LTEPRACH配置参数分析.doc
LTE PRACH参数配置分析 目录目录31引言41.1编写目的41.2文档组织41.3预期读者和阅读建议41.4参考资料41.5缩写术语和常用符号42PRACH信道的配置分析62.1PRACH信道的配置参数62.1.1PRACH配置索引（prach-ConfigurationIndex）62.1.2零相关配置（zeroCorrelationZoneConfig）142.1.3根序列索引（rootSequenceIndex）182.1.4是否为高速状态（highSpeedFlag）212.1.5频率偏移（prach-FrequencyOffset）222.2PRACH信道的参数的配置方法232.2.1PRACH信道参数的配置步骤232.2.2邻小区的PRACH信道的配置233LTE典型 PRACH配置243.1Format 0时 PRACH信道的参数的配置243.1.1密度为1情况下PRACH相关参数配置243.1.2密度为2情况下PRACH相关参数配置263.2Format 4时 PRACH信道的参数的配置274高速模式下配置原则295附录325.1南京规模实验网站址分布图321 引言1.1 编写目的本文档的编写目的是分析PRACH信道的各参数的配置方法及各邻区间如何进行配置。1.2 文档组织本文首先对LTE3.0版本需要配置的PRACH信道的各参数进行了说明和描述，根据网络规划如何确定各参数的取值，并给出相邻小区各参数的配置原则。本文在第2章的后半部分给出了PRACH各参数的配置方法和步骤。第3章给出了高速模式下的零相关和根序列的配置有一定的关联关系。1.3 预期读者和阅读建议本文档的预期读者为LTE网络建设人员和LTE网络优化人员、测试人员等。1.4 参考资料1. LTE无线配置参数分析.doc V1.12. LTE PRACH密度需求分析.docV1.03. LTEUMTS长期演进理论与实践马霓、邬钢等译4. 3GPP TS 36.211 Physical Channels and Modulation 1.5 缩写术语和常用符号英文缩写英文全称中文全称CMCubic Metric立方量度PAPRPeak-to-Average Power Ratio峰均值功率比PRACHPhysical Random Access Channel物理随机接入信道RACHRandom Access Channel随机接入信道2 PRACH信道的配置分析2.1 PRACH信道的配置参数LTE 中PRACH信道的配置参数主要有五个，都是小区级参数分别是：² PRACH配置索引（prach-ConfigurationIndex）² 零相关配置（zeroCorrelationZoneConfig）² 根序列索引（rootSequenceIndex）² 是否为高速状态（highSpeedFlag）² 频率偏移（prach-FrequencyOffset）2.1.1 PRACH配置索引（prach-ConfigurationIndex）2.1.1.1 参数基本信息参数编号取值范围物理单位调整步长0.63默认值：51无无参数名称传送途径作用范围参数出处prach-ConfigurationIndexeNodeB->UECell36.211所属网元及设置途径小区逻辑无线资源参数->物理随机接入信道-> PRACH配置索引不同场景下的差异化配置说明无用于指示小区的PRACH配置索引。该参数指示了PRACH的频域资源索引、时域的无线帧、半帧、子帧的资源占用情况。该参数确定后，小区PRACH的时、频资源即可确定，同时也确定了采用的前导格式（047为前导格式03，4757为前导格式4），其定义见下表（36.211 Table 5.7.1-4）。PRACH configuration Index(See Table 5.7.1-3)UL/DL configuration (See Table 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(1,0,0,0)(1,0,0,1)N/AN/AN/A18(0,0,0,0)(0,0,0,1)(0,0,0,2)(0,0,1,0)(0,0,1,1)(0,0,1,2)(0,0,0,0)(0,0,0,1)(0,0,1,0)(0,0,1,1)(1,0,0,1)(1,0,1,1)(0,0,0,0)(0,0,1,0)(1,0,0,0)(1,0,1,0)(2,0,0,0)(2,0,1,0)(0,0,0,0)(0,0,0,1)(0,0,0,2)(1,0,0,0)(1,0,0,1)(1,0,0,2)(0,0,0,0)(0,0,0,1)(1,0,0,0)(1,0,0,1)(2,0,0,0)(2,0,0,1)(0,0,0,0)(1,0,0,0)(2,0,0,0)(3,0,0,0)(4,0,0,0)(5,0,0,0)(0,0,0,0)(0,0,0,1)(0,0,0,2)(0,0,1,0)(0,0,1,1)(1,0,0,2)19N/A(0,0,0,0)(0,0,0,1)(0,0,1,0)(0,0,1,1)(1,0,0,0)(1,0,1,0)N/AN/AN/AN/A(0,0,0,0)(0,0,0,1)(0,0,0,2)(0,0,1,0)(0,0,1,1)(1,0,1,1)20 / 30(0,1,0,1)(0,1,0,0)N/A(0,1,0,1)(0,1,0,0)N/A(0,1,0,1)21 / 31(0,2,0,1)(0,2,0,0)N/A(0,2,0,1)(0,2,0,0)N/A(0,2,0,1)22 / 32(0,1,1,1)(0,1,1,0)N/AN/AN/AN/A(0,1,1,0)23 / 33(0,0,0,1)(0,0,0,0)N/A(0,0,0,1)(0,0,0,0)N/A(0,0,0,1)24 / 34(0,0,1,1)(0,0,1,0)N/AN/AN/AN/A(0,0,1,0)25 / 35(0,0,0,1)(0,0,1,1)(0,0,0,0)(0,0,1,0)N/A(0,0,0,1)(1,0,0,1)(0,0,0,0)(1,0,0,0)N/A(0,0,0,1)(0,0,1,0)26 / 36(0,0,0,1)(0,0,1,1)(1,0,0,1)(0,0,0,0)(0,0,1,0)(1,0,0,0)N/A(0,0,0,1)(1,0,0,1)(2,0,0,1)(0,0,0,0)(1,0,0,0)(2,0,0,0)N/A(0,0,0,1)(0,0,1,0)(1,0,0,1)27 / 37(0,0,0,1)(0,0,1,1)(1,0,0,1)(1,0,1,1)(0,0,0,0)(0,0,1,0)(1,0,0,0)(1,0,1,0)N/A(0,0,0,1)(1,0,0,1)(2,0,0,1)(3,0,0,1)(0,0,0,0)(1,0,0,0)(2,0,0,0)(3,0,0,0)N/A(0,0,0,1)(0,0,1,0)(1,0,0,1)(1,0,1,0)28 / 38(0,0,0,1)(0,0,1,1)(1,0,0,1)(1,0,1,1)(2,0,0,1)(0,0,0,0)(0,0,1,0)(1,0,0,0)(1,0,1,0)(2,0,0,0)N/A(0,0,0,1)(1,0,0,1)(2,0,0,1)(3,0,0,1)(4,0,0,1)(0,0,0,0)(1,0,0,0)(2,0,0,0)(3,0,0,0)(4,0,0,0)N/A(0,0,0,1)(0,0,1,0)(1,0,0,1)(1,0,1,0)(2,0,0,1)29 /39(0,0,0,1)(0,0,1,1)(1,0,0,1)(1,0,1,1)(2,0,0,1)(2,0,1,1)(0,0,0,0)(0,0,1,0)(1,0,0,0)(1,0,1,0)(2,0,0,0)(2,0,1,0)N/A(0,0,0,1)(1,0,0,1)(2,0,0,1)(3,0,0,1)(4,0,0,1)(5,0,0,1)(0,0,0,0)(1,0,0,0)(2,0,0,0)(3,0,0,0)(4,0,0,0)(5,0,0,0)N/A(0,0,0,1)(0,0,1,0)(1,0,0,1)(1,0,1,0)(2,0,0,1)(2,0,1,0)40(0,1,0,0)N/AN/A(0,1,0,0)N/AN/A(0,1,0,0)41(0,2,0,0)N/AN/A(0,2,0,0)N/AN/A(0,2,0,0)42(0,1,1,0)N/AN/AN/AN/AN/AN/A43(0,0,0,0)N/AN/A(0,0,0,0)N/AN/A(0,0,0,0)44(0,0,1,0)N/AN/AN/AN/AN/AN/A45(0,0,0,0)(0,0,1,0)N/AN/A(0,0,0,0)(1,0,0,0)N/AN/A(0,0,0,0)(1,0,0,0)46(0,0,0,0)(0,0,1,0)(1,0,0,0)N/AN/A(0,0,0,0)(1,0,0,0)(2,0,0,0)N/AN/A(0,0,0,0)(1,0,0,0)(2,0,0,0)47(0,0,0,0)(0,0,1,0)(1,0,0,0)(1,0,1,0)N/AN/A(0,0,0,0)(1,0,0,0)(2,0,0,0)(3,0,0,0)N/AN/A(0,0,0,0)(1,0,0,0)(2,0,0,0)(3,0,0,0)48(0,1,0,*)(0,1,0,*)(0,1,0,*)(0,1,0,*)(0,1,0,*)(0,1,0,*)(0,1,0,*)49(0,2,0,*)(0,2,0,*)(0,2,0,*)(0,2,0,*)(0,2,0,*)(0,2,0,*)(0,2,0,*)50(0,1,1,*)(0,1,1,*)(0,1,1,*)N/AN/AN/A(0,1,1,*)51(0,0,0,*)(0,0,0,*)(0,0,0,*)(0,0,0,*)(0,0,0,*)(0,0,0,*)(0,0,0,*)52(0,0,1,*)(0,0,1,*)(0,0,1,*)N/AN/AN/A(0,0,1,*)53(0,0,0,*)(0,0,1,*)(0,0,0,*)(0,0,1,*)(0,0,0,*)(0,0,1,*)(0,0,0,*)(1,0,0,*)(0,0,0,*)(1,0,0,*)(0,0,0,*)(1,0,0,*)(0,0,0,*)(0,0,1,*)54(0,0,0,*)(0,0,1,*)(1,0,0,*)(0,0,0,*)(0,0,1,*)(1,0,0,*)(0,0,0,*)(0,0,1,*)(1,0,0,*)(0,0,0,*)(1,0,0,*)(2,0,0,*)(0,0,0,*)(1,0,0,*)(2,0,0,*)(0,0,0,*)(1,0,0,*)(2,0,0,*)(0,0,0,*)(0,0,1,*)(1,0,0,*)55(0,0,0,*)(0,0,1,*)(1,0,0,*)(1,0,1,*)(0,0,0,*)(0,0,1,*)(1,0,0,*)(1,0,1,*)(0,0,0,*)(0,0,1,*)(1,0,0,*)(1,0,1,*)(0,0,0,*)(1,0,0,*)(2,0,0,*)(3,0,0,*)(0,0,0,*)(1,0,0,*)(2,0,0,*)(3,0,0,*)(0,0,0,*)(1,0,0,*)(2,0,0,*)(3,0,0,*)(0,0,0,*)(0,0,1,*)(1,0,0,*)(1,0,1,*)56(0,0,0,*)(0,0,1,*)(1,0,0,*)(1,0,1,*)(2,0,0,*)(0,0,0,*)(0,0,1,*)(1,0,0,*)(1,0,1,*)(2,0,0,*)(0,0,0,*)(0,0,1,*)(1,0,0,*)(1,0,1,*)(2,0,0,*)(0,0,0,*)(1,0,0,*)(2,0,0,*)(3,0,0,*)(4,0,0,*)(0,0,0,*)(1,0,0,*)(2,0,0,*)(3,0,0,*)(4,0,0,*)(0,0,0,*)(1,0,0,*)(2,0,0,*)(3,0,0,*)(4,0,0,*)(0,0,0,*)(0,0,1,*)(1,0,0,*)(1,0,1,*)(2,0,0,*)57(0,0,0,*)(0,0,1,*)(1,0,0,*)(1,0,1,*)(2,0,0,*)(2,0,1,*)(0,0,0,*)(0,0,1,*)(1,0,0,*)(1,0,1,*)(2,0,0,*)(2,0,1,*)(0,0,0,*)(0,0,1,*)(1,0,0,*)(1,0,1,*)(2,0,0,*)(2,0,1,*)(0,0,0,*)(1,0,0,*)(2,0,0,*)(3,0,0,*)(4,0,0,*)(5,0,0,*)(0,0,0,*)(1,0,0,*)(2,0,0,*)(3,0,0,*)(4,0,0,*)(5,0,0,*)(0,0,0,*)(1,0,0,*)(2,0,0,*)(3,0,0,*)(4,0,0,*)(5,0,0,*)(0,0,0,*)(0,0,1,*)(1,0,0,*)(1,0,1,*)(2,0,0,*)(2,0,1,*)58N/AN/AN/AN/AN/AN/AN/A59N/AN/AN/AN/AN/AN/AN/A60N/AN/AN/AN/AN/AN/AN/A61N/AN/AN/AN/AN/AN/AN/A62N/AN/AN/AN/AN/AN/AN/A63N/AN/AN/AN/AN/AN/AN/A表格中的含义如下：在prach-FrequencyOffset的基础上指示同一时刻内频分的各个PRACH信道的频率位置；：指示PRACH信道的无线帧位置，0为全部无线帧，1为奇数无线帧，2为偶数无线帧；：指示PRACH信道在无线帧的前半帧或后半帧，0为前半帧，1为后半帧；：指示PRACH信道在“5ms半帧”内的上子帧序号，带*表示在UpPTS上。2.1.1.2 前导码格式与小区半径的关系 随机接入信号是由CP（长度为TCP）、前导序列（长度为TSEQ）和GT (长度为)三个部分组成，前导序列与PRACH时隙长度的差为GT，用于对抗多径干扰的保护，以抵消传播时延。一般来说较长的序列，能获得较好的覆盖范围，但较好的覆盖范围需要较长的CP和GT来抵消相应的往返时延，即小区覆盖范围越大，传输时延越长，需要的GT越大，为适应不同的覆盖要求，36.211协议规定了五种格式的PRACH 循环前缀长度、序列长度、以及GT长度如下表3。Preamble格式和小区覆盖范围的关系约束原则为：小区内边缘用户的传输时延需要在GT内部，才能保证PRACH能正常接收，且不干扰其他的子帧。即需要满足的关系为 ，其中，TTCP 为循环前缀CP的长度； TGT为保护间隔；TRTT为最大往返时间。根据以上关系，可以得到各种格式下所支持小区的最大半径(考虑)如表3：表3前导格式CP长度(Ts/)GT长度()03168/103.132976/96.886.2514.53121024/684.3815840/515.6316.6777.3426240/203.136048/196.886.2529.53321024/684.3821984/715.6316.67100.164448/14.583288/9.37551.406具体可以叙述为：Preamble 格式 0：持续1ms，序列长度800us，适用于小、中型的小区，最大小区半径14.53km，此格式看满足网络覆盖的多数场景。Preamble 格式 1：持续2ms，序列长度800us，适用于大型的小区，最大小区半径为77.34km。Preamble 格式 2：持续2ms，序列长度1600us，适用于中型小区，最大小区半径为29.53km。Preamble 格式 3：持续3ms，序列长度1600us，适用于超大型小区，最大小区半径为100.16km；一般用于海面、孤岛等需要超长距离覆盖的场景。Preamble 格式 4： TDD模式专用的格式，持续时间157.292s（ 2个OFDM符号的突发），适用于小型小区，小区半径1.4km，一般应用于短距离覆盖，特别是密集市区、室内覆盖或热点补充覆盖等场景。它是对半径较小的小区的一种优化，可以在不占用正常时隙资源的情况下，利用很小的资源承载PRACH信道，有助于提高系统上行吞吐量，某种程度上也可以认为有助于提高上行业务信道的覆盖性能。2.1.1.3 RACH容量选择这里用一个简单的模型来估计有限的PRACH资源上的竞争随机接入用户的承载数量。设定在某时间间隔中需要进行随机接入用户数为（用户数足够大，即用户间），随机接入的资源数为（随机接入的资源数由PRACH的密度决定。m表示每10ms内的preambles码数preambles），用户等概率地选择这些资源中的一个，任一用户A的碰撞概率为。用户发生碰撞后，重新进行随机接入时，在这个简单模型中记为一个新用户的接入，则任一用户A选定资源集（共个资源）中某一资源时，其它用户不和该用户发生碰撞，即其它用户都选择其他个资源，其概率约为。即用户A不和其他用户发生碰撞的概率为：时间间隔T内，随机接入的用户数N表示为：从上式可以看出，一定PRACH密度情况下，目标碰撞概率对所支持的随机接入的用户数需求起决定作用。设定用户可以接受的碰撞概率=1%（在LTE中，检测到碰撞后就可以使用回退机制），一个PRACH资源（一个1.08MHz带宽的时频资源）中的64Preambles均用于竞争随机接入，则一个PRACH资源可以接入的用户数个。如果一个无线帧（10ms）内有两个PRACH资源（即密度为2），则每秒钟可以接入的用户数为个。这就是LTE中期望的典型PRACH负载能力。下面两幅图是3GPP相关提案中给出的不同RACH负载下的碰撞概率曲线，其中第二幅图是对第一幅图在碰撞概率低于1%时的缩放。途中横坐标表示1s中内发起RACH的总次数（竞争式），纵坐标表示碰撞概率，64signatures表示10ms周期内共有64个preamble可用，128signatures表示共有128个preamble可用。从第一幅图可以看出如果目标碰撞概率设为低于1%，则每10ms128个preamble可以支持200次/s的竞争式随机接入。进一步考虑将随机接入区分为竞争式的和非竞争式两种情况，为非竞争式随机接入预留preamble。提案R2-070205中给出在假设的话务模型下，小区竞争式随即接入负载和非竞争式随机接入负载随小区覆盖范围内UE数变化而变化的情况，如下图所示。 10002000300040005000600070008000900010000aRACH load12.224.436.748.961.173.385.697.8110.0122.2load for dedicated signatures9.719.429.238.948.658.368.177.887.597.2虽然预留会导致竞争式的preamble个数的减少，但是由于可以通过分配的方式避免碰撞，preamble的使用效率会得到提升。以7000个UE时非竞争随机接入的负载是68.1 access/second为例，这个负载由以下三部分构成：- Call establishment (RT): 1.9- Handover (RT): 8.8 - Handover (NRT): 58.3假设为了切换时能够采用一个异步的方法，一个相同的preamble应该在后续连续5个时刻上被保留，而下行资源分配（下行数据到达）只是需要在1个随机接入时刻上1个专用preamble即可。因此可以采用一个因子5修正切换时的非竞争随机接入负载，从而得到总的非竞争式随机接入负载为： access/second，或者3.37/occasion（假设10ms inter-occasion period）。进一步假设：· 平均需要分配3.37个专用preamble· 每个随机接入时刻的preamble需求到达满足Poisson分布 · 能够接受的专用preamble消耗完的概率是0.5%满足1- P0 P1- Px < 0.5%的x=9，因此预留9个非竞争式preable就可以满足上述7000个UE时的非竞争式的随机接入负载需求。可以看出非竞争式随机接入的preamble利用率大大提高了。补充 根据以上分析，不考虑当小区覆盖范围内的用户数小于7000时，PRACH密度配置为2，在一般情况下式可以满足需求的。如果用户数小于3500则可以考虑将PRACH密度配置为1。2.1.1.4 相邻小区RACH时域、频域的分配原则相邻小区间的PRACH信道的时域或频域位置尽可能错开，因前导格式4是在UpPTS时隙上，且不支持配置频率偏移，多个小区之间时域上、频域上可以选择的不同的时、频域位置较少，建议小半径一般采用Format 0 格式的PRACH。1. 频域相同，时域不同此种情况，“PRACH配置索引（prach-ConfigurationIndex）”参数需配置不同，相邻小区在RACH密度选择相同的情况下，通过三种方式将PRACH的时域配置不同：² 将PRACH配置在不同无线帧上，此情况只适用于RACH密度为0.5。² 将PRACH配置在不同的前后半帧上。² 将PRACH配置在不同的上行子帧序号上，此情况只适用于前导格式03。2. 时域相同，频域不同此种情况，相邻小区的“PRACH配置索引（prach-ConfigurationIndex）”参数可以配置相同， 通过参数“频率偏移（prach-FrequencyOffset）”配置不同，保证给小区的PRACH信道频域位置不同。此方法只适用于前导格式03，前导格式4时，不需要配置“频率偏移（prach-FrequencyOffset）”参数。2.1.2 零相关配置（zeroCorrelationZoneConfig）2.1.2.1 参数基本信息参数编号取值范围物理单位调整步长0.15默认值：4无无参数名称传送途径作用范围参数出处zeroCorrelationZoneConfigeNodeB->UECell36.211所属网元及设置途径小区逻辑无线资源参数->物理随机接入信道->零相关配置不同场景下的差异化配置说明无2）功能描述 该参数指示PRACH前导序列生成使用的循环移位配置的索引值，如下表3（36.211 Table 5.7.2-2:）、表4（Table 5.7.2-3），对于前导格式0-3，本参数的取值范围为0-15，对于前导格式4，本参数的取值范围为0-6， “unrestricted set”或“restricted set”参数“是否为高速状态”由2.1.4节的“是否为高速状态（highSpeedFlag）”指示。表3 for preamble generation (preamble formats 0-3).zeroCorrelationZoneConfig valueUnrestricted setRestricted set00151131821522318264223252638632467385584668959821076100119312812119158131672021427923715419-表4: for preamble generation (preamble format 4).zeroCorrelationZoneConfig value021426384105126157N/A8N/A9N/A10N/A11N/A12N/A13N/A14N/A15N/A2.1.2.2 Ncs与小区半径的关系Ncs与小区半径相关，下面是Ncs和小区半径的关系参见如下公式： （公式1）其中，对于前导格式0-3，对于前导格式4，；对于前导格式0-3，对于前导格式4，；为最大多径时延扩展，是小区边缘UE对抗多径干扰的保护；为光速。原则上，Ncs越大，小区半径越大，以下是根据公式1计算获得的前导格式0-3 、前导格式4，Ncs数值及其对应的最大小区半径(假设)关系表。表5 前导格式03 时Ncs值与支持的最大小区半径zeroCorrelationZoneConfigUnrestricted setRestricted set小区半径小区半径00 119.1km15 1.4km113 1.0 km18 1.7 km
