Downlink channel estimation
[hest,noisest]
= lteDLChannelEstimate(enb,rxgrid)
[hest, noisest]
= lteDLChannelEstimate(enb,cec,rxgrid)
[hest, noisest]
= lteDLChannelEstimate(enb,chs,cec,rxgrid)
[
returns hest
,noisest
]
= lteDLChannelEstimate(enb
,rxgrid
)hest
,
the estimated channel between each transmit and receive antenna and noisest
,
an estimate of the noise power spectral density on the reference signal
subcarriers. See Algorithms.
This example sets up and transmits RMC R.12 (4antenna transmit diversity), models the propagation channel by combining all transmit antennas onto one receive antenna, OFDM demodulates and performs channel estimatation.
Initialize cell parameters for transmission of RMC R.12.
Initialize channel estimator configuration (cec
). The averaging window size is configured in terms of resource elements (REs), in time and frequency. Here cubic interpolation will be used with an averaging window of 1by1 REs. Since we are not adding noise and don't require a noise estimate, there is no need for averaging, so we set the frequency window and time window size to '1'.
Generate a transmit waveform with lteRMCDLTool
function
rc = 'R.12'; enb = lteRMCDL(rc); cec.FreqWindow = 1; cec.TimeWindow = 1; cec.InterpType = 'cubic'; cec.PilotAverage = 'UserDefined'; cec.InterpWinSize = 3; cec.InterpWindow = 'Causal'; txWaveform = lteRMCDLTool(enb,[1;0;0;1]);
Model the propagation channel by combining all transmit antennas onto one receive antenna.
Perform OFDM demodulation.
With the cell parameters defined, channel estimation configured and a received waveform demodulated the channel characteristics for the received resource grid is estimated. Displaying the channel estimate size, we see hest is an MbyNbyNRxAntsbyCellRefP array
rxWaveform = sum(txWaveform,2); rxGrid = lteOFDMDemodulate(enb,rxWaveform); hest = lteDLChannelEstimate(enb,cec,rxGrid); size(hest)
ans = 72 140 1 4
enb
— eNodeB cellwide settingsstructureeNodeB cellwide settings structure that can contain these parameter fields.
Parameter Field  Required or Optional  Values  Description 

NDLRB  Required  Scalar integer (6, ..., 110). Standard bandwidth values are
6, 15, 25, 50, 75, and 100.  Number of downlink (DL) resource blocks (RBs) 
CellRefP  Required  1, 2, 4  Number of cellspecific reference signal (CRS) antenna ports 
NCellID  Required  Nonnegative scalar integer (0,…,503)  Physical layer cell identity 
NSubframe  Required  Nonnegative scalar integer  Subframe number 
CyclicPrefix  Optional 
 Cyclic prefix length 
DuplexMode  Optional 
 Duplexing mode, specified as:

The following parameters
are dependent upon the condition that  
TDDConfig  Optional  0 (default), 1, 2, 3, 4, 5, 6  Uplink or downlink configuration 
SSC  Optional  0 (default), 1, 2, 3, 4, 5, 6, 7, 8, 9  Special subframe configuration (SSC) 
The following parameters
are dependent upon the condition that  
CSIRefP  Required  1 (default), 2, 4, 8  Array of number of CSIRS antenna ports 
CSIRSConfig  Required  scalar integer  Array CSIRS configuration indices. See TS 36.211, Table 6.10.5.21. 
CSIRSPeriod  Optional 
 CSIRS subframe configuration. 
If cec
.Reference
is
set to 'CSIRS'
, the cellwide settings structure, enb
,
can contain the additional fields CSIRefP
, CSIRSConfig
,
and CSIRSPeriod
.
CSIRSbased channel estimation, activated when cec
.Reference
is
set to 'CSIRS'
, is strictly only valid within the
standard for the 'Port714'
transmission scheme.
The optional CSIRSPeriod
parameter
controls the downlink subframes in which CSIRS is present. It is
ither always 'On'
or 'Off'
,
or defined by the scalar subframe configuration index, Icsirs (0...154),
or the explicit subframe periodicity and offset pair, [Tcsirs Dcsirs].
For more information, see TS 36.211, Section 6.10.5.3 [3].
rxgrid
— Resource element array3D numeric matrixResource element array, specified as a 3D numeric matrix of
size MbyNbyNRxAnts
.
The second dimension of rxgrid
can contain any
whole number of subframes worth of OFDM symbols. For a normal cyclic
prefix, each subframe contains 14 OFDM symbols; therefore, N is
a multiple of 14.
Note:
To adhere to the estimation method defined in TS 36.104 [1] and TS 36.141 [2], 
Data Types: double
Complex Number Support: Yes
cec
— Channel estimator configurationstructureChannel estimator configuration, specified as a structure that can contain the following parameter fields.
Parameter Field  Required or Optional  Values  Description  

PilotAverage  Required 
See footnote 1  Type of pilot averaging  
FreqWindow  Required  Nonnegative scalar integer  Size of window in resource elements used to average over frequency during channel estimation  
TimeWindow  Required  Nonnegative scalar integer  Size of window in resource elements used to average over time during channel estimation  
InterpType  Required 
See footnote 2  Type of 2D interpolation used during interpolation. For details,
see
 
The following parameters
are only required if  
InterpWindow  Required 
 Interpolation window type used during channel estimation. Options  
InterpWinSize  Required  Positive scalar number. If  Window size across which to interpolate. The interpolation window size is specified in number of subframes.  
The following parameters
are dependent upon the condition that  
Reference  Optional 
 Specifies point of reference (signals to internally generate) for channel estimation  

chs
— PDSCHspecific channel transmission configurationstructurePDSCHspecific channel transmission configuration, specified as a structure that can contain the following parameter fields.
Parameter Field  Required or Optional  Values  Description 

TxScheme  Required  Default  Transmission scheme, specified as one of the following options.

PRBSet  Required  1 or 2column integer matrix  Zerobased physical resource block (PRB) indices corresponding
to the slot wise resource allocations for this PDSCH.
PRBSet varies per subframe for the RMCs 
RNTI  Required  Scalar integer  Radio network temporary identifier (RNTI) value (16 bits) 
The following parameters
are dependent upon the condition that  
NLayers  Required  1,2,3,4  Number of transmission layers 
When TxScheme
is
set to 'Port5'
, 'Port78'
, 'Port8'
,
or 'Port714'
and cec
.
Reference
is
set to 'DMRS'
, the channel estimation is performed
using UEspecific reference signals and the size of the returned channel
estimate is MbyNbyNRxAnts
byNLayers
.
Alternatively, when cec
.
Reference
is
set to 'CSIRS'
, the channel estimation is performed
using the CSI reference signals (CSI) and the size of the returned
channel estimate is MbyNbyNRxAnts
byCSIRefP
.
For other transmission schemes, the channel estimation is performed
using cellspecific reference signals. The size of the returned channel
estimate is MbyNbyNRxAnts
byCellRefP
.
When TxScheme
is
set to 'Port78'
or 'Port714'
and cec
.
PilotAverage
is
set to 'UserDefined'
, if cec
.
TimeWindow
is
2 or 4 and cec
.
FreqWindow
is
1, the estimator enters a special case where an averaging window of
two or four pilots in time is used to average the pilot estimates.
The averaging is always applied across two or four pilots, regardless
of their separation in OFDM symbols. This operation ensures that averaging
is always done on two or four pilots.It also provides the appropriate
"despreading" operation required for the case of UERS
ports / CSIRS ports which occupy the same time/frequency locations
but use different orthogonal covers to allow them to be differentiated
at the receiver. For the CSIRS and any number of configured CSIRS
ports, given by enb
.
CSIRefP
,
the pilot REs occur in pairs, one pair per subframe, that require
averaging with cec
.
TimeWindow
=
2 and results in a single estimate per subframe. For the UERS with
from 1 through 4 layers (given by chs
.NLayers
),
the pilot REs occur in pairs, repeated in each slot, that require
averaging with cec
.
TimeWindow
=
2 and results in two estimates per subframe, one for each slot. For
from 5 through 8 layers, the pairs are distinct between the slots
of the subframe and the required averaging is cec
.
TimeWindow
=
4, resulting in one estimate per subframe. In these cases, rxgrid
must
contain only one subframe because only a single subframe can be estimated.
hest
— Estimated channel between transmit and receive antennas4D numeric arrayEstimated channel between transmit and receive antennas, returned
as a 4D numeric array of size MbyNbyNRxAnts
byCellRefP
.
Optionally, for UEspecific beamforming transmission schemes, the
array is of size MbyNbyNRxAnts
byNLayers
.
Where M is the number of subcarriers, N is
the number of OFDM symbols, NRxAnts
is the number
of receive antennas, CellRefP
is the number of
cellspecific reference signal antenna ports, and NLayers
is
the number of transmission layers.
Data Types: double
Complex Number Support: Yes
noisest
— Power spectral density estimate on reference signal subcarriersnumeric scalarPower spectral density estimate on reference signal subcarriers, specified as a realvalued numeric scalar.
Data Types: double
The channel estimation algorithm functions as described in the following steps:
Extract the reference signals, or pilot symbols, for a transmitreceive antenna pair from the received grid. Use the reference signals to calculate the leastsquares estimates of the channel response at the pilot symbol positions within a received grid.
Average the leastsquares estimates to reduce any unwanted noise from the pilot symbols.
Using the cleaned pilot symbol estimates, interpolate to obtain an estimate of the channel for the entire number of subframes passed into the function.
The leastsquares estimates of the reference signals are obtained by dividing the received pilot symbols by their expected value. The leastsquares estimates are affected by any system noise. This noise needs to be removed or reduced to achieve a reasonable estimation of the channel at pilot symbol locations.
To minimize the effects of noise on the pilot symbol estimates,
the leastsquares estimates are averaged using an averaging window.
This simple method produces a substantial reduction in the level of
noise found on the pilot symbols. The two pilot symbol averaging methods,
which also define the interpolation method performed to obtain the
channel estimate, are 'TestEVM'
and 'UserDefined'
.
'TestEVM'
— follows the
method described in TS 36.141, Annex F.3.4 [2]. Time averaging is performed across each pilot
symbol carrying subcarrier, resulting in a column vector containing
the time averaged estimates of the channel. Frequency averaging is
then performed using a moving window, maximum size 19. Linear interpolation
is used to estimate the values between the pilot symbols. The estimated
vector is then replicated and used as the entire channel estimate.
Note:
For 
The algorithm differs from the implementation described in
TS 36.141 [2] due to the
number of subframes across which timeaveraging is performed. In TS
36.141 [2], the method requires
10 subframes to be used. The function lteDLChannelEstimate
performs
time averaging across the total number of subframes contained in rxgrid
.
'UserDefined'
— uses an
averaging window defined by you. The averaging window size is in resource
elements. Any pilot symbols located within the window are used to
average the value of the pilot symbol found at the center of the window.
The averaged pilot symbol estimates are then used to perform a 2D
interpolation across a window of subframes. The location of pilot
symbols within the subframe is not ideally suited to interpolation.
To account for this, virtual pilots are created and placed out with
the area of the current subframe. This approach allows complete and
accurate interpolation to be performed. The subframe window can be
specified to be causal
, noncausal
,
or centered
, depending on the data available.
[1] 3GPP TS 36.104. "Base Station (BS) radio transmission and reception." 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (EUTRA). URL: http://www.3gpp.org.
[2] 3GPP TS 36.141. "Base Station (BS) conformance testing." 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (EUTRA). URL: http://www.3gpp.org.
[3] 3GPP TS 36.211. "Physical Channels and Modulation." 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (EUTRA). URL: http://www.3gpp.org.
griddata
 lteDLPerfectChannelEstimate
 lteEqualizeMIMO
 lteEqualizeMMSE
 lteEqualizeZF
 lteOFDMDemodulate
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