Kindly note :- In Ideal case 1000 VOLTE calls maximum supported by FDD 10 Mhz bandwidth.
In practical scnerio it would be lesser calls based on RF condition vs CQI.
To find out How many call that can be supported by VOLTE cell first we need to calculate below :-
1.) VoLTE Call Packet Size Calculation
2.) Radio Resource Calculation for VoLTE packet
3.) No. of RE in one PRB that can carry VOLTE data bits
3.) Modulation type vs CQI value based on that how many data bits will be carried out by one RE.
4.)No. of Max. Theoretical VoLTE user per cell
VoLTE Call Packet Size Calculation:-
VoLTE call packet is size is not fix, it depends on codec used by for voice, UE radio conditions in the cell, eNodeB’s scheduling algorithm, the protocol used , and so on. To avoid these many variable and complex calculation, let’s find out one particular aspect of VoLTE capacity i.e. how many Physical Resource Blocks (PRBs) are required for one VoLTE call over the air interface?
LTE System bandwidth: 20 MHz (100 PRB)Dulpex Mode: FDDMIMO Mode: 2×2Codec: Adaptive Multi Rate -Wide Band Codec (AMR-12.65)RoHC enabled
An AMR-WB 12.65 coder generates about 253 bits of coded speech every 20 ms. In order to deliver each voice sample to the UE needs to add protocol headers such as RTP header (typically 12 bytes), a UDP header (8 bytes), and an IP header (40 bytes).
VoLTE Packet length with all Header= Codec Bits + RTP Header + UDP Header + IP Header
= 253 Bits+ 12*8=96 Bits +8*8=64 Bits + 40*8=320 Bits
This results in the total packet length up to some 733 bits every 20 ms.
When RoHC is enabled, it will replace with RTP, UDP and IP headers with a much smaller RoHC header before the packet is actually transmitted over the air. The length of the RoHC header will vary depending on the particular circumstances, but it will average around 3 bytes, or 24 bits. The RLC and MAC layers will add their own overhead, so the end result is that only about 300 bits is needed for every VoLTE voice call packet at the air interface.
Radio Resource Calculation for VoLTE Packet:-
The above calculation states that about 300 bits is required for VoLTE voice packet. Then here it is required that how many Radio resource it requires to be transmitted over the air. In LTE the unit of radio resources is know as Physical Resource Block (RB) and one PRB has 12 sub carrier and 14 symbols (Normal Cyclic Prefix) over 1 mili second time duration, or 12*14=168 Resource Elements (REs).
Some of REs are occupied by the control symbols (PDCCH) and Pilot symbols (RS) provides about 120 REs available for Data transmission.
LTE downlink modulation support QPSK, 16 QAM and 64 QAM for PDSCH which means each Resource can carry 2Bits, 4 Bits or 6 Bits based on modulation used. But some of these bits will be used for data and some of error control bits (modulation and coding rate shown in figure below). so number of Resource block required for VoLTE packet depend what modulation is applied and modulation depends on the radio condition of the UE. UE reports is RF condition with Channel Quality Indicator (CQI) to eNodeB and using this report eNodeB decide the modulation for particular resource block. The CQI report range is 0-15 where 15 is the best channel condition.
Let’s consider CQI 15 =Good , CQI 7 = Average and CQI 1 = Poor and CQI 15 provides 64 QAM, CQI 7 provides 16 QAM and CQI 1 results QPSK modulation.
When UE reports CQI 15 then eNodeB can use 64QAM modulation and a 948/1024 = 0.926 effective coding rate is applied, which means that each RE holds 6 x 0.926 = 5.55 data bits on average. Considering this a single PRB can then carry 120 x 5.55 = 666 data bits, or the equivalent of two VoLTE voice samples. But LTE scheduler can’t allocate less than one PRB per user, so this will count that one PRB is needed per VoLTE call.While UE reports CQI 7, then eNodeB can use 16QAM modulation and a 378/1024 = 0.369 coding rate, resulting in 4 x 0.369 x 120 = 177 data bits. This means for 300 bits VoLTE data requires two PRBsWith CQI 1 report eNodeB can use QPSK modulation and a 78/1024 = 0.076 coding rate, supporting 2 x 0.076 x 120 = 18 data bits per PRB. So to transmit 300 bits VoLTE data it about 16 PRBs.
Numbers of Max. Theoretical VoLTE user Per Cell :-
In VoLTE voice data is generated every 20 mili second, so if everything is good i.e. there is no re-transmission, then about 20 VoLTE call can share the same set of PRB one after other.
Formula for calculation :-
The maximum number of VoLTE call that can be carried is then determined by=
(Number of Available PRB)/ Number of PRB per VoLTE call) X 20
i.e if 20 mhz bw & CQI 15
=2000 calls ideal scnerio.
For 10 Mhz bw it would be around 100 calls