Most of the channels (e.g, PDSCH, PDCCH, PBCH etc) is for carrying a special information (a sequence of bits) and they have some higher layer channel connected to them, but Reference Signal is a special signal that exists only at PHY layer. This is not for delivering any specific information. The purpose of this Reference Signal is to deliver the reference point for the downlink power.

When UE try to figure out DL power (i.e, the power of the signal from a eNode B), it measure the power of this reference signal and take it as downlink cell power. Another important role of reference signal is to help the receiver demodulate the received signal. Since the reference signal is made up of data known to both transmitter and reciever, the reciever can figure out how the communication channel destort the data by comparing the decoded received reference signal and predefined reference signal, and use the result of this comparison to equalize (post process) the recieved user data. The process for the reciever to perform this comparision and figure out the characteristics of a communication channel is called 'Channel Estimation' which is one of the most critical part of many high-end wireless communication like LTE. (If you are really interested in the detailed procedure, I would strongly suggest you to study the basic concept of channel estimation)

These reference signal are carried by multiples of specific Resource Elements in each slots and the location of the resource elements are specifically determined by antenna configuration.

As LTE gets evolved into higher version, we are getting more and more reference signal which is mapped to a specific antenna port. And we are getting more and more confused as a result -:)

Following shows the reference signals supported by each 3GPP version. The table stops at 36.211 v11.4.0, so the last section on this page says what clause 6.10 has gained since.

For the exact Resource Element locations of each reference signal, refer to following pages.

• RS (Reference Signal) - Cell Specific (Antenna port 0,1,2,3)
• RS (Reference Signal ) - MBSFN (Antenna Port 4)
• RS (Reference Signal ) - UE Specific (Antenna Port 5,7,8,9,10)
• RS (Reference Signal ) - Positioning (Antenna Port 6)
• RS (Reference Signal ) - CSI (Antenna Port 15,16,17,18,19,20,21,22)

To implement this signal, you need to go through two steps - signal generation and resource allocation. The details of signal generation and resource allocation would vary on the type of reference signal. In this page, I would focus mostly on Cell Specific Reference Signal to give you general idea.

Followings are the list of topics in this page.

• Generation of Reference Signal
• Resource Allocation of Reference Signal
• Reference Signal Antenna Port Number vs Transmission Mode
• What 36.211 v19.3.0 adds to these lists
• Reference

Generation of Reference Signal

Signal generation is done by the following procedure. You would notice that Cell ID is a key parameter for the sequence and you would guess the sequence will be unique for each Cell ID.

Another thing you would notice here would be that downlink reference signal is a kind of Gold sequence whereas most of UL reference signal and DL Synchronization signal is based on Zadoff Chu sequence. (c(n) is a Gold Sequence(Psuedo Random Sequence)and you see the reference signal is generated by combining the two Gold Sequences). Following equation is based on  36.211 6.10.1.1

The equation below is the whole of clause 6.10.1.1 in one picture. The top line defines the reference signal symbol itself, the three lines under it define the pseudo-random sequence behind it, and the bottom line is the seed that ties the sequence to a particular cell, slot and symbol.

< LTE Downlink Refence Signal - Sequence Generation >

The symbol itself is one of four points : the top line builds a QPSK symbol out of two consecutive bits of c, so every reference signal symbol has the same magnitude and one of four phases.

The sequence is a Gold sequence : the two lines for x₁ and x₂ are the two shift registers, and c is their sum modulo 2. That is the construction the paragraph above names.

The seed carries the cell identity twice : the bottom line multiplies the physical cell ID by 2 in one term and by 2 again in another, so two cells with different identities get different sequences from the very first symbol.

The seed changes every symbol : the slot number and the symbol number both enter it, so the generator is re-initialised at the start of each OFDM symbol rather than run continuously.

The cyclic prefix appears in the seed too : the note at the lower right sets N_CP to 1 for normal cyclic prefix and 0 for extended, so the two prefix lengths do not share a sequence.

One example of reference signal symbol data is as follows. This is generated by Matlab LTE Toolbox and Refer to Matlab :ToolBox : LTE : Downlink : Cell Specific Reference Signal for the detailed Matlab source. You don't have to try to understand the details of how this is generated (If you really want to understand the details of the signal generation. The only way is to create a program on your own based on the mathematical formula shown above). With this example, you can just build up some intuitive understandings of reference signal properties. See the constellation, it is QPSK, not Zadoff Chu.

The plots below come from that generator run once. They are worth reading for the shape rather than the values, because the point of them is that a reference signal looks like noise until you know the seed.

< LTE Downlink Reference Signal - Matlab Tool Box >

The constellation has four points and nothing between them : the upper left plot shows the four QPSK points at roughly plus and minus 0.7 on each axis, which is the 1 over the square root of 2 in the equation above.

The upper right plot is sparse because the mapping is sparse : the horizontal axis runs to about 1000 resource elements and the marked ones sit in four clusters, since only some resource elements in a slot carry a reference signal.

The lower plot has no pattern in it : against RS number rather than RE index the real and imaginary parts alternate with no visible period, which is what a Gold sequence is for.

Resource Allocation of Reference Signal

Once you have generated the sequence, next step is to allocate each data point of the sequence to a specified resource elements. That is done by the following process. The resulting location of the process is as shown in  Reference Signal section of Downlink Frame Structurepage. Following equation is based on 36.211 6.10.1.2

The picture below is clause 6.10.1.2 laid out the same way. The boxed expression at the top is the resource element, and every arrow into it points at one of the terms that decides where that element sits.

< LTE Downlink Reference Signal - RE Mapping >

The symbol index takes two values only : l is 0 or N^DL_symb minus 3 for antenna ports 0 and 1, and 1 for ports 2 and 3. Ports 2 and 3 therefore appear on one symbol per slot where ports 0 and 1 appear on two.

The subcarrier index steps by 6 : k is 6m plus an offset, so a reference signal sits on every sixth subcarrier of the symbols that carry it.

The offset is where the port and the cell both enter : v depends on the antenna port and, for ports 2 and 3, on the slot number as well. The term v_shift is the physical cell ID modulo 6.

Modulo 6 is what sets the repeat : because the shift is taken modulo 6, two cells whose identities differ by 6 put their reference signals on the same subcarriers.

Note 1 : The DL Reference Signal (Cell Specific Reference Signal) is mainly determined by Physical Cell ID.

Note 2 : The resource element locations for DL reference signal gets different according to Physical Cell ID, but there is possibility that the reference signal location with two different physical cell ID can be same if (PCI1 mod 6) == (PCI2 mod 6). (PCI stands for Physical Cell ID). It means that you should be careful when you allocate the physical cell ID for multiple cells in a specific area. Following is some of examples of Reference Signal Location with different physical cell IDs.

(I created following subframe structure using LTE Resource Grid and edited to fit the topics of this page)

The picture below shows that modulo 6 result. Six subframes are shown side by side, one for each physical cell ID from 0 to 5, and the black cells are the reference signal positions.

< LTE Downlink Reference Signal - SISO - Location based on Physical Cell ID >

Each picture is two slots wide : the header row of every grid reads Slot 0 and Slot 1, and the symbol numbers run 0 to 6 across each slot.

The black cells move down by one row per cell ID : the pattern in the grid for cell ID 1 is the cell ID 0 pattern shifted by one subcarrier, and so on to cell ID 5.

The coloured cells on the left do not move : the yellow, blue and magenta cells sit in the same place in all six grids, while the black ones shift by one row each time.

If you examined the location of the reference signal (black cells) shown above, you may ask "According to the RE mapping formula, the reference signal shift is done by ' CellID mod 6' which imply that the reference signal location would repeat at every 6 CellID interval, but according to the grid shown above it seems the location repeats at every 3 CellID interval, not 6. Because of this, UE may experience some degree of RS interference at every 3 CellID intervals even though RS location is shifted by 'CellID mod 6'. (You may see this kind of interference at Intra Frequency Interference between LTE and LTE with Varying Physical Cell ID (PCI) )

Actually this is a kind of illusion. In reality the location repeats at every 6 cellID, but the interval of the reference signal in frequency domain (vertical direction) repeats at every 3 RE(resource element) and all the reference signal is marked in a same color (black). That's why it looks as if it repeat at every 3 CellID. To remove this confusion, I marked the RS at a symbol and marked in different color with 6 REs in vertical direction(I picked only one symbol.. it was too much work to mark different colors manually in Windows Paint :). Now you may (hopefuly) see the shift by 6 interval.

The picture below is the same six grids with one symbol marked. The red cells mark a single reference signal position and its repeats, which is what the paragraph above is about.

< LTE Downlink Reference Signal - SISO - Location based on Physical Cell ID >

The red cell sits at a different height in each grid : that is the v_shift term working through the six cell identities one step at a time.

Only one symbol is marked : the paragraph above says so, and the black cells elsewhere in each grid are the same reference signal on the other symbols.

Six is the period, not three : reading the red cells across the six grids shows one step per cell ID and no repeat inside the six, which is the answer to the question the paragraph above raises.

Reference Signal Antenna Port Number vs Transmission Mode

Reference Signals are used for various purpose and the type of reference signal being used varies depending on transmission mode. Some of the possible combination of refernece signal and transmission mode are as follows.

The rows below are read left to right as one configuration. The four columns on the right are the interesting ones, because a single transmission mode can use a different reference signal for the control channel, for PDSCH, for channel state measurement and for demodulation.

Note : ',' indicate "AND". (E.g, p0, p1 means that p0 AND p1 are used)

Note : UE Specific means "UE Specific Reference Signal, UE Specific Antenna ports" or is called "DMRS (Demodulation Reference Signal) as well.

Note : "No of Tx" means the number of Tx antenna on eNodeB and "No of Rx" means number of Rx antenna on UE

Note : TM9 can have much more combinations, but I listed only on the combination I have seen until now (Jun 2014)

What 36.211 v19.3.0 adds to these lists

The version table near the top of this page ends at 36.211 v11.4.0, and clause 6.10 has grown since. One whole reference signal type has been added, one existing type has been split four ways, and the CSI reference signal now reaches four times as many antenna ports as the list above gives it.

A fourth downlink reference signal type was added : clause 6.10.3A of v19.3.0 defines the demodulation reference signal for EPDCCH, MPDCCH and SPDCCH, on antenna ports 107 to 110. No row of the version table above carries those ports.

That signal follows its own control channel : 36.211 says it is transmitted on the same antenna port as the EPDCCH, MPDCCH or SPDCCH it belongs to, and only on the physical resource blocks that channel is mapped to.

CSI reference signals now reach 32 antenna ports : v19.3.0 allows 1, 2, 4, 8, 12, 16, 20, 24, 28 or 32 of them, where the list above stops at eight and the table at the end of this page stops at p22.

More than eight ports are built by aggregation : 36.211 says configurations in the same subframe are aggregated to reach the total, the first taken from resourceConfig and the rest from NZP-ResourceConfig-r13.

The MBSFN reference signal was split four ways : clause 6.10.2.1 now has a sequence generation subclause for 15 kHz and 7.5 kHz, another for 1.25 kHz, another for 2.5 kHz and another for 0.37 kHz, each with a mapping subclause to match.

Both clause numbers on this page are still correct : 6.10.1.1 is still sequence generation and 6.10.1.2 is still mapping to resource elements, so both equation pictures point at the right place.

CRS is no longer in quite every downlink subframe : 36.211 now says a UE cannot assume CRS in the fifth OFDM symbol of the special subframe when ssp10-CRS-LessDwPTS is configured. For frame structure type 3 it is present in non-empty subframes only.

Reference

One specification carries every clause and every equation quoted on this page. It was resolved at v19.3.0, which is the latest published version at the time of writing.

• 3GPP TS 36.211 v19.3.0, E-UTRA Physical channels and modulation. Clause 6.10.1 for the cell specific reference signal, 6.10.2 for MBSFN, 6.10.3 for the UE specific one, 6.10.3A for the EPDCCH and MPDCCH demodulation reference signal, 6.10.4 for positioning and 6.10.5 for CSI.