Can eMTC technology support high-quality VoLTE voice services?
In the R13 stage, 3GPP has formulated eMTC and NB-IoT communication standards for Internet of Things and machine communication, aiming at low power consumption, Dalian connection, wide coverage and low cost. What needs to be concerned is that, compared with the traditional mobile communication technology goals, user experience rate improvement is no longer the focus of research and design, but the need for deep coverage, that is to say, in an environment with deeper coverage and weaker signal than the existing mobile communication network, data connection can be established and business can be successfully completed.
Based on the ambitious goal of actively building the ecology of the Internet of Things and embracing the interconnection of all things, China Telecom has provided NB-IoT commercial services in mid-2017, and clearly proposed the eMTC commercial plan for 2018. At present, the commercial network of eMTC is mainly concentrated in the United States. For China Telecom, it is necessary to discuss the business objectives in detail before eMTC is commercialized.
Applicability Analysis of eMTC Business Characteristics to VoLTE Business
The typical difference between eMTC and NB-IoT is that the bandwidth of eMTC terminal can reach 1.08MHz, which is much higher than 200 kHz of NB-IoT terminal. Therefore, the peak speed of eMTC terminal is much higher than that of NB-IoT terminal. Based on the above characteristics, the industry generally believes that eMTC technology can provide relatively low-cost VoLTE terminal solutions, and can have better quality of service. However, through detailed technical analysis, using eMTC technology to support voice services or provide voice solutions, the actual effect may be difficult to be optimistic.
Low Power Characteristic
In order to provide Internet of Things services in power supply constrained environments, like NB-IoT, eMTC also takes low power consumption as its system design goal, expecting to support 10-year maintenance-free terminals based on smaller capacity batteries, and adopts two energy-saving technologies, eDRX (Extended Discontinuous Reception) and PSM (Power Saving Mode), like NB-IoT. For the typical low-frequency Internet of Things business, such as automatic meter reading, because the frequency of business is very low, using these two technologies can make the terminal in a long-term dormant state, only when data transmission is needed, so it can save electricity very much. But this good power-saving effect is only effective for low-frequency business. For VoLTE business, the terminal needs to go through. Often monitor network pages and respond to incoming calls in time, so it is impossible to adopt the power saving mechanism of eDRX and PSM technology.
Wide coverage features
Considering that some Internet of Things terminals are often located deep inside buildings, such as water meters, where the environment is often very weak, eMTC designed repetitive technology to enhance coverage. Through the repetition of upstream and downstream wireless signals, signal energy can be accumulated at the receiving end, thereby enhancing coverage. However, due to the duplication of wireless signals, the average service rate is reduced. That is to say, this coverage enhancement technology is at the cost of service rate reduction. Therefore, eMTC coverage enhancement technology does not bring any benefits to VoLTE, which needs a certain rate guarantee.
Low Cost eMTC Terminal Scheme
The low-cost schemes of eMTC terminals (chips) mainly include: smaller bandwidth and 1.08MHz bandwidth. Although the bandwidth of eMTC terminals is higher than that of NB-IoT, it is much lower than that of ordinary LTE terminals, which can reduce device prices and chip computing capacity requirements, thus reducing the overall price; lower peak rate, lower peak rate than LTE. It reduces chip computing power and Buffer requirements, thus lowering chip price; single terminal receiving antenna reduces the cost of RF devices; half-duplex scheme can save terminal RF duplexers, thereby reducing the cost of terminals.
The influence of half-duplex eMTC scheme on VoLTE
Semi-duplex scheme is the main scheme for equipment manufacturers and chip manufacturers to realize eMTC capability. The 3GPP protocol stipulates that eMTC has two implementations: half-duplex and full-duplex. In order to reduce costs, half-duplex has become the mainstream in the current stage.
Downlink scheduling of half-duplex eMTC terminals
According to the eMTC half-duplex downlink scheduling sequence in Fig. 1, when the signal is good, the terminal needs to wait for two subframes before receiving the data in the downlink PDSCH channel, and then wait for four subframes before the terminal feeds back ACK/NACK to the base station. Upper and downlink switching takes one subframe to prepare the terminal to receive MPDCCH scheduling again. In the optimum case, the number of sub-frames for downlink data transmission of PDSCH is 3 in 10 sub-frames. That is to say, for downlink data transmission, 70% of the time is used for non-data transmission, only 30% of the time is used for downlink data transmission. From the power efficiency of base station, the average power of base station for data transmission is reduced to 30% compared with ordinary LTE. If half of the transmission opportunities are reserved for upstream data transmission, the average power of the base station for downstream data transmission will drop to about 15%, and this is the best environment for signal transmission.
Figure 1. Half duplex eMTC downlink scheduling sequence.
If the terminal is in a poor signal environment, multiple MPDCCH repetitions are necessary to ensure the correct reception of downlink scheduling signaling, then the time proportion of PDSCH in downlink scheduling sequence will be lower. Referring to the current LTE network, LTE terminal demodulates PDCCH at the edge of the cell. If eMTC terminal demodulates MPDCCH correctly at the edge of the cell, a single MPDCCH scheduling takes up two sub-frames. Thus, at the edge of the cell, only about 10% of the time can be used for downlink PDSCH data transmission for a single terminal, and the downlink data receiving capability of the terminal is further reduced by the single antenna receiving scheme of the terminal.
Uplink scheduling of half-duplex eMTC terminals
Figure 2 shows the optimal upstream scheduling sequence in a better signal environment. After receiving MPDCCH scheduling, the terminal must wait for four subframes to transmit the upstream PUSCH data scheduled by MPDCCH, and the switching interval between upstream and downstream transmission must occupy at least one subframe. Upstream data from PUSCH to base station for ACK/NACK confirmation in MPDCH also needs to wait for 4 subframes.
Figure 2 Uplink scheduling sequence for duplex eMTC
At this time, only 3/8 = 37.5% of the terminal time is really used for upstream PUSCH data transmission. Similarly, assuming that up-and down-stream scheduling takes up half of the time, the proportion of time that the terminal actually spends on upstream PUSCH data transmission will be less than 19%.
If MPDCCH must be repeated at the edge of the cell, then one MPDCCH scheduling will occupy two subframes, and the time used for upstream PUSCH transmission will account for about 10%. For upstream, when the terminal transmits at full power, the proportion of PUSCH transmission time is about 10%, which means that the average upstream power is only about 10% of full power.
Comprehensive impact of half-duplex eMTC terminals
For VoLTE, which has a fixed rate requirement, a fixed voice frame is generated every 20 ms to form a fixed data packet every 20 ms. The lower proportion of upstream PUSCH transmission time means that the terminal must send voice data in fewer transmission opportunities. Therefore, compared with the 100% PUSCH transmission time of ordinary LTE terminals, half-duplex eMTC terminals must adopt higher MCS in fewer transmission opportunities to complete voice data transmission. Higher upstream MCS means lower upstream coverage. Low.
Similarly, the lower proportion of downlink PDSCH transmission time of base stations means that base stations must adopt higher MCS to send voice data arriving every 20 ms cycle. Higher downlink MCS means lower downlink coverage.
In the case of full power transmission (which means at the edge of cell), the average power of PUSCH of half-duplex eMTC terminal is reduced to about 10% compared with that of ordinary LTE terminal. It can be simply considered that the coverage of half-duplex eMTC terminal is 10 dB lower than that of LTE terminal upstream. Similarly, within the 1.08MHz bandwidth of terminal capacity, the average power of PDSCH of base station received by terminal is equivalent to 10% of that of ordinary LTE terminal. It can also be simply considered that the downlink coverage of half-duplex eMTC terminal is also reduced by 10 dB compared with LTE terminal. In fact, the downlink signal bandwidth of ordinary LTE terminal is higher than 1.08MHz, so half-duplex eMTC terminal is better than ordinary LTE terminal. The downlink coverage of the terminal decreases by more than 10 dB. It should be pointed out that the number of upstream PRBs supported by PUSCH is only 2-3 when the terminal transmits full power over the edge area. Therefore, the lower bandwidth of eMTC terminal will not affect the performance of upstream VoLTE.
According to the link budget analysis, in the dense urban environment of 1.8 GHz band, the coverage capacity is reduced by 10 dB, which means that the coverage radius of the cell is reduced to about 60%. Therefore, it can be seen that the coverage performance of VoLTE voice service with half-duplex eMTC terminal is bound to face enormous challenges. At present, the industry chain only supports half-duplex eMTC scheme, so we should not take a positive attitude towards using eMTC technology to carry VoLTE voice.
Effect of Full Duplex eMTC Scheme on VoLTE
Compared with half-duplex eMTC scheme, full-duplex eMTC terminal can greatly improve the time share of downlink PDSCH and upstream PUSCH channel transmission. Therefore, the coverage performance of full-duplex eMTC terminal is much better than that of half-duplex eMTC terminal, but this coverage improvement does not mean that it can be completely equivalent to ordinary LTE terminal.
Firstly, the bandwidth of the terminal is only 1.08MHz, which means that at the edge of the cell, the bandwidth that the terminal can receive is much lower than that of the ordinary LTE terminal, and the decline of downlink coverage is obvious. However, as mentioned earlier, this smaller terminal bandwidth has no impact on the upstream in the edge of the cell environment.
Secondly, the single antenna receiving scheme adopted by eMTC terminal in order to reduce the cost affects the downlink coverage performance to a certain extent.
Therefore, even the full-duplex eMTC terminal scheme, it is difficult to fully achieve the coverage performance of ordinary LTE terminals.
Overall consideration of eMTC scheme supporting VoLTE voice scheme
As mentioned above, if we adopt half-duplex eMTC scheme to carry VoLTE voice service, we will face great challenges in network operation, and there is a huge risk that the quality of service is difficult to guarantee. Therefore, for the half-duplex eMTC technology generally supported in the current industry chain, we should carefully consider providing VoLTE terminal solutions.
If we want to use low-cost eMTC chips to provide low-cost VoLTE terminal solutions and intelligent wearing equipment, we must consider the full-duplex eMTC scheme, and make further feasibility evaluation according to the test situation. In view of the lack of full-duplex eMTC terminal chips and equipment in the current industrial chain, operators need to decide whether to promote the development of related industries according to actual needs.