The figure below shows the timing diagram of LTE FDD uplink HARQ process between eNB and UE (X-Series). There are 8 uplink HARQ processes running on both UE and eNB with 4 processes delay. HARQ process length is same as a subframe (1 ms). When UE sends data to eNB, eNB decodes data and checks CRC. eNB then sends acknowledgement (ACK) or not-acknowledged (NACK) to UE after 4 subframes. Based on the eNB response, UE will send new data or retransmit same data with different redundancy version (RV).
On X-Series with multiplexed mode, X-Series samples AUX IO input level at specified delay within 100 us window. If the level is high (with input polarity settings set to positive), it detects as ACK, and if the level is low, input is NACK. This delay specifies timing of eNB’s initial HARQ response from the start of frame. All of subsequent process use the same delay time. To set input monitor window at the center of eNB’s process 0 subframe, the HARQ input delay settings for AUX multiplex mode is set to 138,240 samples (=4.5*30720Ts). Although sampling window has 100 us width, it is recommended to hold input level high/low for the entire subframe length (1 ms) to ensure X-Series detects command.
TDD has different HARQ timing scheme from FDD as it has different uplink and downlink slot allocation based on UL/DL configuration. All of TS 36.141 receiver performance test configuration requires TDD UL/DL configuration 1, which has same number of UL and DL subframe thus each downlink subframes carries one HARQ ACK/NACK (IPHICH=0) to correspond uplink subframe. For UL/DL configuration 0 with more UL subframes than DL, PHICH carries 2 HARQ ACK/NACK which is identified by IPHICH = 0 and 1 to indicate ACK/NACK for 2 UL subframes.
When IPHICH=0, the received HARQ ACK/NACK corresponds to the UL subframe i-k from the following table. For example, when HARQ is received on subframe 1, it refers 1-4=-3, which refers subframe 7 in previous frame.
|
DL subframe (i) |
0 |
1 |
(2) |
(3) |
4 |
5 |
6 (DwPTS) |
(7) |
(8) |
9 |
|
k for IPHICH=0 |
|
4 |
|
|
6 |
|
4 |
|
|
6 |
|
Corresponding UL subframe(i-k) |
|
7 (prev. frame) |
|
|
8 (prev. frame) |
|
2 |
|
|
3 |
Sampling timing for HARQ feedback signal on X-Series will be determined automatically based on the UL/DL configuration for LTE TDD HARQ. HARQ input delay (level triggered and serial bits) must be set to 0 for LTE TDD.
The detected HARQ input can be monitored using marker output type, “Hybrid ARQ State.”
With multiplexed mode, timing advance (TA) input can’t be used.