PBCH: Does the MIB tell the UE how many antennas are used in the cell?

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PBCH: Does the MIB tell the UE how many antennas are used in the cell?

  • Comments 9

Before answering this question let's review the contents of the Master Information Block (MIB). You will find the definition of the MIB in the RRC specification, 36.331. It contains just three parameters: DL Bandwidth, PHICH Configuration and System Frame Number as well as ten spare bits for future expansion. In a previous blog, "PBCH: How quickly can a UE read the MIB?" I discussed at length the SFN. The presence of the system bandwidth in the MIB is a reflection of the PBCH being in the center 1.4MHz regardless of the actual bandwidth of the channel. Why does the UE need the PHICH Configuration information to be in the MIB? Not because it will be receiving ACKs or NAKs immediately, but it must know WHERE this channel is so that it can read the PDCCH.

You have probably noticed that there is no parameter in the MIB for the number of antennas in the cell. The MIB has a CRC however, which is scrambled with one of three sequences which maps to the number of antennas used in the cell. Perhaps then, when the UE calculates the CRC from the decoded MIB it can compare against each of the three descrambled CRCs looking for a match and hence discover the number of antennas - perhaps not. It is tempting to think of this scrambling sequence as a parameter. But it is not a parameter in the same way that a C-RNTI used to scramble the CRC of the PDCCH is a parameter. Ask yourself the question "How does the UE decode the MIB in each of the three possible scenarios, namely, one antenna, two antennas or four antennas?" In the one antenna case there is nothing special the UE has to do, but for both the two and four antenna cases the UE has to be in sync with the base station. LTE uses a specific type of transmit diversity in which both the transmitter and receiver are aware of the method and participate in its application.  For the two antenna case LTE uses Space-Frequency Block Coding (SFBC) and for the four antenna case a combination of SFBC and Frequency Switched Transmit Diversity (SFBC+FSTD) is used. The LTE UE will need to blindly detect the number of antennas by trying each possible antenna configuration in turn, decoding the MIB and descrambling the MIB's CRC with the corresponding antenna mask in order to compare the CRC.

So the next time you are told that the MIB tells the UE how many antennas are used in the cell you'll know exactly what this means.

  • Is it possible for a eNodeB with 2 (or 4) antenna ports capability to transmit the PBCH with a sinlge antenna port? It can still differentiate the antenna ports it had with the CRC mask.

  • to wyxjtu: no not possible. the crc is masked with the actual number of antenna ports used to send the PBCH. in addition, the other control channels,particularly PDCCH, used the same transmission scheme as the PBCH, so if the PBCH is using 1 antenna then the PBCH would also use 1 antenna.

  • Hi John, Thanks a lot for your post and it's really informative. I am confused with the statement , "Why does the UE need the PHICH Configuration information to be in the MIB? Not because it will be receiving ACKs or NAKs immediately, but it must know WHERE this channel is so that it can read the PDCCH." This is conflicting statement form the post  by Vishal Dhar's One Millisecond in the Life of an LTE UE. May be i got it in the wrong sense. Please correct me if i am wrong. Thanks in advance.

  • to Venubabu bandarupalli:  Hi Venubabu.  i dont see a contradiction between the two posts.  Vishal's post deals with a UE that is fully connected, monitoring the PDCCH every ms and possibly sending UL data also.  So he is discussing the PHICH from the perspective of the UE receiving ACK/NAK for a previous UL transmission.  The point that I was emphasising above is that the UE needs to know the amount of resource used by the PHICH in each subframe in order to be able to read the PDCCH. In particular, during the initial power up sequence the UE cannot transmit anything until he reads SIB TYPE 1 and 2. So the UE will not be expecting ACK/NAK from the eNB at this time.  Hope this clears up any confusion.  

    thanks for your comment. John.

  • Is it PHICH or PCFICH?

  • Hi John, I am still confused, as far as I know, there are 3 sequences 0x0000, 0x5555 & 0xffff corresponding to 1, 2 or 4 eNB antennas, so the PBCH CRC will be scrambled with one of these based on the number of antenna's used at the eNB, is it not? If yes, then at the UE side, when UE calculates the CRC for the received MIB and descrambles the received CRC with one of the above scrambling sequence and then compare the descrambled CRC with calculated CRC, it should still be able to tell number of Antenna's right? Even though multiple streams are going in case of MIMO, the PBCH content (MIB+CRC) should remain the same, correct me if I am wrong.

  • Hi Pradeepcec, an LTE cell will use either 1 antenna transmission or 2 or 4 antenna transmit diversity to transmit the PBCH.  Even though the information content would be the same in each case the layer mapping and pre-coding is not, resulting in different symbols being transmitted from the antennas. So the UE must try each possibility until it finds a CRC that is correct. The fact that the CRC is scrambled with the # of antennas is like an extra bonus check.  

    Note that all the other control channels are guaranteed to be transmitted in the same way as the PBCH so the UE only does this trial and error discovery once for the PBCH.

    John

  • Hi John,

    If a eNB has 2 antennas in that case is it compulsory to send PBCH on both antennas or can send PBCH on single antenna. or who guides physical layer about this?

    garv

  • Hi Garv, its possible for eNB to have 2 antennas and only one be used to send PBCH (and therefore all the other control channels). The eNB will be configured by the operator in any case.  However its an unlikely configuration.

    John