Uplink control channel design for 3 gpp lte

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system design about PUCCH in LTE

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Uplink control channel design for 3 gpp lte

  1. 1. The 18th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC07) Uplink Control Channel Design for 3GPP LTE Amitava Ghosh, Rapeepat Ratasuk, Brian Classon, Vijay Nangia Robert Love, Dale Schwent, David Weimin Xiao Wilson Motorola Networks Motorola Labs Motorola Mobile Devices 1501 West Shure Drive, Arlington Heights, IL 1301 E. Algonquin Rd, Schaumburg, IL 60196, 600 North US Highway 45, Libertyville, IL 60004, USA USA 60048, USAAbstract —Long term evolution (LTE) of the UMTS signaling in the presence of data is discussed. Finally,Terrestrial Radio Access and Radio Access Network is conclusions are drawn in Section V.aimed for commercial deployment in 2010. Goals for the II. Overview of SC-FDMA ULevolved system include support for improved systemcapacity and coverage, high peak data rates, low latency, In the uplink, Single-Carrier Frequency Division Multiplereduced operating costs, multi-antenna support, flexible Access (SC-FDMA) is selected to efficiently meet E-UTRAbandwidth operations and seamless integration with performance requirements. SC-FDMA has many similarities toexisting systems. To reach these goals, a new design for the OFDM, chief among them for the uplink is that frequencyair interface is currently being specified in the 3GPP domain orthogonality is maintained among intra-cell users tostandards body. The Uplink (UL) for LTE is based on manage the amount of interference seen at the base station. SC-Single Carrier Frequency Division Multiple Access. The UL FDMA also has a low power amplifier de-rating (Cubic Metriccontrol channel carries non-data associated control / PAPR) requirement, thereby conserving battery life orsignaling like CQI, ACK/NACK, Scheduling request etc. extending range.To maintain the low PA power de-rating, the single carrier The baseline SC-FDMA signal is DFT-Spread OFDMproperty of the UL has to be maintained. As such, special (DFT-SOFDM) [2] as shown in Figure 1. The only differenceconsideration should be given to the UL control channel from OFDM is the addition of the M-point FFT (DFT) in thedesign. This paper discusses in detail the LTE UL control figure which “spreads” M symbols onto the M subcarrierschannel design and its performance. selected by the symbol to subcarrier mapping. The selected subcarriers must also be either adjacent or evenly spaced to I. Introduction maintain the low PA power de-rating. The signal is considered Evolved UMTS Terrestrial Radio Access (E-UTRA) is single carrier as the first M-point FFT and the larger N-point IFFT cancel each other resulting in a single carrier signal in theaimed at commercial deployment around 2010 timeframe. time domain. The receiver can use simple frequency domainLong term goals for the system include support for high peak equalization.data rates (100 Mbps downlink and 50 Mbps uplink), lowlatency (10ms round-trip delay), improved system capacity andcoverage, reduced operating costs, multi-antenna support,efficient support for packet data transmission, flexiblebandwidth operations (up to 20 MHz) and seamless integrationwith existing systems. To reach these goals, a new design forthe air interface is envisioned. Single-Carrier FrequencyDivision Multiple Access (SC-FDMA) is selected to efficientlymeet E-UTRA performance requirements for Uplink (UL). SC-FDMA has many similarities to OFDM, chief among them is Figure 1 - Block diagram for DFT-SOFDM.that frequency domain orthogonality is maintained among An advantage for DFT-SOFDM as a SC-FDMA techniqueintra-cell users. SC-FDMA also has a low power amplifier de- is that the numerology can match the OFDM downlink, withrating (Cubic Metric / PAPR) requirement, thereby conserving excellent spectral occupancy due to the IFFT providing pulsebattery life or extending range. The UL control channel is shaping of the signal. Table 1 provides the uplink subframedesigned to carry non-data associated control signaling like numerology for different spectrum allocations. The OFDMCQI, ACK/NACK, Scheduling Request (SR) etc. To maintain numerology provides for an additional vacant DC subcarrier tothe single carrier property of the UL special consideration simplify some receiver architectures; a vacant subcarriershould be given to UL control channel design. This paper cannot be used with DFT-SOFDM without affecting the lowprovides a detailed view on LTE UL control channel design PA de-rating property of DFT-SOFDM. In this case, normaland its performance. cyclic prefix is applied as shown in the table. An alternate The paper is organized as follows. In Section II, anoverview of E-UTRA uplink structure and frame format is frame structure with extended cyclic prefix length of 16.67 µsprovided. Section III, provides a summary of the uplink control is available to handle large cells.signaling. In Section IV, multiplexing details of uplink control1-4244-1144-0/07/$25.00 ©2007 IEEE.
  2. 2. The 18th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC07) III. UL Control Signaling Table 1. Parameters for UL transmission scheme. In principle, uplink control signaling can be divided into two Spectrum SC-FDMA CP duration Allocation (µs/#of occupied subcarriers (µs) categories: data-associated and data non-associated control (MHz) /samples) signaling. Data-associated control signaling is always transmitted with and used in the processing of data packet. 20 66.67/1200/2048 Examples of this control signaling include transport format, new data indicator, and MIMO parameters. In LTE it was 15 66.67/900/1536 agreed that this type of control signaling is not necessary. 10 66.67/600/1024 Control signaling not associated with data is transmitted (4.69 µs) × 12, independently of uplink data packet. Examples of this control (5.21 µs) × 2 signaling include ACK/NACK, CQI, and MIMO codeword 5 66.67/300/512 feedback. When users have simultaneous uplink data and 3 66.67/144/256 control transmission, control signaling is multiplexed with data prior to the DFT to preserve the single-carrier property in 1.4 66.67/72/128 uplink transmission. In the absence of uplink data transmission, this control signaling is transmitted in a reserved The physical uplink shared channel is defined by one frequency region on the band edge as shown in Figure 3. Notesubframe and the parameters NTx and k0, used in the generation that additional control regions may be defined as needed.of the SC-FDMA signal. The variables NTx and k0, determiningthe transmission bandwidth and the frequency hoppingpattern, respectively, are under control of the uplink schedulerand may vary on a per-sub-frame basis. The number of SC-FDMA symbols in a slot depends on the cyclic prefix lengthconfigured by higher layers. The uplink slot format (a sub-frame consists of two slots) with normal cyclic prefix (CP) isshown in Figure 2 with seven SC-FDMA symbols. For frameswith extended cyclic prefix, only six SC-FDMA symbols arepresent. The uplink supports QPSK, 16-QAM and 64-QAMmodulation. Figure 3. Control regions for uplink. Tcp Td Allocation of control channels with their small occupied LB LB bandwidth to carrier band edge resource blocks reduces out of Data RS carrier band emissions cause by data resource allocations on inner band resource blocks and maximizes the frequency 0.5 ms diversity benefit for frequency diverse control channel Figure 2. Uplink slot format. allocations while preserving the single carrier property of the uplink waveform. This FDM allocation of control resources to Two types of reference signals (RS) are supported on the outer carrier band edge allows an increase in the maximumuplink - (a) demodulation reference signal, associated with power level as shown in Figure 4 as well as maximizes thetransmission of uplink data and/or control signaling and (b) assignable uplink data rate since inserting control regions withsounding reference signal, not associated with uplink datatransmission used mainly for channel quality determination if consecutive subcarriers in the central portion of a carrier band requires that the time+frequency resources on either side of thechannel dependent scheduling is used. Orthogonality of control region to be assigned to different UEs.reference signals is obtained via frequency domainmultiplexing onto distinct set of sub-carriers. The RS 28.0 QPSK - 5MHz, Band Edge RBs for Datasequence length is equal to the number of sub-carriers in the Max Power Level (dBm) 27.0 16QAM - 5MHz, Band Edge RBs for Dataresource blocks. The RS sequence is generated either by QPSK - 5MHz, Band Edge RBs for Ctltruncation or cyclic extension of ZC (Zadoff-Chu) sequences 26.0 16QAM - 5MHz, Band Edge RBs for Ctldepending on the allocation size. It was observed that for a 25.0 Max Power Practical Limitation due to EVMgiven size, neither truncation nor cyclic extension was the best. and other considerations 24.0Many options exist for selecting either truncation or cyclicextension RS construction method, including: 23.0 1. Choose the method that for a given resource block (RB) 22.0size minimizes the amount of truncation or cyclic extension, 0 2 4 6 8 10 12 14 #RBs of size 25 subcarriers 2. Choose the method that for a given RB sizemaximizes the number of sequences with Cubic Metric <= the Figure 4. Increase in maximum power level if control istarget data modulation (e.g., QPSK). mapped to band edge.
  3. 3. The 18th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC07)Table 2 provides the required quality targets for uplink control perform joint channel estimation and decoding. This in turnsignaling. depends on the number of CQI bits to be supported. Two types of receivers are possible - Table 2. Uplink control signaling target quality. • Type 1: Channel estimation is first done based on the Event Target quality reference signals, and then CQI decoding is performed based on these channel estimates. ACK miss detection (1e-2) • Type 2: Channel estimation and decoding is done DTX to ACK error (1e-2) jointly using all possible CQI codewords. While this NACK to ACK error (1e-4) receiver is more complicated than Type 1 receiver, complexity is manageable for the CQI codeword CQI block error rate FFS (1e-2 – 1e-1) length being considered.A. Channel Quality Information Performance comparison between the two receiver types is shown in Figure 4 with Type 2 outperforming Type 1 receiver The CQI structure is shown in Figure 5. The transmission by approximately 2-3 dB. This is because, for this receiver,spans the entire 1ms sub-frame and up to six users may be channel estimation is aided by CQI codeword detection.multiplexed within the sub-frame via different cyclic shifts of a However, as shown in Figure 5, two reference signals per slotConstant Amplitude Zero Auto-Correlation (CAZAC) were chosen so as not to mandate particular receiversequence, e.g. Zadoff-Chu sequence. Data is modulated on top architecture at the Node B.of the CAZAC sequence using QPSK modulation. 0 10-bit CQI, TU, QPSK, Non-Ideal Chan Est 10 3 km/h 120 km/h 350 km/h -1 10 BLER Receiver Type 2 Receiver Type 1 -2 (24,10), 1 RS (20,10), 2 RS Figure 5. CQI channel structure. 10The number of CQI bits may vary between 5-10 bits dependingon whether wideband or narrowband CQI reports aretransmitted. However, larger CQI reports may be transmitted -3using multiple subframes. In addition, repetition may be used 10 -15 -10 -5 0 5to ensure reliable reception from cell edge users. An example SNR (dB) per antennaof CQI performance is shown in Figure 6 for various coding Figure 7. CQI performance with advanced receiver.schemes. 0 CQI (5-bit, 10-bit), TU, QPSK, Receiver Type 2, Non-Ideal Chan Est B. ACK/NACK 10 Figure 8 illustrates the ACK/NACK channel structure. Note that in this case only acknowledgment is present (no CQI -1 or data). To provide the maximum number of multiplexed 10 users, both frequency domain and time domain code multiplexing are used. In the frequency domain, different cyclic shifts of a CAZAC sequence are used to differentiate BLER users. For instance, with sequence length of 12 corresponding -2 10 to one resource block, 6 available cyclic shifts are possible. In 5-bit CQI, (32,10) Reed-Muller the time domain, block spreading is used to further multiplex -3 10 5-bit CQI, Convolutional additional users. For instance, within each cyclic shift of a 5-bit CQI, (24,12) Golay 10-bit CQI, (32,10) Reed-Muller Zadoff-Chu sequence, reference signals are multiplexed using 10-bit CQI, Convolutional 10-bit CQI, (24,12) Golay DFT code of length 3 while the acknowledgments are -4 0017 multiplexed using Walsh-Hadamard code of length 4. As a 10 -20 -15 -10 -5 0 result, acknowledgments from 18 different users may be SNR per antenna (dB) multiplexed within one resource block. The ACK/NACK is Figure 6. CQI performance with various coding schemes. then modulated onto the frequency and time-spread sequence. Both 1-bit and 2-bit acknowledgements are supported usingIt should be noted that the number of reference signals required BPSK and QPSK modulations.depends on the feasibility of using an advanced receiver to
  4. 4. The 18th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC07) 0 5-bit CQI, TU (3 km/h), Receiver Type 2, Non-Ideal Chan Est 10 CQI BLER - (20,5) Code 1-bit ACK/NACK SER -1 10 Error Rate -2 10Figure 8. ACK/NACK structure - users are multiplexed using different cyclic shifts and time-domain spreading. -3 10Figure 9 shows performance of 1-bit acknowledgments from18 multiplexed users. Although not shown here, for 2-bitacknowledgments the performance is approximately 3dB -4 10worse. -12 -10 -8 -6 -4 -2 0 2 SNR (dB) per antenna 0 10 ACK/NACK Performance - 18 users, GSM-TU (3 km/h) Figure 10. Performance of 5-bit CQI and 1-bit ACK/NACK 3 km/h (BPSK) at TU 3 km/h. IV. Multiplexing of Control and Data -1 10 To preserve the single-carrier property of uplink transmission, L1/L2 control signaling must be multiplexed with data prior to the DFT when both data and control are to be transmitted in the same TTI. This may be performed as shown BER -2 10 in Figure 11 where uplink data is uniformly punctured to provide room for control signaling. Naturally, in case of turbo -3 coding, puncturing is only performed on the parity bits. Since 10 the Node B has prior knowledge of uplink control signaling transmission, it can easily de-multiplex control and data packets. In addition, a power boosting factor may be applied when data is punctured to ensure similar data packet -4 10 -20 -18 -16 -14 -12 -10 -8 SNR (dB) performance to when control is absent. This is especially Figure 9. Performance of 1-bit acknowledgments (BPSK) at important in the case of re-transmission since the data MCS TU 3 km/h. cannot be changed due to synchronous H-ARQ operation in the uplink. This appropriate power boosting factor (in the order ofC. CQI + ACK/NACK 0.5-1.5dB) can be calculated based on the coding rate When CQI and ACK/NACK are to be transmitted reduction resulting from puncturing. With appropriate powersimultaneously, they are coded separately and multiplexed in a adjustment there should be little effect on the H-ARQTDM fashion. This allows greater control of CQI and performance at the receiver. Of course, power boosting is notACK/NACK error requirements, and the ability to multiplex possible when the UE is power-limited (e.g. at the cell edge).ACK/NACK into CQI reports that are transmitted in multiplesub-frames (either for large CQI report or through the use of Puncturing / Sub-carrier CPrepetition) once CQI transmission has started. Figure 10 Data Insertion DFT Mapping IFFT Insertionillustrates the performance of 5-bit CQI + ACK/NACK under Gain N TX symbolsTU 3 km/h channel with realistic channel estimation. In this Control Factor Size-NTX Size-N FFTcase, one SC-FDMA symbol per slot was used forACK/NACK. Figure 11. Multiplexing of control signaling with data. As an alternative, scheduling restriction may be used to Figure 12 illustrates typical performance degradation due toensure that CQI and ACK/NACK will not be transmitted in the turbo-code puncturing for both QPSK and 16-QAM. From thesame sub-frame. However, this may place unnecessary and figure, it is seen that the performance loss depends on thecomplicated constraint on the scheduler. Alternately, only initial coding rate. However, it may be observed that inACK/NACK can be transmitted (CQI is not transmitted in the general the amount of resources required to accommodatesub-frame). This may result in some scheduling and resource control information is small and less than 1dB degradation canallocation efficiency loss as some CQI reports will be lost. be expected. As a result, appropriate power boosting should be comfortably accommodated unless the UE is already in a power-limited situation (e.g. cell edge transmission).
  5. 5. The 18th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC07) 4 variable size which must be taken care of by the rate-matching QPSK 3.5 16-QAM algorithm. 3 CQI Coding Repetition Puncturing Penalty (dB) 2.5 2 MUX Modulation 1.5 ACK Coding Repetition 1 0.5 Figure 14. Mapping to multiple codewords. 0 Since control is multiplexed with data prior to the DFT, -0.5 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 appropriate modulation and coding selection for control is Code Rate required for reliable reception. As a result, the amount of Figure 12. Performance loss due to puncturing (turbo code). coded data to be punctured is variable based upon the MCS selected for control. In this case, rate matching may be done in Since both control and data must be transmitted with the one step. With one-step rate matching, the number of bitssame power, reliable reception of control information can be punctured for control is factored in when computing theachieved through appropriate selection of modulation and effective coding rate.coding. Since these control fields are generally small,codeword mapping is use to provide additional protection. V. ConclusionsSubsequent to codeword mapping, repetition (if necessary) and This paper provided an overview of the UL control channelmodulation selection are performed according to information design for 3GPP LTE.about the channel. Obviously, this selection can be tied to theMCS of the data block to aid in the decoding. In addition, it REFERENCESshould also depend on the uplink data transmission method (L- [1] 3GPP TR 25.913, Requirements for Evolved UTRA (E-UTRA) andFDMA or L-FDMA with hopping). This is because these two Evolved UTRAN (E-UTRAN), v.7.3.0, March 2006.localized transmission methods have different target error rates [2] 3GPP TR 25.814, Physical Layer Aspects for Evolved UTRA, v.2.0.0,for the same selected MCS. As a result, control power June 2006. [3] R1-070777, “E-UTRA Multiplexing of UL Control Signaling with Data,”requirement relative to the two transmission methods is Motorola, RAN1#48, St. Louis, USA, Feb 2007.different. [4] R1-070394, “Multiplexing of L1/L2 control signals between UE’s in the Two possible codeword mappings for the control signaling absence of data,” Nokia, RAN1#47bis, Sorrento, Italy, Jan. 2007.are as follows – [5] R1-070782, “Multiplexing of UL L1/L2 control signals in the absence of data,” Motorola, RAN1#48, St. Louis, USA, Feb 2007. (a) Single codeword: In this case, all control fields are [6] R1-070162, “EUTRA UL L1/L2 Control Channel Mapping,” Motorola,mapped into a single codeword (i.e. jointly coded) as shown in RAN1#47bis, Sorrento, Italy, Jan. 2007.Figure 13. If all fields are not present, dummy input values are [7] R1-070778, “CQI Feedback Overhead with CDM Uplink Control Channel Region,” Motorola, RAN1#48, St. Louis, USA, Feb 2007.inserted which are then ignored at the Node B. Alternatively, [8] R1-070275, “Ack/Nack transmission without reference signal overheadinthe UE may use the available fields to transmit some additional E-UTRA UL,” TI, RAN1#47bis, Sorrento, Italy, Jan. 2007.information based on an agreed upon methodology (e.g. UE [9] R1-070472, “Uplink control Signaling – Summary of e-mailthat does not support MIMO may transmit wideband CQI in discussions”, Ericsson, RAN1#47bis, Sorrento, Italy, Jan. 2007.the MIMO field). This results in codeword of the same length Note – 3GPP documents may be downloaded from ftp://ftp.3gpp.orgwhich may simplify the multiplexing and de-multiplexingprocess. However, with this approach it may be difficult tosatisfy performance requirements of different control fields.Also, overhead is higher. (CQI, ACK, MIMO, ...) Coding Repetition Modulation Figure 13. Mapping to one codeword. (b) Multiple codewords: In this case, each control field isindividually mapped to a codeword with its own repetitionfactor as shown in Figure 14. This allows individualadjustments of transmission energy using different coding andrepetition so that performance of each control field can becontrolled. However, this results in a control portion of

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