Title:J2017W-02-02.indd p14 2018/01/31/ 水 11:22:04 2 Terrestrial Communication Technology Research and Development Cyclic Prefix Guard Time BS Reference signal Data signal uency 250 s 250 s Request Ａ One resource is dedicated 500 s (= 1 slot duration) Ｃ Freq Ａ to one UE. FFig. 2 Basic structure of the radio frame [3] Ｂ Data ＡＢＣ Grant ＵＥ ＵＥ ＵＥ Time １ ２２３ Latency of 10 ms or more caused by transmission with grant in UL. Ａ Zadoff-Chu sequence Conventional system ２０６ ２２３，１ ２０４ Ｂ Zadoff-Chu sequence (18-sample cyclic shift) １８８ ２２３，１ １８７ Ｃ BS Shortened TTI for latency reduction Zadoff-Chu sequence (36-sample cyclic shift) • UE Identification • Interference Cancellation Correlation with Zadoff-Chu sequence without cyclic shift Estimates on channel impulse responses are available without Ｃ One resource is shared by Frequency interference when these responses are terminated within 16.7 s. Ａ multiple Ues. Ｂ Delay 16.7s 16.7s Transmission without grant FFig. 3 Reference signal structure example ＵＥ ＵＥ ＵＥ Time New radio access system the maximum propagation delay time of valid multi-paths. FFig. 1 Outline of radio access technology that realizes simultaneous Among the channel models [4] for developing the technical connectivity and low latency specifications of the 5G radio access, the TDL-A channel model includes the longest delayed path in a non-line of between UE signals. Those technologies will be explained sight environment, and the maximum delay time is ap- below. Further, in the designing of this radio access tech- proximately 3.5 µs. Further, assuming the BS-UE distance nology, based on a scenario [2] premised on its use with of 1,153 m and urban macro cell environment, propagation IoT devices, the channel bandwidth will be set at 1.08 MHz. delay time from UE to BS is 3.8 µs. Considering the above, it would be necessary for the orthogonal sequence cyclic 2.1 Structure of radio frame shift time in the reference signal to be equal to or larger Here we discuss the structure of the radio frame for than 7.3 µs, and also considering the synchronization error uplink data transmission from UE to BS. Figure 2 shows between BS and UE, 18 samples equivalent to 16.7 µs are the basic structure of the radio frame. The time slot has a set as the cyclic shift fo the orthogonal sequence. In order total length of 500 µs and is composed of a 250 µs reference to accommodate simultaneous connection of 12 UEs, it signal and a 250 µs data signal. The reference signal is used would be necessary for the sequence length to be 216 or for the purposes of identifying the UE terminals and esti- more, and in the current R&D the Zadoff-Chu sequence mating the channel impulse response between the UE and length is set at 223. As was shown in Fig. 2, a 20-length BS. The data signal is used to transfer data sent from UE. sample (equivalent to 18.5 µs) cyclic prefix (CP) si added Transmission delay is minimized by sending a reference to a sequence that has been cyclically expanded to a length signal and a data signal without intermission. of 225. Figure 3 shows an example of the structure of the Here we discuss the structure of the reference signal. reference signal when cyclic shift has been applied. By By using orthogonal sequences for the reference signal, assigning different cyclic shift umn bers for each UE termi- even when signals are being sent by multiple UE terminals nal, it becomes possible to identify the UE even when the on the same time and frequency resource—in addition to BS simultaneously receives reference signals from multiple identifying the sending UE—estimation of the channel UEs. In the grant-free transmission scheme, a UE-specific impulse response necessary for the data demodulation and cyclic shift number is assigned to each UE. decoding is conducted. In the current R&D at hand, we Next, we discuss the structure of the data signal. employ the Zadoff-Chu sequence for the orthogonal se- Figure 4 shows the processing of information bits for the quence, which is also used in LTE-A. When using this data signal in each UE terminal. After the information bits sequence, in order to estimate the channel impulse response are scrambled, 8 bit cyclic redundancy check (CRC) encod- without interference between signals from UE terminals, it ing is performed. Following this, turbo encoding (coding is necessary to assign each UE terminal with a Zadoff-Chu rate: 1/2 or 1/3; constraint length: 4) is performed. After sequence having a cyclic shift time equal to or longer than QPSK or 16-QAM modulation, CP is inserted and the 14 Journal of the National Institute of Information and Communications Technology Vol. 64 No. 2 (2017).