necessary reception information from Site 1 to Site 2 through X2-interface, the processing of macro diversity received signals can be performed in the serving Site-2. In this study both cases are studied, and the Intra-Site macro diver-sity reception is mainly used as a reference to the Inter-Site macro diverdiver-sity reception.
UL CoMP operation for all the UEs in the network is practically infeasible.
In this study the selection of CoMP UEs, i.e. UE-A and UE-D in Figure 4.2, is based on the RSRP measurement. The RSRP is defined as the lin-ear average over the power contributions of the resource elements that carry DL cell-specific reference signals1within the considered measurement frequency bandwidth [16][93]. The measurement reporting is assumed to be UE-assisted and network-decided, where the UE measures the RSRP from the neighboring eNBs and reports the RSRP measurements back to the serving eNB. Based on the measurements, the serving eNB will make the decision on whether a certain UE i should be macro diversity received or not according to the criteria:
RSRPiServeLink−RSRPiOtherLinks ≤X [dB] (4.1) where RSRPServeLink is the RSRP measurement taken from the serving eNB, RSRPOtherLinks is the RSRP measurement from one of the other surrounding eNBs except the serving eNB and X is called triggering threshold or Combining Window Size (CWS).
If a certain UE fulfills the criteria stated in Equation 4.1, then the UE is defined as ‘Macro Diversity UE’ with the corresponding eNB.
4.3 Macro Diversity Reception
Diversity is one of the MIMO technology applications. Depending on whether the diversity is implemented in the transmitter or the receiver, it can be deter-minated as transmitter diversity, i.e. Multiple Input Single Output (MISO) and receiver diversity, i.e. Single Input Multiple Output (SIMO). In this study the receiver diversity reception is considered. The basic idea behind receiver
diver-1Downlink Reference Signal contains Cell-Specific RS used by the UE for cell search, initial acquisition and DL channel quality measurements, UE-Specific RS used by the UE to estimate the DL channel to decode the eNB DL transmission and MBSFN RS for broadcasting channel.
Uplink Reference Signal contains Demodulation RS used by the eNB to estimate the UL channel to decode the UE UL transmission and Sounding RS used by the eNB to estimate the UL channel conditions for each user to decide the best UL scheduling.
60 Uplink CoMP in the Form of Macro-Scopic Combining
sity is that multiple antennas receive different versions of the same signal and the chance of all these copies of signal being in a deep fade is small. Therefore, the reception reliability is enhanced. The combining schemes, such as MRC, Selection Combining (SC) or Equal Gain Combining (EGC), can be utilized to combine the received diversity signals. In this study the MRC scheme is studied in details and the SC scheme is utilized as a reference.
Considering the received signal model with single transmit antenna from UEi as:
ri = p
Ptx,ihisi+
Nu
X
j=1,j6=i
pPtx,jhjsj+n (4.2)
= p
Ptx,ihisi+ω (4.3)
where ri = [r1 r2· · ·rNr]T is the received signal vector from UE i on the Nr coordination cell receive antennas. hi = [h1 h2· · ·hNr]T is the channel vector of UEi on theNr receive antennas. si is the transmitted signal of UE i with transmit power of Ptx,i. Nu is the number of UEs transmitting on the identical PRB. sj is the transmitted signal of interference UE j with transmit power of Ptx,j. hj = [h1 h2· · ·hNr]T is the channel vector associated with the j -th interfering UE. n = [n1 n2· · ·nNr]T is the noise vector on the Nr receive antennas andω represents the inter-cell interference plus noise vector over the transmission PRBs.
For the MRC, both phase correction and amplitude weighting estimations are required at the receiver. The MRC combiner output signal can be expressed as:
bsi=hHi ri=hHi p
Ptx,ihisi+hHi ω (4.4) And the SINR of MRC combiner output is:
γmrc = E
hHi√
ptx,ihisi
2
E
hHiω
2 =
ptx,i hHi hi
2
E sisHi En
hHiωωHhi
o = ptx,i
hHi hi
2
hHi E{ωωH}hi
=
ptx,i hHihi
2
σω2hHiUNrhi
= ptx,i
hHi hi
2
σω2hHi hi
= ptx,i
hHi hi
σω2 (4.5)
=
Nr
X
a=1
ptx,i|hi,a|2
σ2ω (4.6)
=
Nr
X
a=1
γi,a (4.7)
4.3 Macro Diversity Reception 61
whereUNr denotes anNr×Nridentity matrix. The inter-cell interference plus noise power is assumed to be spatially and temporally white with varianceσω2. hi,a is the channel of UEi at a-th receive antenna.
It is intuitive to note that the term hHi hi =
Nr
X
a=1
|hi,a|2 is actually the sum of the signal channel powers across all theNrreceive antennas and the total MRC output SINRγmrcis the sum of SINR γi,a at each antenna.
Unlike the MRC scheme, for the SC scheme, the receive antenna with the high-est SINR is chosen for further processing. The simple SC scheme only needs a measurement of signal power, whereas the phase correction and amplitude weighting information is not required. The SINR of SC combiner output is:
γsc= max
a∈[1,···,Nr]{γi,a} (4.8)
As seen from Equation 4.8, the SC scheme wastes the signal energy by discarding (Nr−1) copies of the received signal. This drawback is avoided by the MRC scheme, as shown in Equation 4.7, which exploits all available signal copies by multiplying a complex weight in each signal copy and then sums them up.
According to [43] and presented in Figure 2.1, the user plane protocol stack in LTE includes the protocols MAC, RLC, and PDCP. The MAC layer scheduler decides the transport block size of the MAC Protocol Data Unit (PDU) to be transmitted over the air interface. Based on this decision the RLC protocol provides on request of the MAC layer a RLC PDU of appropriate size to the MAC protocol either by concatenation or segmentation of PDCP PDUs. From the Equation 4.2 and 4.5, for the whole MAC transport block, the received SINR for UE i with MRC can then be expressed as:
γmrcmac =
Nr
X
a=1
Ptx,i NPRB
NPRB
X
p=1
|hi,a,p|2
NPRB
X
p=1
Nu
X
j=1,j6=i
Ptx,j NPRB
|hj,a,p|2+σ2n
(4.9)
where NPRBis the number of assigned PRB andσn2 represents noise power.
With respect to the system-level simulations, the UL macro diversity reception
62 Uplink CoMP in the Form of Macro-Scopic Combining
is also naturally applied to the CSI measurements, which has been discussed in Section 2.2. By applying the CSI macro diversity, better channel knowledge can be acquired for the macro diversity users to perform the LA selection and PS allocation.