6.2.2
SRS
Channel dependent scheduling, in both the time and frequency domain, is a key LTE technology. The RS allow for UL channel estimation on the subcarriers, which are currently used by the UE’s PUSCH. The intention with the SRS is for the network to estimate the channel quality of the uplink frequencies, which are currently not used by UE’s PUSCH transmission. The sounding reference signals can also be used to estimate the timing of UE transmissions and to derive timing control commands for UL time alignment.
SRS are transmitted independently of the UE’s PUSCH transmission, i.e. a UE may transmit the SRS also in subframes, where it does not have any data transmission. Furthermore, the bandwidth of SRS can be, and typically is, different from that of the UE’s PUSCH.
The SRSs are transmitted within the last DFT-s-OFDM block of a subframe as shown in Figure 6.7. The SRS resources are shared by a number of UEs by their multiplexing in the time, frequency and orthogonal codes domain:
• SRS in the time domain.
Different UEs may by configured to transmit SRS in different subframes by providing the UE with SRS periodicity and SRS subframe offset. The period- icity of the SRS transmission is selected from the set{2, 5, 10, 20, 40, 80, 160, 320} ms or subframes.
• SRS in the frequency domain.
UE may be configured to transmit SRS in the whole or a fraction of the UL carrier bandwidth. For example, if srs-BandwidthConfig = 2 in a cell with 5 MHz UL bandwidth then some UEs in the cell may be configured to transmit SRS in the bandwidth of 24 RBs, some in the bandwidth 4 RBs and some in the bandwidth of 4 RBs.
• SRS orthogonal codes.
Similar to the RS, the SRS is a Zadoff-Chu sequence. With cyclic shifts up to 8 shift can be configured, which implies that up to 8 UEs can transmit SRS in the same time and in the same bandwidth but with different orthogonal (independent) sequences.
Example of the SRS allocation is illustrated in Figure 6.7.
6.3
PUCCH
PUCCH supports uplink L1/L2 control signalling, which carriers one (or more) of the following singling information:
• HARQ acknowledgements related to reception of DL-SCH transport. HARQ
acknowledgements are sent by PUCCH format 1A, 1B or PUSCH.
• Scheduling requests, used by the terminal to request UL-SCH resources in case
it does not have a valid scheduling grant. Scheduling request are transmitted on the PUCCH format 1.
Figure 6.7: UL SRS.
• Channel Quality Indicator (CQI) indicating the downlink channel quality per-
ceived by the terminal. CQI is used by the network for DL modulation and coding scheme selection. The CQI reports are transmitted periodically on PUCCH format 2 or aperiodically on PUSCH. UE reporting is discuss in section 7.5.
As illustrated in Figure 6.8, these resources are located at the edges of the total available system bandwidth. Each such resource consists of 12 subcarriers (one re- source block) within each slot of an uplink subframe. To provide frequency diversity, these frequency resources are frequency hopping on the slot boundary, that is one L1/L2 control resource consists of 12 subcarriers at the upper part of the spectrum within the first slot of a subframe and an equally sized resource at the lower part of the spectrum during the second slot of the subframe or vice versa, and it is referred to as a resource block pair.
If more resources are needed for the uplink L1/L2 control signalling, for example, in case of very large overall transmission bandwidth supporting a large number of users, additional resources blocks can be assigned next to the previously assigned resource blocks.
6.3 PUCCH
6.3.1
PUCCH format 1A/1B
PUCCH format 1A and 1B are used for transmission of HARQ acknowledgements.
• Format 1A supports one bit acknowledgement to one code word.
• Format 1B supports two bits acknowledgement to two code words sent to the
UE during one subframe, which is the case of spacial multiplexing.
One (format 1A) or two (format 1B) acknowledgement bits are modulated using BPSK or QPSK, respectively, resulting in one complex number (modulation sym- bol).
A length-12 Constant Amplitude Zero AutoCorrelation (CAZAC) sequence is ap- plied to each symbol in order to spread the symbol over 12 symbols sent on different subcarriers of an RB. Different cyclic shift of the length-12 CAZAC sequence are applied by different users, therefore 12 UEs feedbacks can be transmitted over the same subcarriers in the same time. Then scrambling is applied to all the symbols, see Figure 6.9. Different scrambling codes are used in the two different slots within one subframe.
The 12 complex numbers are further multiplied by an orthogonal cover sequence. Orthogonal cover sequences are applied to both the four information symbols in a slot as well as to the three reference signal symbols. Thus, with three reference symbols per slot, up to three orthogonal cover sequences can be used. This implies three different UEs acknowledgements can be transmitted at the same cyclic shift of the length-12 CAZAC sequence resulting in up to 3· 12 = 36 UEs with PUCCH format 1A/1B sharing one resource block pair.
The same PUCCH structure is used in the two slots of a subframe. To further ran- domise the inter-cell interference between PUCCH resource blocks, cyclic shift hop- ping (per OFDM symbol) and orthogonal cover hopping (per slot) are used.
Figure 6.9: PUCCH format 1.
HARQ acknowledgements are transmitted at a fixed time after the reception of a DL-SCH transport block (4 subframes in case of FDD). Furthermore, the PUCCH resource to use is derived from the index of the first control channel element in the
PDCCH used for scheduling the downlink transmission (or from RRC signalling in case of persistent scheduling).
6.3.2
PUCCH format 1
PUCCH format 1 is used for transmitting scheduling requests. The overall structure is similar to that used for HARQ acknowledgements. Each active terminal is assigned a dedicated resource for scheduling request through RRC signalling, providing the possibility to request an uplink grant every x subframe.
If the UE do not want more scheduling, then it will not transmit anything on the dedicated resources.
6.3.3
PUCCH format 2
PUCCH format 2 is used for CQI reports. The CQI reports are coded to 20 bits and scrambled. The scrambling sequence depends on the CellID, slot number and Cell RNTI (C-RNTI). The scrambled bits are then modulated using QPSK, resulting in 10 complex valued symbols, see Figure 6.10. Each of the QPSK symbols (assuming normal cyclic prefix) is multiplied by a cyclically shifted length-12 CAZAC sequence and transmitted in one DFT-s-OFDM symbol. As the same underlying principle of cyclically shifted CAZAC sequences is used for PUCCH format 2 as for format 1A/1B, CQI from different terminals can be transmitted on the same time-frequency resource by assigning different cyclic shifts. In theory, it is possible to use 12 different cyclic shifts, hence twelve different UE’s CQI can be transmitted in the same resource block pair.
Figure 6.10: PUCCH format 2.
It is also possibility for one UE to send CQI reports together with ACK/NACK. In that case format 2A or 2B is used. However, it is also possible to mix different