4X4 MIMO with CA

MIMO:
As the name suggests, MIMO stands for Multiple input and multiple output. In any wireless communication, MIMO plays a major role in effectively using the network. In case of Mobile communication where CA (Carrier Aggregation) is known to increase throughput by aggregating  multiple bandwidths by aggregating multiple bands, MIMO can be a catalyst to achieve peak throughput like 1GBPS

There are 3 broad categories for  MIMO configuration so far in mobile industry

1. 2x2 MIMO
2. 4x4 MIMO
3. 8x8 MIMO

DL MIMO:

There are totally 3 parameters which collectively decide the MIMO configuration. 

A. Transmission mode

B. Number of Tx antenna (from Enb side) 

C. Number of layers (from UE side)

Transmission mode and Num of Tx antenna are Layer3(L3) parameters where as Number of active layers are Layer1(L1) parameter as it doesn't involve any role of MAC layer.

Layer-3 analysis

Step-1: When a UE sends a LTE service request, the eNB will respond with ‘RRCConnectionSetup’ message. It will also include the transmission mode and Max number of antennas to be used for Pcell.

Step-2: If the network supports CA and if MIMO is available in the current geographical area, eNB will send ‘RRCConnectionReconfiguration’ message. It will include the Transmission mode and max number of antennas to be used for Scell-1 and Scell-2.

Step-3: Depending on Transmission mode, the UE will scan network and download data as per the pre-defined DCI formats from 3gpp. 

Step-4: If the transmission mode supports closed loop for any component carrier then depending on the Rank, CQI and PMI values, the network will apply the codeword before sending the data to UE.

Step-5: If the transmission mode is open loop for any component carrier then PMI will not be considered by eNB and codeword will not be applied before sending the data.

Detecting Pcell DL MIMO:

1. First we need to know what is the transmission mode by which we can understand if it is SM/TD

2. Then we need to get the rank, if the rank is 4 then we can we have 4 unique streams at UE

3. But to confirm if it is 4x4 we need to get the max number of antennas at DL and UL

4. For DL we need to get the ‘nTx antenna’ parameter from RRC connection setup message under 'physical config dedicated' block. (Ref 3GPP 36.213 V14)

5. For UL we need to get no of layers active which can be achieved only if we are able to read L1 messages .

Used cases

If nTx = 4 and rank is 4 then two possibilities 4x4

If nTx = 4 and rank is 3 then two possibilities 4x3 and 3x3

If nTx = 4 and rank is 1, 2 then many combinations are possible but to get the exact combination, we have to rely on number of layers.

If ntx = 2 and rank is 2 then it is 2x2

If ntx = 2 and rank is 1 then it is 2x1 or 2x2

Detecting Scell DL MIMO:

1. First we need to know what is the transmission mode by which we can understand if it is SM/TD

2. Then we need to get the rank, if the rank is 4 then we can we have 4 unique streams

3. But to confirm if it is 4x4 we need to get the max number of antennas at DL and UL

4. For DL we need to get the ‘antennaPortsCount an’ parameter from RRC connection Reconfiguration message under 'PhysicalconfigdedicatedScell' block. (Ref 3GPP 36.213 V14)

5. For UL we need to get no of layers active which can be achieved only by L1 messages

Used cases

If antennaPortsCount an = 4 and rank is 4 then two possibilities 4x4

If antennaPortsCount an = 4 and rank is 3 then two possibilities 4x3 and 3x3

If antennaPortsCount an = 4 and rank is 1, 2 then many combinations are possible but to get the exact combination, we have to rely on number of layers.

If antennaPortsCount an = 2 and rank is 2 then it is 2x2

If antennaPortsCount an = 2 and rank is 1 then it is 2x1 or 2x2

UL MIMO:

Code word Splitting:

eICIC (enhanced Inter-cell interference coordination)

  eICIC (enhanced Inter-cell interference coordination)

·         eICIC introduced in 3GPP release 10

·         eICIC introduced to deal interference issues in Heterogeneous Networks (HetNet)

·         eICIC mitigates interference on traffic and control channels

·         eICIC uses power, frequency and also time domain to mitigate intra frequency interference in heterogeneous networks

ICIC technology evolution

In time domain approach, a subframe can take MBSFN subframe approach.

    MBSFN subframe

·         MBSFN subframes and almost blank subframes that we mentioned previously could be used to reduce interference to the co-channel PDCCH in the other cell layer.

·         The macrocell and picocell can configure their MBSFN and almost blank subframes in a cooperative manner in the time domain. For MBSFN, the macrocell configures subframe N as an MBSFN subframe with only one OFDM symbol used for the control region, while the picocell configures this subframe as a normal subframe.

·         Thus, only one OFDM symbol in the control region at the picocell would suffer the improved compared with the situation where all of the three OFDM symbols are the Victims.

·          The almost blank subframe (ABS) refers to the subframe that carries only the CRS in the whole subframe region. Similar to the previous MBSFN, the interference of the control channel could be alleviated. For example, we assume that the eNBI sends the ABS pattern and the RNTP, UL HII (high interference indicator), and UL 01 (overload indicator) to the eNB2.
·         We also assume that the DL subframe n is configured as an ABS. The ABS may have no other signals that are transmitted in ABSs except the CRS.

·          If the primary synchronization signal (PSS), secondary synchronization signal (SSS), physical broadcast channel (PBCH), system information block (SIB) 1, paging, and the positioning reference signal (PRS) coincide with an ABS.

·         They will be transmitted in the ABS. In CoMP scenarios, an aggressor cell informs a victim cell of certain ABSs in a time-domain manner, and based on this information the victim cell can perform appropriate user scheduling with RRC signalling to UEs for related RRM and CSI measurements .

 HII(High Interference indicator in ABS)

·         In the DL subframe, which is configured as an ABS by eNBI, eNB2 can do PDSCH scheduling without being concerned with the interference from eNBI regardless of the RNTP message.

·          It is worth mentioning that the performance of the victim PDCCH and PDSCH could be improved further if the UE is enabled with the capability to detect and cancel the neighbour cell's CRS.

·         Interference from the macro, and the PDCCH performance at the picocell could be another method of decreasing the interference is power reduction or muting on the portion of symbols of the macrocell that overlapped the control region of the picocell by scheduling and power control.

·          From the previous discussion we can see that Rel-8 and Rel-9 frequency domain ICIC and LTE-A time domain ICIC can coexist.

·         Each eNB can send both Rel-8 and Rel-9 and LTE-A ICIC messages to its neighbouring cell. Further, possible LTE-A eICIC methods include space domain coordination, network power control, network interference cancellation, network coding, network MIMO, and net-work beam-forming, which means an aggressor cell informs a victim cell of certain kinds of network information in a space, time, code, or frequency domain manner, so as to be utilized at a victim cell for appropriate avoidance schemes.