Tuesday, 16 July 2013

What is PUCCH Mixed Mode in LTE and PUCCH Format Types in LTE




What are different PUCCH formats and PUCCH Mixed Mode in LTE?

We should know what all PUCCH formats are available in LTE or LTE-A, before exploring PUCCH Mixed Mode.
Basically PUCCH formats are of two types Format 1 and Format 2 (Format 3 is introduced in LTE advance release 10, which uses modulation scheme QPSK and number of bits used as 48 per subframe).
 
PUCCH Format 1 (Rel 8):

Format Type
Control Information
Modulation Scheme
No. of bits / Subframe
1
SR (Scheduling Request)
Not Applicable
Not Applicable
1a
HARQ ACK/NACK
BPSK
1 bit
1b
HARQ ACK/NACK (for MIMO)
QPSK
2 bits

PUCCH Format 2:

Format Type
Control Information
Modulation Scheme
No. of bits / Subframe
2
CSI (Channel State Info.)
QPSK
20 bits
2a
CSI+HARQ ACK/NACK
QPSK+BPSK
21 bits
2b
CSI+HARQ ACK/NACK (for MIMO)
QPSK + BPSK
22 bits

Location of PUCCH resources are on the edge of bandwidth allocated. To provide frequency diversity, PUCCH frequency resources are frequency hopping on the slot boundary (mentioned in below figure).
Mapping of modulation symbols for the physical uplink control channel

Why the location of PUCCH resources are on the edge of bandwidth? Here is the answer, to assign the contiguous RBs to single terminal for PUSCH data transmission along with increased frequency diversity experience by control signaling.  

You might be thinking that what could be the maximum value of m. The value of m depends on the number of UEs in the eNB or Macro eNB coverage area. To control more UEs, more control signaling with more PUCCH RBs would be required and hence value of m will be more.  Maximum value of m could be equivalent to the maximum number of RBs (in case of 10MHz bandwidth, it is 50), but it is not practical.

Now how do we derive the value of m?
Index m is derived from higher layer parameter, Refer 36.211 section 5.4.3 ( N1_PUCCH, N_RB_SC, N_UL_RB, N2_RB, c, Ncs, delta_pucch_shift) for Format 1.

Index m is derived from higher layer parameter, Refer 36.211 section 5.4.3 (N2_PUCCH, N_RB_SC, N_UL_RB) for Format 2. 
Where,
N1_PUCCH is Resource index for PUCCH formats 1/1a/1b.
N_RB_SC is Resource block size in the frequency domain, expressed as a number of subcarriers.
N_UL_RB is Uplink bandwidth configuration, expressed in multiples of N_RB_SC.
N2_RB is Bandwidth available for use by PUCCH formats 2/2a/2b, expressed in multiples of N_RB_SC.
Ncs is Number of cyclic shifts used for PUCCH formats 1/1a/1b in a resource block with a mix of formats 1/1a/1b and 2/2a/2b.
N2_PUCCH is Resource index for PUCCH formats 2/2a/2b.

You can explore more about the calculation of m here .

So, what is PUCCH Mixed Mode? In my view, PUCCH mixed mode occurs,  if same resource block is shared between two or more UEs to transmit the PUCCH format 1 by first (or second ) UE and the PUCCH format 2 by second (or first) UE.

The actual meaning of PUCCH Mixed Mode is some UE are transmitting either SR or HARQ ACK/NACK in the same resource block whiles other transmitting CQI/PMI/RI with or without HARQ ACK/NACK in the same resource block.
   
To enable PUCCH mixed mode, Ncs parameter value should not be set as 0 (should be in between 1..7) and resource index parameter should be same for both UE profile configuration. Also, at most one resource block in each slot can support mix of format 1 and 2 (Example: m=0 in slot 1 and m=0 in slot 2 of subframe, in above figure).   

What is the benefit of using this PUCCH Mixed Mode in LTE? 

It would not be suffice to allocate different RBs for different format type for smaller cell bandwidth (Example 1.4MHz, out of 6 RBs 2 RBs will be used for PUCCH for different formats). To minimize this overhead , it would be preferred to mix the format 1 and format 2 in same resource block. However to achieve this some phase rotation are used for guard to separate ACK/NACK and CQI , hence the efficiency in this mixed  mode is slightly lower. 

Questions are welcome.

8 comments:

  1. Hello, thank u for the detail of PUCCH.

    I got 1 question, please answer me if u know about that.

    I wonder how we can know the limit of n_PRB (the number of available RBs for PUCCH).

    Is that related to system bandwidth, and is the number of available RBs also set, once system bandwidth is determined?

    I've tried to find out from spec (36.211), but it was hard to find this information.

    u said on the link (the calculation of m), "Normally PUCCH Format 1 is located at a little less extreme edge, comparing with PUCCH format 2"

    it means the maximum n_RB depends on the PUCCH format? How does it work?

    (Actually, I wanted to know the maximum n_PRB for system bandwidth 10MHz, and I've been trying to find it, but failed)

    Where can I find these information? please help me. thanks.

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  2. Hi Pil,

    Please find my response inline(I will update you on these questions more after some time):
    > I guess number of available RBs for PUCCH is determined by operator or in network planning and then implemented by layer 1 (physical Layer). Area where number of UEs are huge we need more PUCCH resource to send SR,CQI and HARQ by all UEs. So all I can say is it depends upon number of UEs and also to some extend BW available (Like in area where BW availability is very less we use Mixed mode concept).
    > PUCCH formats only specify the way UEs are reporting SR, HARQ , CQI or combination of these . Total number of RBs (PUCCH+PUSCH) is only determined by system BW.

    Thanks for asking these questions. If you have different view on this, please let me know. I will update you on this If I will get more suitable answer.

    Thanks

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  3. This comment has been removed by the author.

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  4. Hi Abhishek,

    Thank you for the detailed explanation. I am trying to configure the number of PUCCHs on frequency domain. Right now I have four PUCCHs two on both ends of the system bw in frequency domain. Is it possible to configure more PUCCHs per slot for example 0, 1 and 2 RBs in slot 1 and 47, 48 and 49 RBs in slot 2 for 10 MHz Bw?

    Thank you
    Sanjeev

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  5. Hi Sanjeev,

    You can definitely increase PUCCH RBs by increasing NRBCqi parameter for PUCCH format 2/2a/2b reporting. Also for location of PUCCH in a subframe , There is a single index, m, derived from the PUCCH resource index and other parameters that specifies the location of the PUCCH in time/frequency. When m is 0, the PUCCH occupies the lowest RB in the first slot and the highest RB in the second slot of a subframe. When m is 1, the opposite corners are used—the highest RB in the first slot and the lowest RB in the second slot.

    Please go through below links:

    http://lteuniversity.com/get_trained/expert_opinion1/b/lauroortigoza/archive/2012/05/29/format-2-pucch-capacity-calculations.aspx

    http://www.mathworks.in/help/lte/ug/uplink-control-channel-format-1.html

    Also its not necessary that these PUCCH resources have to be on edge of RBs, it could be anywhere and it depends on customer requirement.

    Thanks for visiting .

    Please let me know if you have any doubt.

    Regards,
    Abhishek

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  6. Hi,
    How this PUCCH info is mapped on PUSCH when there is uplink grants available?,
    in such case whether it use the same PRBs of PUCCH ?

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  7. Hello Sir,
    I would like more details on Format 1/1a/1b. Index m is derived from higher layer parameter, Refer 36.211 section 5.4.3 ( N1_PUCCH, N_RB_SC, N_UL_RB, N2_RB, c, Ncs, delta_pucch_shift) for Format 1/1a/1b. However, I am confused abour N1_PUCH and Pucch_SR-Resource Index. N1_PUCCH is Resource index for PUCCH formats 1/1a/1b.
    My Question is PUCCH-SR-Resource Index given within N1_PUCCCH? Example: If N1_PUCCH = 8, N_RB_SC = 12, Pucch Delta Shift is 1, SR Periodicity = 10. So PUCCH-SR-Resource Index shall not be greater than the value of N1_PUCCH i.e 8

    ReplyDelete