Don joined Award Solutions in 2005, bringing his knowledge and experience in mobile wireless technologies to bear in the planning, development and delivery of technical training seminars. Don specializes in wireless telecommunications networks, focusing on air interface and core network standards, wireless and Internet applications, and advanced wireless network solutions, such as ad hoc and mesh networking.Don has over 30 years of hands-on experience in the telecommunications and wireless industries. He began his career in Ottawa, Canada, with Nortel Networks (then Bell-Northern Research) as a call processing software designer. He moved to Richardson, Texas, in 1983, as one of the initial team responsible for designing and developing Nortel’s wireless product line. He rose quickly through the ranks, first as a development manager, then as a senior project manager, and then as a director of advanced wireless technology, involved in all aspects of the design of Nortel’s AMPS, TDMA and CDMA products. In his final role at Nortel, Don was responsible for a small technology group investigating advanced networking technologies, including self-organizing wireless mesh networks.Don is currently involved in developing and delivering courses for Award’s 4G (LTE) technology curriculum at many leading telecommunications companies. In addition to technology classes, Don conducts network planning and evolution sessions for large wireless service providers to help RF and core network engineers understand and plan for upcoming technology changes and enhancements such as VoLTE and LTE Advanced.Don received his Bachelor of Science degree in Computer Science (First Class Honors) from the University of British Columbia in Vancouver, Canada. He holds 9 patents in various areas of wireless technology.
VoLTE (Voice over LTE) will be a reality soon, as operators around the world complete their field tests and prepare to roll out voice and other IMS-based services to the eagerly awaiting public.
Actually, in real life, the public probably doesn’t care about VoLTE per se. All they want is good voice calls to complement their high-speed wireless data services. VoLTE is poised to deliver the goods, but how does it compare with other wireless voice solutions? We can answer that question in a number of ways, but let’s start with something easy: 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 eNB’s scheduler algorithm, the protocol 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 air interface?
Let’s assume for the moment that the operator has deployed 10 MHz LTE radio channels. This is fairly typical (at least in the US), and provides 50 PRBs per millisecond on the downlink (somewhat less on the uplink, depending on the PUCCH configuration). 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.
The AMR-WB 12.65 coder generates 253 bits of coded speech every 20 ms (a net data rate of 12.65 kbps, hence the name). In order to deliver each voice sample to the UE, additional protocol headers are needed: 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.
RoHC, 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.
VoLTE vs. PRBs
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 and the downlink reference signals, leaving about 120 REs per PRB to carry data on the downlink. 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. Let’s see what happens under good (CQI = 15), average (CQI = 7) and poor (CQI = 1) situations.
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:
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:
Nonetheless, this exercise provides some insight 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. 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.
Thanks for your Details. you mentioned that AMR Codec generates 253 bits for 20ms Speech and after overheads it will roughly come around 300 bits for 20ms. So for 1ms it will around 15bits only. For any CQI values 1 PRB will be enough for transmitting 15bits. Whereas you calculated how many PRB's required for transmitting 300bits in 1ms. I don't understand on this . Can you plse bit explain on that?
Thanks, Shankar Ganesh