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 challenge 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 operator’s network when it comes to us, here we will have a deep dive into the VoLTE voice calls and to discover the possibilities of the VoLTE, both to benefits Huawei & Operators. VoLTE Architectures Before we starts, let’s have a look into the complete network architectures proposed by 3GPP on this VoLTE, which is to replace the CSFB currently, that’s 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. 2014-06-12
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VoLTE (Voice over LTE) will be a reality soon, as operators around the world are completing their field tests and prepare to roll out voice and other IMS-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 doesn’t 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 let’s start with something easy, which is [how many VoLTE calls can an LTE cell support?] VoLTE Packet Size As it turns out, that question doesn’t have a simple answer. It depends on a lot of variables, including the voice coder choices, the RF conditions in the cell, the Huawei eNB’s scheduler algorithm, the 3GPP protocol releases options, and so on. To keep this discussion at manageable levels, let’s 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? Let’s 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). Let’s 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 overhead consumption 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. The RLC 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).
NSCRB Subcarriers = 12
Physical Resource Block
N RB DL
Resource Element NSymbDL
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. Subframe
PDSCH Symbols PDSCH Symbol Mapping Reserved for Control
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x
R
x
R
R
x
R
x
x
R
x
R
R
x
R
x
x
R
x
R
R
x
R
x
x
R
x
R
R
x
R
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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 which will be feedback by the User on the uplink, that will be Channel Quality Indicator, CQI table below. 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
Let’s 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 = 666 data bits, or the equivalent of two VoLTE voice samples. LTE can’t
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allocate less than one PRB per user, though, so we’ll 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 wouldn’t 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 20 ms 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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6
QPSK
602
1.1758
7
16QAM
378
1.4766
8
16QAM
490
1.9141
9
16QAM
616
2.4063
VoLTE10Conclusions 64QAM
466
2.7305
Nevertheless, this64QAM document provides hints into what the operators can 11 567 some3.3223 expect to see when VoLTE is turned on in the field. Under good RF conditions, 12 deliver VoLTE 64QAMpackets quickly 666 LTE can and3.9023 efficiently, with enough capacity left over users. Under poor 13for other 64QAM 772 conditions, 4.5234LTE will struggle to support even a handful of users. 14
64QAM
873
5.1152
The reality is that, in general, VoLTE is expected to have a call capacity 15 64QAM 948solutions, 5.5547 comparable to other wireless voice 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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