LTE defines an optional sounding reference signal (SRS) in the UL. What is it for? UL channel quality, timing advance, and more. SRS is transmitted by the UE using a known sequence, similar to UL demodulation reference signal (DM RS), so the eNB can use it to estimate the UL channel quality. You may have a question: UL DM RS is already there and the eNB can decode the UL information with its assistance, what's special about SRS? Well, UL DM RS is transmitted together with the UL data and both locate exactly in the same RBs, so the channel quality information the eNB extracts from the UL DM RS is for that transmission. In terms of SRS, it may be transmitted periodically in a wider bandwidth (beyond PUSCH RBs allocated for UL data transmission) and when there is no UL data transmission, so the channel information obtained from SRS is a good input to UL scheduler. It's like CQI report from UE for DL scheduler. Also, since SRS can be transmitted periodically, the eNB can use it to check the UE timing alignment status and send time alignment command to the UE accordingly.
Where is SRS located in a UL PHY frame? It is transmitted in the last symbol of a subframe if scheduled.
Figure 1: Illustration of SRS location in UL PHY frame
In the time domain, SRS can transmit once or periodically based on eNB scheduling. If periodically, the UE-specific periodicity can be 2 / 5 /10 / 20 / 40 / 80 / 160 / 320 ms as defined in TS36.213 section 8.2. The eNB defines the SRS transmit time instance (specific subframes) in TS 36.211 section 126.96.36.199, which is shared by all UEs.
In the frequency domain, the SRS from a UE is transmitted in the unit of 4RBs, defined in TS36.211 section 188.8.131.52-1/2/3/4. For example, with system bandwidth of 50 RBs, we should check Table 184.108.40.206-2. There are 8 possible configurations. Here we show the configuration 1 on the left and the configuration 2 on the right. With CSRS = 1, the SRS for a UE can occupy 4 RBs, 8 RBs, 16 RBs, or up to 48 RBs. It's a tree-based structure and the allocation scheme would be similar to OVSF allocation in WCDMA. So, multiple UEs share the RBs in frequency domain.
Figure 2: Example of SRS bandwidth configuration
You may notice that the same RBs are shared by two UEs. Why? It's because SRS uses Interleaved SC-FDMA (IFDMA) where consecutive subcarriers can be allocated to several UEs alternatively. LTE decides to multiplex two UEs in one RB as shown in Figure 3, i.e., subcarriers in one RB are allocated to two UEs alternatively. This structure provides comb-like effect and expands the SRS channel bandwidth of a UE. Frequency hopping can also be used to help cell edge UE (with power limitation) spread sounding channel bandwidth by using different frequency locations in different subframes/transmit opportunities.
Figure 3: Multiplexing of two UEs in one RB (12 subcarriers over one slot)
Next, what is content of the SRS? SRS uses the same Zadoff-Chu sequence as UL demodulation reference signal (DM RS). Since the cyclic shift versions of the Zadoff-Chu sequence are orthogonal, several UEs (up to 8) can transmit using different cyclic shifts on the same physical radio resource.
In summary, SRS can be used to estimate UL channel quality and provide input to UL scheduler, as well as help timing advance. SRS is transmitted in the last symbol of a subframe periodically and occupies one comb leg of several RBs (multiple of 4). The content of SRS is Zadoff-Chu sequence. Multiple levels of UE multiplexing are supported: subframe (time), RBs (frequency/ RB level), comb leg (frequency/ subcarrier level), cyclic shift (code).
I am now study spec about SRS.
I could not understand one thing clearly in the spec.211: srsMaxUpPts
for example, the cell SRS bandwidth set :80,40,20,4
when srsMaxUpPts = 1, no PRACH, then in the UpPts, how do the SRS bandwiths change?
Form my understanding to the word in the spec, I think only the largest bandwidth changes.
so , the cell SRS bandwidth set is chenged to 96,40,20,4.
What's your opinion?
Hello, Thanks for the article. I have a question for you.
If suppose UE is given an UL grant to transmit PUSCH on RBs 10,11,12,13, but at the same time it needs to transmit SRS on RBs 31,32,33,34, will the UE able to send both(PUSCH and SRS) or will it drop the SRS as UL transmission will not be on contiguous RBs as required by SC-FDMA?