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.
- 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 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:
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.
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.
