VoLTE Calculation

8/11/2019 VoLTE Calculation

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2014-06-12 HUAWEI Confidential Page1, Total7

How Big is VoLTE Voice Call?

Global Challenges

The third generation mobile telecommunication technologies and the

increasing popularity of Smartphone have greatly driven user demand for

mobile broadband services. Explosive growth of data traffic and the cha llenge

of increasing network capacity force mobile carriers to upgrade their networks

and increase the network transmission rate. The LTE technology is the best

option for all the operators across the globe.

In the LTE era, the time has come for mobile operators to move to voice over

LTE (VoLTE). Globally, voice is still the major revenue source for the

telecommunications industry, and will probably still be when LTE goes

mainstream. However, basic voice may not be enough to remain relevant in an

LTE environment, because subscriber communication is shifting from plain

voice to rich & quality voice.

So, What is VoLTE? and how does the VoLTE will impact operators network

when it comes to us, here we will have a deep dive into the VoLTE voice callsand to discover the possibilities of the VoLTE, both to benefits Huawei &

Operators.

VoLTE Architectures

Before we starts, lets have a look into the complete network architectures

proposed by 3GPP on this VoLTE, which is to replace the CSFB currently,

thats experiencing delay, long latency, and also no significant voice quality

improvements compare to legacy 2G/3G CS calls.

Below is the complete network architecture of VoLTE interworking between

EUTRAN, EPC, CS Domain, PS Domain & IMS network.


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Huawei VoLTE network consists of the operation support layer, service layer,

core layer, access layer, and terminal layer. In the LTE or 2G/3G networks,

subscribers can use CSFB, Single Radio, and Dual Radio terminals to access

the VoLTE network.

Huawei VoLTE solution builds IMS and LTE on a live CS domain to provide

E2E quality of service (QoS) guarantee, high-quality voice and video calls, and

rich data services. With this solution, carriers can evolve their 2G/3G networks

to LTE networks to extend their business from offering voice-only to

multimedia-rich voice. Subscribers can use various LTE terminals such as

CSFB, Single Radio, and Dual Radio terminals to access an LTE network or

2G/3G network. When subscribers move out of LTE coverage, the LTE

network smoothly hands over calls to a 2G/3G network. Centralized service

provisioning, network management, and charging are available in the VoLTE

architecture.


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VoLTE (Voice over LTE) wi ll be a reality soon, as operators around the world

are completing their field tests and prepare to roll out voice and otherIMS-based services to the eagerly awaiting public and to gain the leading

position in their respective countries in order to win the market shares.

Actually, in real life, the public probably doesnt care about VoLTE. All they

want is good quality, high definition HD voice calls to complement their

high-speed wireless data services. VoLTE is positioned to deliver the goods,

but how does it compare with other wireless voice solutions such as 2G/3G CS

calls? We can answer that question in a few manners, but lets start with

something easy, which is [how many VoLTE calls can an LTE cell support?]

VoLTE Packet Size

As it turns out, that question doesnt have a simple answer. It depends on a lot

of variables, including the voice coder choices, the RF conditions in the cell,

the Huawei eNBs scheduler algorithm, the 3GPP protocol releases options,

and so on. To keep this discussion at manageable levels, lets concentrate on

one particular aspect of VoLTE capacity: how many Physical Resource Blocks

(PRBs) are needed to deliver the traffic for one VoLTE call over a typical LTE

Uu air interface?

Lets assume for the moment that the operator has deployed channel

bandwidth of 10 MHz LTE radio channels(Which is most operators are

deploying their LTE services at phase 1 stage). This is fairly typical to provides

50 PRBs per millisecond on the downlink (somehow it will be lesser than 50

PRBs resources on the uplink, due to the PUCCH configuration & limitations).

Lets further presume that VoLTE is configured to use the Adaptive Multi-Rate

Wideband (AMR-WB) 12.65 coder, and that Robust Header Compression

(RoHC) is enabled over the air interface which to reduce the overheadconsumption over the LTE air interface.

Huawei VoLTE scheduling are based on 20ms per Time-Transmission-Interval,

TTI( Huawei Proprietary), and the AMR-WB 12.65 coder generates 253 bits of

coded speech every 20 ms (a net data rate of 12.65 kbps). In order to deliver

each voice services to the UE, additional protocol headers are needed, such

as an RTP header (typically 12 bytes), a UDP header (8 bytes), and an IPv6

header (40 bytes). This brings the total packet length up to some 733 bits

every 20 ms.


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RoHC (Robust Overhead Compression), however, 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. TheRLC and MAC layers will add their own overhead, so the end result is that the

air interface will have to transport roughly 300 bits of data for every VoLTE

packet scheduled to one User.

VoLTE vs. PRBs

Now we need to relate the above data size back into our LTE Air Interface

resources. A single PRB has 12 subcarriers and 14 symbols over the course of

1 ms, or 12 x 14 = 168 resource elements (REs).

Some of those REs are occupied by the PDCCH (Assuming max 3 symbols

are used for PDCCH) and the downlink reference signals RS, leaving about

120 REs per PRB to carry data on the downlink.

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Each RE carries 2, 4 or 6 coded bits, depending on the modulation scheme in

effect (QPSK, 16QAM or 64QAM, respectively), but some of those bits will be

data bits, and some will be error protection bits. So how many data bits will fit

in a single PRB? That depends on the specific RF conditions in the cell whichwill be feedback by the User on the uplink, that will be Channel Quality

Indicator, CQI table below.

Lets see what happens under good (CQI = 15), average (CQI = 7) and poor

(CQI = 1) situations.

CQI 15 transmissions use 64QAM modulation and a 948/1024 = 0.926

effective coding rate, which means that each RE holds 6 x 0.926 = 5.55

data bits on average. A single PRB can then carry 120 x 5.55 = 666data bits, or the equivalent of two VoLTE voice samples. LTE cant

CQI Index Modulation Code Rate x 1024 Efficiency

0 out of range

1 QPSK 78 0.1523

2 QPSK 120 0.2344

3 QPSK 193 0.3770

4 QPSK 308 0.6016

5 QPSK 449 0.8770

6 QPSK 602 1.1758

7 16QAM 378 1.4766

8 16QAM 490 1.9141

9 16QAM 616 2.4063

10 64QAM 466 2.7305

11 64QAM 567 3.3223

12 64QAM 666 3.9023

13 64QAM 772 4.5234

14 64QAM 873 5.1152

15 64QAM 948 5.5547


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allocate less than one PRB per user, though, so well count this as one

PRB per VoLTE call.

CQI 7 transmissions use 16QAM modulation and a 378/1024 = 0.369

coding rate, resulting in 4 x 0.369 x 120 = 177 data bits. In other words,two PRBs are needed to carry a single VoLTE voice sample.

CQI 1 transmissions use QPSK modulation and a 78/1024 = 0.076

coding rate, supporting 2 x 0.076 x 120 = 18 data bits per PRB. This

means that a single VoLTE packet requires about 16 PRBs.

VoLTE by the Numbers

So how many VoLTE calls can we squeeze into a 10 MHz LTE channel? Voice

samples are generated every 20 ms, so if everything lines up exactly right (and

no retransmissions are needed), then twenty VoLTE calls can share the same

set of PRBs, one after the other. The maximum number of VoLTE calls that

can be carried is then determined by:

((Number of Available PRBs) / (Number of PRBs per VoLTE Call)) x 20

Here are the results, per CQI:

Thus, how realistic are these numbers? There are many presumptions built in

to this calculation, most of which wouldnt hold true out in the real world:

All users in a cell would not report exactly the same CQI value, and a

cell where every UE reports CQI value 1 is basically unusable.

VoLTE packet arrivals would not be perfectly distributed across the 20ms coding intervals.

Most packets would require at least one HARQ retransmission,

especially at lower CQI values, which consumes additional PRBs.

Some capacity needs to be reserved for non-VoLTE (data) subscribers.

The uplink has a lower capacity than the downlink, in terms of the

number of PRBs available and the efficiency of the transmissions.


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VoLTE Conclusions

Nevertheless, this document provides some hints into what the operators can

expect to see when VoLTE is turned on in the field. Under good RF conditions,LTE can deliver VoLTE packets quickly and efficiently, with enough capacity

left over for other users. Under poor conditions, LTE will struggle to support

even a handful of users.

The reality is that, in general, VoLTE is expected to have a call capacity

comparable to other wireless voice solutions, like UMTS and CDMA2000 1x,

on the order of 200 to 300 users per cell under 10MHz bandwidth. The

challenge for the operators is to manage the end-to-end voice quality, and to

juggle the conflicting demands of voice and data users.

Happy reading Folks

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VoLTE Calculation