Section: 5.7.1 Time and frequency structure
Content: The physical layer random access preamble, illustrated in Figure 5.7.1-1, consists of a cyclic prefix of length and a sequence part of length. The parameter values are listed in Table 5.7.1-1 and depend on the frame structure and the random access configuration. Higher layers control the preamble format. Figure 5.7.1-1: Random access preamble format Table 5.7.1-1: Random access preamble parameters The transmission of a random access preamble, if triggered by the MAC layer, is restricted to certain time and frequency resources. These resources are enumerated in increasing order of the subframe number within the radio frame and the physical resource blocks in the frequency domain such that index 0 correspond to the lowest numbered physical resource block and subframe within the radio frame. PRACH resources within the radio frame are indicated by a PRACH configuration index, where the indexing is in the order of appearance in Table 5.7.1-2 and Table 5.7.1-4. For non-BL/CE UEs there are up to two PRACH configurations in a cell. The first PRACH configuration is configured by higher layers with a PRACH configuration index (prach-ConfigurationIndex) and a PRACH frequency offset (prach-FrequencyOffset). The second PRACH configuration (if any) is configured by higher layers with a PRACH configuration index (prach-ConfigurationIndexHighSpeed) and a PRACH frequency offset (prach-FrequencyOffsetHighSpeed). For BL/CE UEs, for each PRACH coverage enhancement level, there is a PRACH configuration configured by higher layers with a PRACH configuration index (prach-ConfigurationIndex), a PRACH frequency offset (prach-FrequencyOffset), a number of PRACH repetitions per attempt (numRepetitionPerPreambleAttempt) and optionally a PRACH starting subframe periodicity (prach-StartingSubframe). PRACH of preamble format 0-3 is transmitted times, whereas PRACH of preamble format 4 is transmitted one time only. For BL/CE UEs and for each PRACH coverage enhancement level, if frequency hopping is enabled for a PRACH configuration by the higher-layer parameter prach-HoppingConfig, the value of the parameter depends on the SFN and the PRACH configuration index and is given by - In case the PRACH configuration index is such that a PRACH resource occurs in every radio frame when calculated as below from Table 5.7.1-2 or Table 5.7.1-4, - otherwise where is the system frame number corresponding to the first subframe for each PRACH repetition, corresponds to a cell-specific higher-layer parameter prach-HoppingOffset. If frequency hopping is not enabled for the PRACH configuration then . For frame structure type 1 with preamble format 0-3, for each of the PRACH configurations there is at most one random access resource per subframe. Table 5.7.1-2 lists the preamble formats according to Table 5.7.1-1 and the subframes in which random access preamble transmission is allowed for a given configuration in frame structure type 1. The start of the random access preamble shall be aligned with the start of the corresponding uplink subframe at the UE assuming , where is defined in clause 8.1. For PRACH configurations 0, 1, 2, 15, 16, 17, 18, 31, 32, 33, 34, 47, 48, 49, 50 and 63 the UE may for handover purposes assume an absolute value of the relative time difference between radio frame in the current cell and the target cell of less than . The first physical resource block allocated to the PRACH opportunity considered for preamble formats 0, 1, 2 and 3 is defined as . Table 5.7.1-2: Frame structure type 1 random access configuration for preamble formats 0-3 For frame structure type 2 with preamble formats 0-4, for each of the PRACH configurations there might be multiple random access resources in an UL subframe (or UpPTS for preamble format 4) depending on the UL/DL configuration [see table 4.2-2]. Table -3 lists PRACH configurations allowed for frame structure type 2 where the configuration index corresponds to a certain combination of preamble format, PRACH density value, and version index, . For frame structure type 2 with PRACH configuration indices 0, 1, 2, 20, 21, 22, 30, 31, 32, 40, 41, 42, 48, 49, 50, or with PRACH configuration indices 51, 53, 54, 55, 56, 57 in UL/DL configuration 3, 4, 5, the UE may for handover purposes assume an absolute value of the relative time difference between radio frame in the current cell and the target cell is less than . Table -3: Frame structure type 2 random access configurations for preamble formats 0-4 Table 5.7.1-4 lists the mapping to physical resources for the different random access opportunities needed for a certain PRACH density value, . Each quadruple of the format indicates the location of a specific random access resource, where is a frequency resource index within the considered time instance, indicates whether the resource is reoccurring in all radio frames, in even radio frames, or in odd radio frames, respectively, indicates whether the random access resource is located in first half frame or in second half frame, respectively, and where is the uplink subframe number where the preamble starts, counting from 0 at the first uplink subframe between 2 consecutive downlink-to-uplink switch points, with the exception of preamble format 4 where is denoted as (*). The start of the random access preamble formats 0-3 shall be aligned with the start of the corresponding uplink subframe at the UE assuming and the random access preamble format 4 shall start before the end of the UpPTS at the UE, where the UpPTS is referenced to the UE's uplink frame timing assuming. The random access opportunities for each PRACH configuration shall be allocated in time first and then in frequency if and only if time multiplexing is not sufficient to hold all opportunities of a PRACH configuration needed for a certain density value without overlap in time. For preamble format 0-3, the frequency multiplexing shall be done according to where is the number of uplink resource blocks, is the first physical resource block allocated to the PRACH opportunity considered and where is the first physical resource block available for PRACH. For preamble format 4, the frequency multiplexing shall be done according to whereis the system frame number and whereis the number of DL to UL switch points within the radio frame. For BL/CE UEs, only a subset of the subframes allowed for preamble transmission are allowed as starting subframes for the repetitions. The allowed starting subframes for a PRACH configuration are determined as follows: - Enumerate the subframes that are allowed for preamble transmission for the PRACH configuration as where and correspond to the two subframes allowed for preamble transmission with the smallest and the largest absolute subframe number , respectively. - If a PRACH starting subframe periodicity is not provided by higher layers, the periodicity of the allowed starting subframes in terms of subframes allowed for preamble transmission is . The allowed starting subframes defined over are given by where - If a PRACH starting subframe periodicity is provided by higher layers, it indicates the periodicity of the allowed starting subframes in terms of subframes allowed for preamble transmission. The allowed starting subframes defined over are given by where - No starting subframe defined over such that is allowed. Each random access preamble occupies a bandwidth corresponding to 6 consecutive resource blocks for both frame structures. Table 5.7.1-4: Frame structure type 2 random access preamble mapping in time and frequency