Reprinted: https: //rf.eefocus.com/article/id-LTE%20delay
For mobile communication concerned services, the most important delays are end to end delay, i.e., send and receive ends for connection has been established, data packets are generated from the transmission side to the receiving side correctly receives delay. Depending on the business model can be divided into single-end delay and the delay for return trip delay, wherein the delay for a single data packet generating means via a wireless network from a transmitting end properly receiving end reaches a further delay means delay for packet back is generated from the transmitter to the destination server receives the packet and returns the corresponding data packet is received correctly transmitting end until the reply packet delay.
The main conventional mobile communication is a communication between people, along with miniaturization and intelligent hardware device, future mobile communication more high speed connections between applications' and objects "and" object and the objects. " Machine communication (Machine Type Communication, MTC) business wide range of applications, such as mobile medical, car networking , smart home, industrial control, environmental monitoring and other applications will drive the explosive growth of MTC system, a large number of devices to access the network, to achieve true the "all things Internet", infinite vitality for mobile communications. At the same time, a broad range of applications MTC system will bring new challenges for mobile communications technology, such as real-time cloud computing, virtual reality, online gaming, telemedicine, smart transportation, smart grid, remote real-time control and other services sensitive to delay for delay put forward higher requirements, the existing LTE system can not meet the demand, the need for research.
This paper describes the future demand for MTC traffic delay, delay analyzes the existing LTE system, describes the key technology to reduce latency.
MTC service delay requirements analysis
Future MTC data transmission delay is further reduced when the fast time communication application response time than the system constraints, you can get real-time communications experience. Here are four typical time-constrained applications:
● human muscle response time in 0.5s ~ 1s, which means that people click on a link, if the connection can be established in 0.5s time, people can achieve real-time web browsing experience.
Auditory ●: When a sound signal in the 70ms ~ 100ms may be ready to receive, one can achieve real-time call. Considering the speed of sound, which means that when two people distance of more than 30m, the two rely solely on sound waves can not be achieved in real time.
● Vision: human vision resolution is generally not more than 100Hz, which means that as long as the image update rate of not less than 100Hz (delay of no more than 10ms), people can get a seamless video experience.
● touch: this area to achieve real-time, delay requirement is limited to a few ms level, including the use of mobile applications involving target 3D, virtual reality, intelligent traffic control business security, smart grid and so on.
Proposed in the industry-end delay of the system should reduce the current more than five times, and, when considering the needs of the 5th generation mobile communication system that RTT (Round Trip Time, loop delay) in the order of magnitude of 1ms. Real-time games, M2M, sensor alarm or event detection scenarios should be the focus of research, some scenes required for delay of no more than 100ms, which, based on the sensor alarm or event detection scene there is a minimum of delay requirements of 2ms.
Accordingly, ultra-low latency scenarios to consider MTC system delay of the air interface delay in milliseconds.
Delay Analysis conventional LTE system
ITU- R of the set target transmission delay unidirectional delay target 10ms. LTE / LTE-A system to meet the delay requirements and ITU with a certain margin, the one-way packet transmission delay is less than 5ms. In the connected state the following physical downlink shared channel line (Physical Downlink Shared Channel, PDSCH) transmitting downlink data and a physical uplink shared channel (Physical Uplink Shared Channel, PUSCH) for transmitting uplink data example for delay analysis.
In the LTE FDD system, subframe n, the base station using a physical downlink control channel (Physical Downlink Control Channel, PDCCH) scheduling downlink data transmission, the terminal feeds back ACK / NACK information on the subframe n + 4, the base station receiver processing delay minimum of 1ms, the base station may be the fastest in subframe n + 5 data retransmission schedule, shown in Figure 1, a single transmission time is 1ms, the minimum time for the retransmission of 5ms.
In the LTE FDD system, when a terminal has data transmission needs to wait for sending a scheduling request (Schedule Request, SR) subframe n, the terminal transmits a scheduling request message to the base station on the subframe n, the base station in a subframe fastest after the uplink data scheduling grant information n TX 2 +, the terminal receives an uplink data scheduling grant information in subframe n + 2, +. 6 on the respective transmitting uplink data n in the subframe, the base station the feedback in a subframe n + 10 ACK / NACK information to the terminal, the terminal in subframe n + 14 on the retransmission of the uplink data, as shown particularly in FIG. 2, to complete a data transfer from the data transmission requirements, without regard to the scheduling request wait time of the sub-frame, single transmission delay is 6ms, a retransmission time is 14ms.
Low latency Technical Analysis
As can be seen from the existing LTE air interface delay analysis, the main factors affecting the air interface delay long data transmission, data transmission resource request waiting time, and the feedback delay caused by data processing, the following four kinds of factors for reducing the air interface the extension of the program.
Reducing the transmission data length
In the existing LTE system in units of subframes for data scheduling, LTE sub-frame length is 1ms, and therefore, the minimum length of the data transmission 1ms, length in order to reduce the data transmission time, there are two possibilities. One is to reduce the sub-frame length, as redesigned subcarrier spacing and a subframe including the OFDM symbol number, such that a sub-frame duration when the corresponding shorter length, thereby reducing the data transmission. For example, the sub-frame length is compressed to 1/4 of the conventional LTE sub-frame length, i.e. 0.25ms, considering the compression ratio of the respective processing time, particularly as the compression effect shown in Table 1, about 75% longer be compressed.
另 一种方案是以OFDM符号为单位进行数据调度传输，此时，最小数据传输长度为1个OFDM符号，按照现有LTE的OFDM符号长度计算，一个OFDM符号 长度为66.67ηs，如果考虑相应处理时间等比例压缩，具体压缩效果如表2所示，相对于现有1ms的数据传输可以压缩大概92%左右，如果进一步结合帧 结构的修改，如子载波间隔变化，可以进一步降低OFDM符号的长度，实现更低时延压缩。
另 外，增强HARQ反馈也有助于重传时延降低。传统的HARQ只反馈ACK/NAK信息，增强的HARQ可以额外反馈接收的BER估计信息，结合该信息和信 道反状态信息，调度器在进行冗余版本选择、MCS选择等方面可以更有针对性，使数据一次重传后被正确解码的概率大为提高，从而进一步降低数据传输时延。
数据传输资源请求导致的时延降低
LTE 系统中，当终端有数据传输需求时，需要先发送调度请求，基站才能分配资源让终端进行上行数据传输，这一过程导致上行数据传输时延明显大于下行数据传输时 延，如表3所示。另外，发送调度请求配置终端发送数据的资源，也会额外增加时延，因此，如果基站可以预分配资源终端，终端在有数据传输时直接在预先分配的 资源上传输数据，可以减少调度请求过程，从而使得上行数据传输时延与下行数据传输时延相当，这样可以实现上行数据单次传输时延压缩大概17%，一次重传时 延压缩36%，再结合上述数据传输时延降低方案可以进一步降低上行数据传输时延。
调度时延降低
After the existing LTE control channel are mainly located in the first n OFDM symbols of a subframe, or frequency-division multiplexing the PDSCH (the length of a subframe), specifically shown in Figure 3, the LTE system is only decoded data in a downlink control channel It can send data, since the position regulating control channel, resulting in increased data decoding delay. Further, a region corresponding to the terminal downlink control channel in one subframe is only one, if the local scheduler miss, can only wait for the next scheduled region, which leads to a delay waiting for data scheduling. In order to reduce scheduling delays, need to introduce more flexible downlink control region is provided, as shown in FIG. 4, as far as possible so that there is downlink data transmission control region, and may be received in advance when decoding the downlink data channel, control channel, to reduce the waiting reception time, thereby reducing the delay due to waiting for decoding the downlink control region and a downlink control channel, and waits for data reception result, and ultimately reduce the data transmission delay.
Reduce processing delay
For reducing the processing delay, to reduce time delay in addition to the algorithm implemented by the hardware and an outer, also contemplated by Advanced adaptive coding to reduce the processing delay codec, such as when SNR when high, convolutional coding, when the comparison SNR when low, the use of Turbo coding.
This article describes the reduction of the air interface delay technique, by a frame structure compressing and method for OFDM symbols scheduling based, and a terminal independent scheduling, can significantly reduce the air interface data transmission delay, Further, flexible control locale and advanced adaptive coding, further air interface delay may be reduced to meet the needs of different services, improve performance of the future mobile communication system.
Subsequent link adaptation may be considered in conjunction with optimization techniques, to ensure air to be reduced latency data port under some reliability study premise to meet ultra-low latency high reliability, so that the mobile communication system having a wider application scenario, enhance the user experience.