 1
`PRB (180 kHz). Multiple of those resources can be allocated when needed (FDM separation between multiple PRB).
`We also propose that slot based frequency hopping is applied to data-non-associated control signalling when the UE has
`no data to transmit. We think that both CDM based schemes are needed when multiplexing UEs having only L1/L2
`control signalling.
`Multiplexing Example #1
`Figure 3 shows the multiplexing principle of the proposed scheme. The considered block-wise spreading provides 4
`orthogonal resources in 180 kHz frequency band each having symbol rate of 24 ks/s. Each of the orthogonal resources
`can be divided into 6 sub-resources using another CDM technique based on cyclic shifts of modulated CAZAC
`sequence. Thus the total number of multiplexed users equals to 24 per allocated PRB, each with symbol rate of 2 ks/s. It
`can be noted that these resources are well suited for transmitting data-non-associated control information such as
`ACK/NACK and very limited amount of CQI.
`
`When comparing the number of users provided by different multiplexing schemes, we notice that with FDM type of
`multiplexing we can support only 6 simultaneous UEs per one PRB. The same applies for CDM in the case of
`modulated CAZAC sequences. Using the block-wise spreading we can have only 4 simultaneous resources (using the
`presented arrangement). Finally, we note that both CDM techniques are needed in order to support as many as 24
`UEs/PRB.
`
`With respect to the reference signals needed by the coherent method, we note that 24 orthogonal reference signals are
`needed when 24 UEs are multiplexed into a certain PRB. As discussed earlier, the LB#1 and LB#7 of the slot are used
`for pilot transmission together with LB#4. This is beneficial from the performance point of view since the operation
`point in terms of SNR decreases when the spreading is applied. To optimize the link performance in the spread system
`the pilot energy needs to be increased. Another motivation is that with increased number of pilot symbols we can
`generate more orthogonal pilot signals.
`
`With respect to multiplexing of reference signals, we utilize the same multiplexing schemes for both pilot and data
`channels. We apply the block-wise spreading with SF=3, for the allocated pilot blocks (LB#1, LB#4, LB#7). 3
`orthogonal pilot resources can be provided with this arrangement. Each of the orthogonal resources is divided into 6
`sub-resources using different cyclic shifts of a CAZAC code. Thus the total number of reference signals equals to 18
`per allocated PRB. We note that the number of pilot resources set the practical limit to the number of multiplexed users.
`
`
`
`BlackBerry Exhibit 1005, pg. 3
`
`

`
`pilot
`data
`
`time
`
`Hadamard spreading, SF=4
`
`freq.
`
`1 PRU
`
`
`
`
`
`1 slot
`
`Figure 3. Proposed Multiplexing Scheme
`
`
`Multiplexing Example #2
`Figure 3 is the starting point also for this multiplexing example. We are still doing the block-wise spreading with SF of
`four (four orthogonal resources in 180 kHz frequency band each having symbol rate of 24 ks/s). Now only part (0-2 out
`of four) of the orthogonal resources are used by the another CDM technique based on cyclic shifts of modulated
`CAZAC sequence. The remaining resources (4-2) of 24 ks/s can be used to convey a bit larger amount of L1/L2 control
`information, e.g., CQI (or ACK/NACK + CQI). It should be noted that the number of multiplexed UEs is smaller than
`with previous example. This has an impact also on pilot signal design discussed previously. An example of MCS set for
`the CQI resource in case where a single PRB is allocated for data-non-associated control signalling is given in Table 1.
`Maximum allowed modulation and coding scheme can be based on the propagation conditions, e.g., average SINR
`whereas the actual MCS is based on amount of bits in data-non-associated control channel. We note that it is also
`possible to scale up the number of bits carried in a single resource (shown in Table 1) by allocating more than 1 PRB
`for a single resource.
`
`Table 1. An example of modulation and coding set for data-non-associated control channel
`MCS
`Modulation/coding Spreading factor Block repetition
`number
`scheme
`(SF)
`factor (BRF)
`1
`BPSK 1/6
`4
`2
`2
`BPSK 1/3
`4
`2
`3
`QPSK 1/3
`4
`2
`4
`QPSK 1/2
`4
`2
`5
`QPSK 1/2
`4
`1
`
`number of bits/1 ms TTI
`2.0
`4.0
`8.0
`12.0
`24.0
`
`Processing gain
`9.0 dB
`9.0 dB
`9.0 dB
`9.0 dB
`6.0 dB
`
`
`
`
`
`5. Code Allocation
`The orthogonality properties related to the CDM techniques must be carefully taken into account. As was discussed
`above there are 12 sub-carriers corresponding to LBs. There are two sets of six codes available, cyclic shifts (1, 3, 5, …,
`11) and (2, 3, 6, …, 12) when only six out of 12 codes are used. These two sets of codes can be applied under different
`spreading codes of block-wise spreading (Hadamard). This arrangement provides improved orthogonality between the
`used resources and it can be applied for both pilot and data transmission. The improved orthogonality is caused by the
`fact that there is actually a double protection between these resources.
`
`BlackBerry Exhibit 1005, pg. 4
`
`

`
`6. Performance Evaluation
`The orthogonality properties of CDM techniques presented in Chapter 4 are shown in Figure 4. The figure shows the
`required SNR for ACK/NACK BER of 1% as a function of power difference between UEs having different cyclic shift
`and different Walsh code. In this example the desired UE utilize the 1st cyclic shift whereas the interfering UE has the
`the 3th cyclic shift. Additionally separated Walsh codes are applied to both pilot and data blocks. As can be seen the
`maximum allowed power difference is from 20 to 35 dB depending on the UE speed. Thus, we propose that both CDM
`techniques should be considered in LTE UL.
`
`v=3 km/h
`v=120 km/h
`v=250 km/h
`
`
`
`0
`
`5
`
`10
`
`15
`
`20
`I/C [dB]
`
`25
`
`30
`
`35
`
`-6
`
`-7
`
`-8
`
`-9
`
`-10
`
`-11
`
`-12
`
`Required SNR for Ack/Nack BER 1%
`
`Figure 4 Required SNR for ACK/NACK BER 1 % as function of interference ratio
`
`7. Conclusions
`In this contribution we discussed how to multiplex the data-non-associated signalling, such as ACK/NACK and CQI
`caused by the DL transmission in LTE UL in the case when UE does not have UL data to transmit. We compared the
`feasibility of FDM and two different CDM-based techniques, block-wise spreading and modulated cyclic shifts of a
`CAZAC sequences. We think that both CDM based schemes are needed when multiplexing UEs having only L1/L2
`control signalling. FDM multiplexing is utilized between different PRBs. We think that slot based frequency hopping is
`needed with data-non-associated control signalling when the UE has no data to transmit.
`
`References
`[1] TR 25.814, 3GPP
`
`[2] R1-061675, “Data-non-associated L1/L2 Control Channel Structure for E-UTRA Uplink”, NTT DoCoMo
`
`[3] R1-061862, “Uplink Non-data-associated Control Signalling”, Ericsson
`
`[4] R1-061802, “Multiplexing and Link analysis of UL control channels”, Qualcomm
`
`[5] R1-062065, “L1/L2 Uplink Control Mapping & Numerology”, Motorola
`
`[6]
`
`R1-062012, “Multiplexing of Distributed (“Sounding”) Reference Signals for CQI Measurement and Scheduling
`in EUTRA Uplink”, Texas Instruments
`
`[7] R1-062353, “UL Reference Signal Structure”, Nokia
`
`[8]
`
`R1-063380, "Multiplexing of L1/L2 Control Signalling when UE has no data to transmit ", Nokia
`
`BlackBerry Exhibit 1005, pg. 5