R1-070870
`3GPP TSG RAN WG1 Meeting #48
`St. Louis, USA, February 12 – 16, 2007
`(Original R1-070108)
`
`Source:
`Title:
`Agenda Item:
`Document for:
`
`NTT DoCoMo, NEC, Panasonic, Sharp, Toshiba Corporation
`Transmission Power Control in E-UTRA Uplink
`6.11
`Discussion and Decision
`
`1. Introduction
`Transmission power control (TPC) is a key technique to achieve link adaptation. TPC is also effective
`in decreasing interference to other users. This paper presents TPC schemes for the physical channels in
`the E-UTRA uplink. First in Section 2, we focus on the overall structure of TPC for various uplink
`physical channels. Then we focus on the necessity of inter-cell TPC based on overload indicator for the
`uplink shared data channel and list the candidate methods.
`
`2. TPC Scheme for Uplink Physical Channels
`In this section, our views on the overall structure of TPC for various uplink physical channels are
`presented.
`
`2.1. Non-synchronized Random Access Channel (RACH)
` Preamble part
`• Open-loop-type slow TPC is applied to the non-synchronized RACH preamble similar to that in W-
`CDMA. The transmission power of the RACH preamble is decided based on the uplink interference
`power and path loss between a UE and a Node B, which is calculated from the measured average
`received signal power (or SINR) and transmission power information at the Node B. In slow TPC,
`distance-dependent path loss and shadowing variation are compensated and the instantaneous
`fading variation remains.
`• Power ramping is applied when retransmission is performed.
`
` 
`
` L2/L3 message part
`• The basic view is that the transmission power of message part, i.e. the initial transmission of the
`shared data channel, is decided based on the transmission power of the preamble part after power
`ramping by applying a pre-determined power spectrum density (PSD) offset.
`• The transmission power offset between the preamble and message parts may be dynamically
`controlled by using RACH response considering CQI (or power headroom) information conveyed
`by preamble.
`
`
`2.2. Sounding Reference Signal
`During the RRC_CONNECTED state after the initial access using the non-synchronized RACH, the
`uplink sounding reference signal is periodically transmitted for CQI measurement and tracking of the
`uplink timing control. Our view is that the uplink sounding reference signal is the baseline physical
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`channel for the TPC during the RRC_CONNECTED state. Thus, the transmission powers of the other
`physical channels are determined based on the transmission power for the sounding reference signal.
`•
`Initial transmission power of the sounding reference signal is decided based on open-loop control
`(similar to non-synchronized RACH preamble).
`• Consecutive TPC is based on a closed-loop. TPC command is periodically transmitted from Node
`B.
`• The control interval of the closed-loop TPC will be the same or longer than the transmission
`interval of the sounding reference signal.
`• TPC controls the PSD of the sounding reference signal.
`
`2.3. L1/L2 Control Channel without Shared Data Channel Transmission
`• The transmission PSD of the L1/L2 control channel for the UE without shared data channel
`transmission is determined by adding a pre-determined offset value to the PSD of the sounding
`reference signal as shown in Fig. 1. The PSD offset value is different according to the contents of
`the L1/L2 control channel, e.g., ACK/NACK, CQI or both.
`
`
`
`PSD
`
`Pre-determined offset
`
`Sounding reference signal
`L1/L2 control (in sub-frame without data)
`
`Tx bandwidth for sounding reference signal
`
`Frequency
`
`Tx bandwidth for L1/L2 control
`
`Figure 1 – Closed-loop TPC for L1/L2 control channel based on sounding reference signal
`
`
`2.4. Shared Data Channel
`• The transmission PSD is decided based on the PSD of the sounding reference signal and offset
`value similar to that for the L1/L2 control channel without data transmission.
`• However, different from the case of the L1/L2 control channel without data, the PSD offset value is
`controlled not only by the serving-cell but also by non-serving cells using overload indicator (See
`Fig. 2). Thus, we consider the necessity for inter-cell TPC. Our views on the inter-cell TPC are
`described in Section 3.
`
`Intra-cell TPC
`
`Inter-cell TPC
`
`Desired signal
`
`Interference to non-serving cell
`
`Serving cell
`
`UE
`
`TPC command
`
`
`Figure 2 – Intra-cell and inter-cell TPC schemes for shared data channel
`
`Overload indicator
`
`Non-serving cell
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`2.5. L1/L2 Control Channel with Shared Data Channel Transmission
`• The same transmission power as that for the shared data channel is assumed.
`•
`In addition to TPC, adaptive modulation and coding (AMC) is applied [1]. AMC mainly controls
`the repetition factor. The modulation scheme candidates are BPSK and QPSK.
`• Additional control signaling for AMC on the L1/L2 control channel is unnecessary by defining the
`corresponding relation between the modulation and coding scheme (MCS) of the shared data
`channel and that of the L1/L2 control channel.
`
`
`3. Inter-cell TPC Scheme for Shared Data Channel
`In this section, we present our views on the necessity for the inter-cell TPC based on overload indicator
`and the candidate methods for achieving inter-cell TPC.
`
`3.1. Necessity of Inter-cell TPC
`E-DCH [2] provides high-speed packet data transmission employing Node B scheduling and hybrid
`ARQ with soft-combining. However, the SINR-based closed-loop transmission power control and
`inter-Node B soft handover were used as well as W-CDMA. Thus, the transmission power of a UE is
`controlled by the serving and non-serving cells. Excessive inter-cell interference is suppressed by the
`overload indicator from a non-serving cell. As a result, stable operation is achieved considering the
`interference margin of neighboring cells as well as the serving cell.
`E-UTRA is a full packet based radio access scheme. Thus, we cannot expect continuous
`transmission power control for burst transmissions in E-UTRA. Our view is that the baseline
`deployment of E-UTRA is one-cell frequency reuse, which is the same as that in W-CDMA. This is a
`different assumption than in GSM/GPRS [3] which typically uses seven-cell frequency reuse or even a
`higher reuse factor. E-UTRA needs a more precise other-cell interference control than GSM/GPRS to
`reduce the transmission power margin at the UE in order to increase the throughput. Figure 3 shows an
`example of the interference power variation observed in a multi-cell environment. A severe fluctuation
`in the interference power up to approximately 15 dB significantly influences the effect of link
`adaptation such as AMC and the scheduling gain. Accordingly, to achieve efficient and stable
`transmission of the uplink shared data channel, TPC considering the overload indicator from the non-
`serving cell is necessary.
`
`Alternatively we can consider the possibility of transferring other-cell interference information
`via the backhaul from a non-serving cell to a serving cell. However, in this case the feedback delay of
`the other-cell interference information would be very long and would not be guaranteed according to
`the network deployment. An air-based overload indicator would seem to be more promising to achieve
`TPC considering other-cell interference with a short delay.
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`1.08-MHz bandwidth
`8 users per sector
`
`RR scheduling
`PF scheduling
`
`20
`
`15
`
`10
`
`19 cell model (inter-cell distance = 500 m, penetration loss = 20 dB)
`Slow TPC without considering other cell interference power
`
`05
`
`0
`
`Normalized interference power (dB)
`
`50
`
`100
`
`200
`
`250
`
`300
`
`150
`Sub-frame
`
`Figure 3 – Example of interference power variation in multi-cell environment
`
`
`3.2. Candidate Methods for Inter-cell TPC Based on Overload Indicator from Non-
`serving Cells
`The candidate transmission schemes for the overload indicator are categorized into the following two
`approaches.
`• Overload indicator common to all UEs, e.g., in [4] and [5]
`Individual overload indicator for each UE, e.g., in [6]
`•
`In the following, we describe inter-cell TPC schemes for the respective approaches on overload
`indicator transmission.
`
` 
`
` Inter-cell TPC based on overload indicator common to all UEs
`Figure 4 shows the inter-cell TPC based on overload indicator common to all UEs that was proposed in
`[4].
`
`
`Serving Node B measures
`received S(I)NR using
`sounding reference signal
`
`
`ServingServing
`
`cellcell
`
`Data channel
`TX power/Hz:
`PTx = Pintra – Δoffset
`
`Closed-loop
`slow TPC
`
`Scheduling
`grant
`(TPC bits)
`
`Non-serving Node B
`measures IoT level
`
`IoT threshold
`
`
`Non-Non-
`
`servingserving
`
`cellcell
`
`IoT
`
`Overload indicator
`(common to all
`surrounding cells)
`
`Sounding reference
`signal and UE feedback
`(e.g., power headroom)
`
`UE
`
`UE at cell edge
`Figure 4 – Inter-cell TPC with cell-common overload indicator
`
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`In this method, the overload indicator is generated based on the Node B measurement of the
`total amount of inter-cell interference. Each UE determines whether its transmission power is
`decreased or sustained according to the decoding results of the overload indicator. In this case, the
`effect of decreasing the “margin” of the uplink transmission power by using inter-cell TPC becomes
`smaller, since all UEs decrease the transmission power regardless of the contribution to the total
`amount of the inter-cell interference if the overload indicator indicates to decrease the transmission
`power.
`Therefore, [5] proposed to control the amount of the offset for the transmission power reduction
`
`by inter-cell TPC based on the path loss difference between the serving cell and the neighbouring cells,
`in order to restrict the application of inter-cell TPC based on the overload indicator to the UE, which
`cause severe interference to the neighbouring cells. More specifically, for the UE having a closer path
`loss to the neighbouring cell as to serving cell, the amount of offset of the transmission power
`reduction by inter-cell TPC is set to be larger since that the UE would cause severe interference to that
`particular neighbouring cell. It is expected that the proposed method in [5] can decrease the “margin”
`of the uplink transmission power by using inter-cell TPC compared to the original proposal in [4].
`
`It should be noted that from the operator point of view, it is desirable that the network directly
`controls the transmission power of the UE (the best way is for the UE to follow the TPC command
`from the network when determining its transmission power). However, the method in [5] allows the UE
`to determine its transmission power by itself based on the received overload indicator and path loss
`measurement. Therefore, in order to achieve more network-initiative inter-cell TPC, we propose the
`following modification to the method in [5].
`• Option 1 (see Fig. 5(a))
` Serving Node B provides the threshold for path loss difference to UEs
` All UEs with a path loss difference less than the pre-decided threshold value should
`follow the overload indicator.
`• Option 2 (see Fig. 5(b))
` The UEs report the measured path loss difference to the serving Node B beforehand
` Then, the serving Node B informs each UE of the range of the inter-cell TPC based on
`overload indicator (power offset unit value).
`
`
`In Option 2, the network can control the UE transmission power more directly than in Option 1. A
`potential issue in Option 2 is the increased control delay due to the feedback of the measured path loss
`difference. Which option is more appropriate is FFS.
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`
`ServingServing
`
`cellcell
`
`
`ServingServing
`
`cellcell
`
`Broadcast
`threshold for pass
`loss difference
`
`UEUE
`
`UEUE
`
`UEUE
`
`UEUE
`
`Report of pass
`loss difference
`
`UEUE
`
`UEUE
`
`TPC based on overload indicator
`is activated only for the UE that
`satisfies the threshold decision
`(a) Option 1
`(b) Option 2
`
`Figure 5 – Options to achieve network-initiative inter-cell TPC based on common overload indicator
`
`Indication of power offset unit
`value for inter-cell TPC
`
` 
`
` Inter-cell TPC based on individual overload indicator for each UE [6]
`Figure 6 shows the inter-cell TPC based on individual overload indicator for each UE [6]. The Node B
`measurement of the interference power for each UE is achieved through the following procedures.
`• We assume that all cells have information related to the CAZAC sequences of the reference signals
`in the uplink used in the surrounding cells as system information.
`• Each cell, as a non-serving cell, measures the received signal power and interfering power level of
`the shared data channel using the reference signal, which employs the cell-specific CAZAC
`sequence of the surrounding cells in each resource unit at each or every other sub-frame duration.
`• Since E-UTRA uplink is based on SC-FDMA, the overload indicator based on the measured
`interfering power level for the respective cell-specific CAZAC sequence is assumed to be an
`individual one for each UE. This is especially true when the update interval of the overload
`indicator is 1 msec.
`This method can directly control the transmission power of the UE, which is causing severe
`interference to the neighboring cell. Meanwhile, since the Node B must measure the uplink inter-cell
`interference separately for different cells and must transmit individual multiple overload indicators in
`the downlink, the complexity of the interference measurement at Node B and the signaling overhead in
`the downlink are increased.
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`Serving Node B measures
`received S(I)NR using
`sounding reference signal
`
`
`ServingServing
`
`cellcell
`
`Data channel
`TX power/Hz:
`PTx = Pintra – Δoffset
`
`Closed-loop
`slow TPC
`
`Scheduling
`grant
`(TPC bits)
`
`
`Non-Non-
`
`servingserving
`
`cellcell
`
`Non-serving Node B
`measures IoT level
`per cell
`
`IoT threshold
`per cell
`Cell 2 Cell 3
`
`Cell 1
`
`Overload indicators
`(selective transmission is possible
`to multiple surrounding cells)
`
`Sounding reference
`signal and UE feedback
`(e.g. power headroom)
`
`UE
`
`UE at cell edge
`
`Figure 6 – Inter-cell TPC with individual overload indicator for each UE
`
`
`3.3. Transmission of Overload Indicator
`If the overload indicator is transmitted every 10-msec or at a longer interval, the overload indicator
`should be based on the average inter-cell interference. Accurate inter-cell TPC targeting should be
`employed to decrease the transmission power only for the UEs that cause problematic inter-cell
`interference. Therefore, our preference is sub-frame-level overload indicator transmission irrespective
`of the inter-cell TPC methods described in Section 3.2. In this case, as shown in Fig. 7, by establishing
`the timing relation between the uplink shared data channel at each sub-frame and the transmission of
`the overload indicator in the downlink from the non-serving cells pre-decided, each UE can follow only
`the overload indicator that corresponds to the previous transmission of that UE.
`
`
`Pre-determined time interval
`First packet
`Tx power
`transmission
`reduction
`Sub-frame
`
`Uplink transmission
`to serving cell
`
`Second packet
`transmission
`
`Downlink reception
`from non-serving cell
`
`Time
`
`Time
`
`Overload indicator
`
`Figure 7 – Sub-frame-level inter-cell TPC based on pre-determined time interval
`
`
`
`4. Conclusion
`This paper presented TPC schemes for the uplink physical channels. In particular, we propose applying
`the overload indicator-based inter-cell TPC to the shared data channel in a manner similar to that for
`the E-DCH to achieve stable and efficient radio resource assignment. Several candidate inter-cell TPC
`schemes are presented. The simulation results on the throughput improvement by using inter-cell TPC
`is presented in [7].
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`Reference
`[1] 3GPP, R1-060320, NTT DoCoMo, NEC, Sharp, Toshiba Corporation, “L1/L2 Control Channel
`Structure for E-UTRA Uplink”
`[2] 3GPP, TS 25.309, “FDD Enhanced Uplink Overall Description Stage 2”
`[3] 3GPP, TS 45.008, “GSM/EDGE, Radio Access Network; Radio subsystem link control (Release
`7)”
`[4] 3GPP, R1-063446, Qualcomm Europe, “Analysis of Inter-cell Power Control for Interference
`Management in E-UTRA UL”
`[5] 3GPP, R1-063478, Lucent Technologies, “Uplink Scheduling with Inter-Cell Power Control, with
`Extensions to Interference Coordination”
`[6] 3GPP, R1-063316, NTT DoCoMo, Sharp, Toshiba Corporation, “Transmission Power Control in
`E-UTRA Uplink”
`[7] 3GPP, R1-070871, NTT DoCoMo, NEC, Panasonic, Sharp, Toshiba Corporation, “Investigations
`on Inter-cell Transmission Power Control based on Overload Indicator in E-UTRA Uplink”
`
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