system design about PUCCH in LTE

1. The 18th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC'07)
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, USA
Abstract —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 UL
evolved system include support for improved system
capacity and coverage, high peak data rates, low latency, In the uplink, Single-Carrier Frequency Division Multiple
reduced operating costs, multi-antenna support, flexible Access (SC-FDMA) is selected to efficiently meet E-UTRA
bandwidth operations and seamless integration with performance requirements. SC-FDMA has many similarities to
existing systems. To reach these goals, a new design for the OFDM, chief among them for the uplink is that frequency
air interface is currently being specified in the 3GPP domain orthogonality is maintained among intra-cell users to
standards 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 Metric
control channel carries non-data associated control / PAPR) requirement, thereby conserving battery life or
signaling 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 OFDM
property of the UL has to be maintained. As such, special (DFT-SOFDM) [2] as shown in Figure 1. The only difference
consideration should be given to the UL control channel from OFDM is the addition of the M-point FFT (DFT) in the
design. This paper discusses in detail the LTE UL control figure which “spreads” M symbols onto the M subcarriers
channel 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 the
aimed at commercial deployment around 2010 timeframe.
time domain. The receiver can use simple frequency domain
Long term goals for the system include support for high peak
equalization.
data rates (100 Mbps downlink and 50 Mbps uplink), low
latency (10ms round-trip delay), improved system capacity and
coverage, reduced operating costs, multi-antenna support,
efficient support for packet data transmission, flexible
bandwidth operations (up to 20 MHz) and seamless integration
with existing systems. To reach these goals, a new design for
the air interface is envisioned. Single-Carrier Frequency
Division Multiple Access (SC-FDMA) is selected to efficiently
meet 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 technique
intra-cell users. SC-FDMA also has a low power amplifier de-
is that the numerology can match the OFDM downlink, with
rating (Cubic Metric / PAPR) requirement, thereby conserving
excellent spectral occupancy due to the IFFT providing pulse
battery life or extending range. The UL control channel is
shaping of the signal. Table 1 provides the uplink subframe
designed to carry non-data associated control signaling like
numerology for different spectrum allocations. The OFDM
CQI, ACK/NACK, Scheduling Request (SR) etc. To maintain
numerology provides for an additional vacant DC subcarrier to
the single carrier property of the UL special consideration
simplify some receiver architectures; a vacant subcarrier
should be given to UL control channel design. This paper
cannot be used with DFT-SOFDM without affecting the low
provides a detailed view on LTE UL control channel design
PA de-rating property of DFT-SOFDM. In this case, normal
and its performance.
cyclic prefix is applied as shown in the table. An alternate
The paper is organized as follows. In Section II, an
overview of E-UTRA uplink structure and frame format is frame structure with extended cyclic prefix length of 16.67 µs
provided. Section III, provides a summary of the uplink control is available to handle large cells.
signaling. In Section IV, multiplexing details of uplink control
1-4244-1144-0/07/$25.00 ©2007 IEEE.

2. The 18th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC'07)
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. Note
subframe 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, determining
the transmission bandwidth and the frequency hopping
pattern, respectively, are under control of the uplink scheduler
and may vary on a per-sub-frame basis. The number of SC-
FDMA symbols in a slot depends on the cyclic prefix length
configured by higher layers. The uplink slot format (a sub-
frame consists of two slots) with normal cyclic prefix (CP) is
shown in Figure 2 with seven SC-FDMA symbols. For frames
with extended cyclic prefix, only six SC-FDMA symbols are
present. The uplink supports QPSK, 16-QAM and 64-QAM
modulation. 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 maximum
uplink - (a) demodulation reference signal, associated with
power level as shown in Figure 4 as well as maximizes the
transmission of uplink data and/or control signaling and (b)
assignable uplink data rate since inserting control regions with
sounding reference signal, not associated with uplink data
transmission 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 the
channel dependent scheduling is used. Orthogonality of
control region to be assigned to different UEs.
reference signals is obtained via frequency domain
multiplexing onto distinct set of sub-carriers. The RS 28.0
QPSK - 5MHz, Band Edge RBs for Data
sequence length is equal to the number of sub-carriers in the
Max Power Level (dBm)
27.0 16QAM - 5MHz, Band Edge RBs for Data
resource blocks. The RS sequence is generated either by QPSK - 5MHz, Band Edge RBs for Ctl
truncation or cyclic extension of ZC (Zadoff-Chu) sequences 26.0
16QAM - 5MHz, Band Edge RBs for Ctl
depending on the allocation size. It was observed that for a 25.0 Max Power Practical Limitation due to EVM
given size, neither truncation nor cyclic extension was the best. and other considerations
24.0
Many options exist for selecting either truncation or cyclic
extension RS construction method, including: 23.0
1. Choose the method that for a given resource block (RB) 22.0
size 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 size
maximizes the number of sequences with Cubic Metric <= the Figure 4. Increase in maximum power level if control is
target data modulation (e.g., QPSK). mapped to band edge.

3. The 18th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC'07)
Table 2 provides the required quality targets for uplink control perform joint channel estimation and decoding. This in turn
signaling. 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 slot
Constant Amplitude Zero Auto-Correlation (CAZAC)
were chosen so as not to mandate particular receiver
sequence, 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. 10
The number of CQI bits may vary between 5-10 bits depending
on whether wideband or narrowband CQI reports are
transmitted. However, larger CQI reports may be transmitted -3
using multiple subframes. In addition, repetition may be used 10
-15 -10 -5 0 5
to ensure reliable reception from cell edge users. An example SNR (dB) per antenna
of 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 using
It 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. The 18th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC'07)
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
10
Figure 8. ACK/NACK structure - users are multiplexed using
different cyclic shifts and time-domain spreading. -3
10
Figure 9 shows performance of 1-bit acknowledgments from
18 multiplexed users. Although not shown here, for 2-bit
acknowledgments the performance is approximately 3dB -4
10
worse. -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 of
C. 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 power
simultaneously, they are coded separately and multiplexed in a adjustment there should be little effect on the H-ARQ
TDM fashion. This allows greater control of CQI and performance at the receiver. Of course, power boosting is not
ACK/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 multiple
sub-frames (either for large CQI report or through the use of
Puncturing / Sub-carrier CP
repetition) once CQI transmission has started. Figure 10 Data
Insertion
DFT
Mapping
IFFT
Insertion
illustrates the performance of 5-bit CQI + ACK/NACK under Gain
N TX symbols
TU 3 km/h channel with realistic channel estimation. In this Control Factor
Size-NTX Size-N FFT
case, one SC-FDMA symbol per slot was used for
ACK/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 to
ensure that CQI and ACK/NACK will not be transmitted in the turbo-code puncturing for both QPSK and 16-QAM. From the
same sub-frame. However, this may place unnecessary and figure, it is seen that the performance loss depends on the
complicated constraint on the scheduler. Alternately, only initial coding rate. However, it may be observed that in
ACK/NACK can be transmitted (CQI is not transmitted in the general the amount of resources required to accommodate
sub-frame). This may result in some scheduling and resource control information is small and less than 1dB degradation can
allocation 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. The 18th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC'07)
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 bits
same power, reliable reception of control information can be punctured for control is factored in when computing the
achieved 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. Conclusions
Subsequent to codeword mapping, repetition (if necessary) and This paper provided an overview of the UL control channel
modulation selection are performed according to information design for 3GPP LTE.
about the channel. Obviously, this selection can be tied to the
MCS of the data block to aid in the decoding. In addition, it REFERENCES
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process. However, with this approach it may be difficult to
satisfy 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 is
individually mapped to a codeword with its own repetition
factor as shown in Figure 14. This allows individual
adjustments of transmission energy using different coding and
repetition so that performance of each control field can be
controlled. However, this results in a control portion of