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)

  

               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

                         

    The Resource Element(RE) in an Antenna Port ,corresponding to the CRS in other antenna port will kept empty. 

     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).

   

       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).

              

                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,

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:

                                     Fig4: RSRQ Range

          
                  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.     

              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.

             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