Introduction
How a new technology developed in lab,perform in actual field? It is essential to do the field testing to ensure that
the technology performs to the expected level in the real scenario. The field
test help to identify the field issues and fine tune the network to the
expected level before offering the service to the actual customer. All the
technology has certain parameter to evaluate the field performance. LTE is also
have.
RSRP,RSSI,RSRQ &SINR
are the four important Radio Frequency parameter used to measure and identify
the signal strength and quality of LTE
signals in field. For an easy understanding of these parameters, we should
have knowledge on LTE frame, Resource Block, Resource elements and Reference
signal.
LTE frame have a length of 10ms in time domain. This 10ms frame is divided into 10 equal sub-frames of length 1ms each. Each sub-frame has been divided into 2 slots of 0.5ms. A slot of 0.5ms is again divided into 7 equal Resource elements (RE). The duration of each RE is 66.67us. The carriers are modulated by actual data. This modulated data is carried by this Resource element. Based on the modulation used , this data may be of 2,4 or 6 bits. This RE is also called as OFDMA symbols. This is case of LTE frame in time domain. In frequency domain, the maximum bandwidth is 20 MHz in LTE. This can have 1200 subcarriers, with 15 KHz spacing between each subcarrier. (1.4,3,5,10,15 and 20 MHz are the different bandwidth blocks available in LTE)
LTE frame have a length of 10ms in time domain. This 10ms frame is divided into 10 equal sub-frames of length 1ms each. Each sub-frame has been divided into 2 slots of 0.5ms. A slot of 0.5ms is again divided into 7 equal Resource elements (RE). The duration of each RE is 66.67us. The carriers are modulated by actual data. This modulated data is carried by this Resource element. Based on the modulation used , this data may be of 2,4 or 6 bits. This RE is also called as OFDMA symbols. This is case of LTE frame in time domain. In frequency domain, the maximum bandwidth is 20 MHz in LTE. This can have 1200 subcarriers, with 15 KHz spacing between each subcarrier. (1.4,3,5,10,15 and 20 MHz are the different bandwidth blocks available in LTE)
For the easiness of
resource allocation, 12 subcarrier in frequency domain and 0.5ms in time domain
are grouped to make a block, which is known as the Resource Block (RB). So in the
case of 20MHz bandwidth, we have 100 RB s. Each RB has 84 Resource Elements.
But in LTE the minimum resource allocated to a UE is 2 RB s which is called as
1 PRB (Physical Resource Block). In other words minimum one PRB ( 12 subcarrier
and 1ms) will be allocated for one UE. So a maximum of 100 different UEs can
get Resource from eNb for a 1ms transmission time.
Fig1:
LTE Frame, Sub carriers, Resource Block, Resource Elements & Reference Signal
Double Click on the picture to Enlarge |
Reference signal is nothing but a pattern which is known to both the
transmitter and receiver. In LTE, this reference signal is specific for a cell.
Hence this is known as Cell specific
Reference signal (CRS).The transmitter will send this reference signal at a
predefined frequency and time domain within the LTE resource grid. The time
domain of the CRS signal is fixed as 0,4 OFDMA symbols of all Resource Blocks.
The frequency domain of the CRS has depend on the PCI of the cell.
The CRS pattern of One Antenna Port
system is shown in the below fig. The Pattern varies for multiple antenna Port
System.
Fig2: Cell
Specific Reference Signal (CRS) Pattern for One Antenna Port System
Double Click on the picture to Enlarge |
Due to geographical variations, this signal may be altered while reaching to the receiver. As the pattern of the reference signal is known to the receiver, it can easily make the changes to the received signal to reproduce the actual signal send by the transmitter. As the geographical condition of a cell will be fixed, the same changes can happen to all signal transmitted in the particular cell. So reference signal do help the receiver to identity the changes that need to impose to the received signal to reproduce the actual signal that has been transmitted.
Now we are clear on LTE Frame,Subcarriers , Resource elements, Resource Block and Reference signal. So we can go to the discussion of RSRP,RSSI,RSRQ and SINR.
RSRP(Reference Signal Received Power):
As
the name indicated, this is the parameter which gives the strength of the Reference
signal, in specific the strength of the Cell Specific Reference signal (CRS).
The details of CRS are already discussed in the introduction.
RSRP is the Linear average power of the resource elements which carries
the CRS ( Cell specific Reference signal) within the allotted LTE frequency
bandwidth.
For a given LTE bandwidth Assume ,
‘c’ is the number of
subcarriers ,
K is the number of CRS for a
period of 66.66us and
‘P’ the Tx power .
This power (P) will be equally divided to ‘c’ sub carriers or RE’s . So each carrier have power Pc =P/c. Assume
there is no power boost for CRS. So CRS also have the same power of other RE’s.
i.e;Power of CRS resource elements (Pcrs) =Pc).
i.e;Power of CRS resource elements (Pcrs) =Pc).
As per the above definition, we can calculate the,
RSRP
at the output port of transmitter =
Linear Average of CRS power= (K*Pcrs) /K= (K*Pc) /K. (Assume no power boost for CRS).
Linear Average of CRS power= (K*Pcrs) /K= (K*Pc) /K. (Assume no power boost for CRS).
The signal from the transmitter output port is fed to the Antenna. The
Antenna converts this to electromagnetic waves and this reaches to the UE.
RSRP
at the UE = RSRP at the output
port of transmitter + Antenna Gain- Path Loss
= EIRP - Path Loss,
= EIRP - Path Loss,
where ,Path
Loss = 20log (f)+20log(d) +32.45 ('d' is distance in KM and 'f 'is the frequency
in MHz).
Fig 3 : RSRP Range
To have a better clarity on the calculation of the RSRP, Refer the Fig1 & read the case study, mentioned in the conclusion.
RSSI (Received Signal Strength indicator):
From
the above discussion, it is very clear that only the power of REs those carries
Reference Signal are considered for calculating the RSRP. But RSSI is the total
power of all signals within a given bandwidth. This includes power of all
carriers, Interference signals and power of noises within an allotted frequency
bandwidth. In other-words it is the
sum of serving cell power, Interference cell power and noise power.
RSSI(
Received signal strength indicator) is defined as “ the sum of power of
all RE s in the entire bandwidth +interference power(Pi) + noise power(Pn)
= (c x RSRP) + Pi +Pn , where ‘c’
is the number of carriers in a given frequency bandwidth. (Like 1200
subcarriers in 20MHz)
RSSI(Watt)= RSRP*12*N+Noise (Ni)(in watt)+Interference(in watt)
Where ‘N ‘is the number of Resource Blocks
RSSI (dBm)= RSRP (dBm) +10log(12*N)+Noise
(Ni)(in dBm)+Interference(in dBm)
RSRQ (Reference Signal Received Quality):
In
the above section we had discussed about the signal strength. Like Signal
strength, Signal quality is also equally important in LTE.
As the name indicating, RSRQ is a
radio parameter which is used to measure the Reference signal Received Quality.
It is the ratio of RSRP and RSSI , measured over one Resource
Block( RB ).
RSRQ = RSRP/(RSSI/N), where N is the number of Resource block in the given bandwidth over which the RSSI is
measured.
The
denominator gives the RSSI of one resource block. As the RSRP is a linear
average of CRS resource elements, the numerator is also for one resource block. So
that the ratio is for 1 resource block.
So if the load /Interference/Noise
increases, the value of RSSI shall increase. This will make the RSRQ value
worse. Thus we can clearly say RSRQ as a measure of interference & Noise.
The range of RSRQ in terms of
Reported and Measured value are mentioned below:
To have a better clarity on the calculation of the RSRQ, Refer the Fig1 & read the case study, mentioned in the conclusion.
SINR (Signal to Interference and Noise Ratio):
Signal to interference and Noise ratio is measured over the allotted bandwidth and normalized to 1 sub carrier bandwidth. It is not defined in 3GPP and not reported to the network by the UE. It is measured by the UE and is reported in Drive test tools. The SINR can be converted to CQI and reports to the network.
As this is a ratio , SINR is expressed in dB. To get a clear idea , we can take an example. If the signal power is 40W and Noise power is 20w, the SINR is 3dB. which means the actual signal power is double of the noise power. If there is 2 antenna the SINR will be doubled that is 3dB increase.
S
SINR = S/(I+Np)
Where, S=Power of Useful Signal, I= Average Interference Power,
Np= Average Noise Power.
Range: >20 dB is considered as excellent.
· Conclusion::
In the above discussion
we got a theoretical concept on RSRP,
RSSI and RSRQ. To have easy understanding ,we need to take a case study.
Case Study:: No Load condition
Consider a UE is located at a distance
of 5Km from the eNb which is operating in 2300Mhz band. The bandwidth is 20MHz
& the eNb transmission power is 40W. Assume Antenna Gain is 17dBi & the
Noise & interference power is of 10dBm at the UE location. Please calculate
the RSRP,RSSI & RSRQ at the UE.
Calculation:-
Fig5: RSRP,RSSI and RSRQ Calculation in LTE
Double Click on the picture to Enlarge |
As there is no load, only CRS resource elements will transmit. As the band width is of 20 MHz, this LTE system has 100 Resource block. Each resource block has 2 CRS. So 200 CRS in total for 100RBs. The total 40W power is divided into the 1200 subcarriers. So the CRS subcarrier will get a power of 0.0333W. The linear average of this power among 200CRS will give 0.0333W, which is equal to the RSRP in Watt and 15.23 in dBm. This is fed to the antenna of Gain of 17dBi . The EM waves coming from the antenna is of 32dBm . This is transmitted in the open space which will introduce a path loss of 113.66dBM.
So the RSRP at the UE= 15dBm+17dBi-113.66 =-81.44dBm.
So the RSSI at the UE =38.24dBm+17dBi-113.66dBm-10dBm
So the RSRP at the UE= 15dBm+17dBi-113.66 =-81.44dBm.
RSSI is the measured
over the entire 20 MHz bandwidth. As this case have zero load, only 200 CRS
resource elements will transmits.
So
RSSI= RSRP*200= 0.0333W*200 =6.6667W =38.24dBm. A 17dBi antenna gain is added
to this.The path loss off 113.66dBm & 10dbm of noise & interference power
need to subtract to get the RSSI at the
UE .So the RSSI at the UE =38.24dBm+17dBi-113.66dBm-10dBm
= -68.42dBm.
The RSRQ is a ratio of RSRP and RSSI over 1 resource block.
So the path loss will come in the numerator and denominator and can be
cancelled. In the No load condition only 2 RE s those carry CRS
only will transmit.
So the CRS will transmit with 50% of the total power.That is -3dB (half Power). This is the maximum possible power possible for RSRP. So -3dB is the highest & best value of RSRQ in LTE.
So the CRS will transmit with 50% of the total power.That is -3dB (half Power). This is the maximum possible power possible for RSRP. So -3dB is the highest & best value of RSRQ in LTE.
At full load condition the RSRQ value for this case can go up
to -11dB. It will further degraded even
below -19.5dB based on the increase in the interference level.
Thanks and Reg
Aneesh.M
Thanks and Reg
Aneesh.M