上行PRACH同步过程.docx
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上行PRACH同步过程.docx
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上行PRACH同步过程
上行RACH信道知识
上行RACH信道作用
上行PRACH信道(PhysicalRandomAccesschannel)上行随机接入信道,用于UE与网格侧的目标小区进行初始通信的第一条消息内容,其主要作用就是获取目标小区的上行同步.
上行RACH信道资源preamble原理
手机要取得与UE的上行同步,手机就要在PRACH信道上进行上行同步,然而上行同步是手机通过发送上行同步码来获得的.每个小区都有64个上行同步码,在相同的频率情况下,如果两个小区的上行同步码相同,就会产生严重的上行同步干扰,就无法成功获得上行同步.
那Preamble码是由什么产生的呢,协议定义了两种ZC序列(长度分别为839bit和139bit),前面讲PSS的ZC序列的特性时,ZC序列具有很好的自相关性和互相关性.任意两条ZC序列都是相互正交无干扰的.
这两种ZC序列的长度为839bit时,其preambleformat(Preamble格式)为0~3,preambleformat与小区覆盖半径有关.
Preambleformat(table5.7.2-1)
指preamble的长度
0–3
839,序列个数=长度-1=838个
4
139,序列个数=长度-1=138个
Preambleformat
TCP(us)
Tpreamble-sequence(us)
TGT(us)
总时长
小区覆盖距离(km)
0
103
800
100
1ms
14
1
684
800
520
2ms
77
2
203
1600
200
2ms
29
3
684
1600
720
3ms
100
4
15
133
9
157us
1.4
Preambleformat的格式与小区半径的关系如下:
以format0以例:
小区半径=min(GT,CP}*光速/2=min(103us,100us)*光速/2=100*10^(-6)*3*10^8/2=150000=15km
参数ra-PreambleIndex/root-SequenceIndex:
preamble的rootsequenceindex索引编号,当PRACH格式为0~3时,根序列为0~837,其838个,分为32个大的逻辑组,每个逻辑索引唯一对应一个物理序列;当PRACH格式为4时,根序列为0~137,其138个,分为7个逻辑组,每个逻辑索引也是唯一对应一个物理序列;由小区配置可用的preamblesequence-index编号,在SIB中广播,一般小区每隔8使用一个根序列(rootsequence重新排序得到逻辑号,考虑preamble的峰均比),排序规律:
每两个每对相加=839.
Preamble逻辑根序列号索引
Preamble物理根序列号索引
(按逻辑根序列号升序排列)
Format0~3(36.211table5.7.2-4)
0–23
129,710,140,699,120,719,210,629,168,671,84,755,105,734,93,746,70,769,60,779,
2,837,1,838
24–29
56,783,112,727,148,691
30–35
80,759,42,797,40,799
36–41
35,804,73,766,146,693
42–51
31,808,28,811,30,809,27,812,29,810
52–63
24,815,48,791,68,771,74,765,178,661,136,703
64–75
86,753,78,761,43,796,39,800,20,819,21,818
76–89
95,744,202,637,190,649,181,658,137,702,125,714,151,688
90–115
217,622,128,711,142,697,122,717,203,636,118,721,110,729,89,750,103,736,61,778,55,784,15,824,14,825
116–135
12,827,23,816,34,805,37,802,46,793,207,632,179,660,145,694,130,709,223,616
136–167
228,611,227,612,132,707,133,706,143,696,135,704,161,678,201,638,173,666,106,733,83,756,91,748,66,773,53,786,10,829,9,830
168–203
7,832,8,831,16,823,47,792,64,775,57,782,104,735,101,738,108,731,208,631,184,655,197,642,191,648,121,718,141,698,149,690,216,623,218,621
204–263
152,687,144,695,134,705,138,701,199,640,162,677,176,663,119,720,158,681,164,675,174,665,171,668,170,669,87,752,169,670,88,751,107,732,81,758,82,757,100,739,98,741,71,768,59,780,65,774,50,789,49,790,26,813,17,822,13,826,6,833
264–327
5,834,33,806,51,788,75,764,99,740,96,743,97,742,166,673,172,667,175,664,187,652,163,676,185,654,200,639,114,725,189,650,115,724,194,645,195,644,192,647,182,657,157,682,156,683,211,628,154,685,123,716,139,700,212,627,153,686,213,626,215,624,150,689
328–383
225,614,224,615,221,618,220,619,127,712,147,692,124,715,193,646,205,634,206,633,116,723,160,679,186,653,167,672,79,760,85,754,77,762,92,747,58,781,62,777,69,770,54,785,36,803,32,807,25,814,18,821,11,828,4,835
384–455
3,836,19,820,22,817,41,798,38,801,44,795,52,787,45,794,63,776,67,772,72
767,76,763,94,745,102,737,90,749,109,730,165,674,111,728,209,630,204,635,117,722,188,651,159,680,198,641,113,726,183,656,180,659,177,662,196,643,155,684,214,625,126,713,131,708,219,620,222,617,226,613
456–513
230,609,232,607,262,577,252,587,418,421,416,423,413,426,411,428,376,463,395,444,283,556,285,554,379,460,390,449,363,476,384,455,388,451,386,453,361,478,387,452,360,479,310,529,354,485,328,511,315,524,337,502,349,490,335,504,324,515
514–561
323,516,320,519,334,505,359,480,295,544,385,454,292,547,291,548,381,458,399,440,380,459,397,442,369,470,377,462,410,429,407,432,281,558,414,425,247,592,277,562,271,568,272,567,264,575,259,580
562–629
237,602,239,600,244,595,243,596,275,564,278,561,250,589,246,593,417,422,248,591,394,445,393,446,370,469,365,474,300,539,299,540,364,475,362,477,298,541,312,527,313,526,314,525,353,486,352,487,343,496,327,512,350,489,326,513,319,520,332,507,333,506,348,491,347,492,322,517
630–659
330,509,338,501,341,498,340,499,342,497,301,538,366,473,401,438,371,468,408,431,375,464,249,590,269,570,238,601,234,605
660–707
257,582,273,566,255,584,254,585,245,594,251,588,412,427,372,467,282,557,403,436,396,443,392,447,391,448,382,457,389,450,294,545,297,542,311,528,344,495,345,494,318,521,331,508,325,514,321,518
708–729
346,493,339,500,351,488,306,533,289,550,400,439,378,461,374,465,415,424,270,569,241,598
730–751
231,608,260,579,268,571,276,563,409,430,398,441,290,549,304,535,308,531,358,481,316,523
752–765
293,546,288,551,284,555,368,471,253,586,256,583,263,576
766–777
242,597,274,565,402,437,383,456,357,482,329,510
778–789
317,522,307,532,286,553,287,552,266,573,261,578
790–795
236,603,303,536,356,483
796–803
355,484,405,434,404,435,406,433
804–809
235,604,267,572,302,537
810–815
309,530,265,574,233,606
816–819
367,472,296,543
820–837
336,503,305,534,373,466,280,559,279,560,419,420,240,599,258,581,229,610
Preamble逻辑根序列号索引
Preamble物理根序列号索引
(按逻辑根序列号升序排列)Format4(36.211table5.7.2-5)
0–19
1
138
2
137
3
136
4
135
5
134
6
133
7
132
8
131
9
130
10
129
20–39
11
128
12
127
13
126
14
125
15
124
16
123
17
122
18
121
19
120
20
119
40–59
21
118
22
117
23
116
24
115
25
114
26
113
27
112
28
111
29
110
30
109
60–79
31
108
32
107
33
106
34
105
35
104
36
103
37
102
38
101
39
100
40
99
80–99
41
98
42
97
43
96
44
95
45
94
46
93
47
92
48
91
49
90
50
89
100–119
51
88
52
87
53
86
54
85
55
84
56
83
57
82
58
81
59
80
60
79
120–137
61
78
62
77
63
76
64
75
65
74
66
73
67
72
68
71
69
70
-
-
PRACH的根序列产生公式:
是根ZC序列的长度,u为第u个根序列编号
,然后通过
循环移位
这两种ZC序列,长度分别为839bit和139bit.这两种序列协议把它称为根序列RootSequence,以839bit的序列为例,这种序列的个数=长度-1=838个.这838个序列中任意两个根序列都是相互正交和无干扰的.拿其中任何一个序列,由于序列上有839个点,选择任意一个点作为起点逆时针转一圈,都能得到一条序列,这样一条序列就叫一个Preamble,那总共838个根序列,每个根序列上有839个点,故每条根序列能产生839条Preamble,则总共的Preamble总数=838*839=703082条Preamble.任意两条Preamble都是相互正交和无干扰的.
参数
:
也是与小区覆盖半径决定的,比如小区覆盖半径为14公里时,Ncs索引取值一般可以是11(12.52KM)或10(10.09KM),它是Preamble的Cyclicshift,目前只支持非限制集,限制集只适用于高速场景。
若取值为11,则表示在此圆上每隔93个点取一个,每两个循环移位间隔为93,由于长度为839,因此这条根序列能产生=int(839/9)=9,即一条根序列可产生9个preamble,因此需要ubound(64/9)=8个根序列,因此规划时每小区的root-sequence差8,
=11。
的大小决定小区半径,例取11,则小区半径={93*preamble时长/preamble长度/2}=800us*间隔93*C光速/839/2=13.3km,其中800us*间隔93就是指间隔的时延。
NCS配置(preambleformat4见36.211table5.7.2-3)
NCS值
0
2
1
4
2
6
3
8
4
10
5
12
6
15
上行RACH信道资源preamble种类个数及划分细则
每个小区都有64(编号为0~63)个Preamble码,协议把它划分为2组,用于UE与网络侧的随机接入同步过程.一组是网络侧eNB指定分配给UE的preamble码,这种称为专用或非竞争的,这个码其它用户不能使用;另一组是UE自已随机选的,大家都可以用,这种称为竞争性的随机接入码.有参数指定随机接入码的个数numberOfRA-Preambles,此参数在SIB2中下发.如果此参数为48个(编号为0~47),则剩下的就是专用的Preamble码(48~63).同时协议又将竞争的分为GroupA和GroupB两组.具体如下图:
如果是竞争性的Preamble,那到底选用GroupA还是GroupB呢?
在此需要提醒一下,假如eNB在随机接入用户同时很多的时候,eNB对GroupB比GroupA的优先级要高点.因此eNB选择GroupB时,需满足两个条件:
1).UE的信号要好,这样才能保证优先处理时的可靠性更高(链路损耗要小些)
pathloss pathlossThreshold=Pmax-preambleInitialReceivedTargetPower–deltaPreMsg3–messagePowerOffsetGroupB 2).手机中有较大的上行数据块要发送 (preamble中需要承载的消息量+MACheader大小)>messageSizeGroupA,有些厂家将此参数写为(raSmallVolUl) 上行RACH信道资源preamble频域位置 前面讲过,PRACH资源占频域的上行6个PRB,fomat0~3格式的Preamble的长度为839bit,每个子载波的带宽为1.25KHZ,则占用频域带宽为=7.5KHZ*839=1.048M,占用连续6个PRB(1.08M)的带宽;长度为139bit的Preamble的子载波的带宽为7.5JHZ,则同样占用7.5KHZ*139=1.0425M(连续6个PRB)的带宽. 在20MHZ的带宽下,上行总共有100个PRB,那PRACH到底在哪几个PRB呢.这与PRACHConfigIndex和上下行子帧配置有关.参数PRACHConfig是指PRACH的format格式,其中包含了一个无线帧内PRACH信道的个数(称为PRACH密度Density,1代表每无线帧出现一次,0.5代表第2个无线帧出现一次,其它按此类推). PRACH configuration Index Preamble Format Density (Per10msDRA) Version VRA PRACH configuration Index Preamble Format Density (Per10ms DRA) Version VRA 0 0 0.5 0 32 2 0.5 2 1 0 0.5 1 33 2 1 0 2 0 0.5 2 34 2 1 1 3(目前配置) 0 1(preamble时长1ms) 0 35 2 2 0 4 0 1 1 36 2 3 0 5 0 1 2 37 2 4 0 6 0 2 0 38 2 5 0 7 0 2 1 39 2 6 0 8 0 2 2 40 3 0.5 0 9 0 3 0 41 3 0.5 1 10 0 3 1 42 3 0.5 2 11 0 3 2 43 3 1 0 12 0 4 0 44 3 1 1 13 0 4 1 45 3 2 0 14 0 4 2 46 3 3 0 15 0 5 0 47 3 4 0 16 0 5 1 48 4 0.5 0 17 0 5 2 49 4 0.5 1 18 0 6 0 50 4 0.5 2 19 0 6 1 51 4 1 0 20 1 0.5 0 52 4 1 1 21 1 0.5 1 53 4 2 0 22 1 0.5 2 54 4 3 0 23 1 1 0 55 4 4 0 24 1 1 1 56 4 5 0 25 1 2 0 57 4 6 0 26 1 3 0 58 N/A N/A N/A 27 1 4 0 59 N/A N/A N/A 28 1 5 0 60 N/A N/A N/A 29 1 6 0 61 N/A N/A N/A 30 2 0.5 0 62 N/A N/A N/A 31 2 0.5 1 63 N/A N/A N/A PRACHconfigurationIndex(SeeTable5.7.1-3) TDDPRACH的时间的频域位置UL/DLconfiguration(SeeTable4.2-2) 0 1 2 3 4 5 6 0 (0,1,0,2) (0,1,0,1) (0,1,0,0) (0,1,0,2) (0,1,0,1) (0,1,0,0) (0,1,0,2) 1 (0,2,0,2) (0,2,0,1) (0,2,0,0) (0,2,0,2) (0,2,0,1) (0,2,0,0) (0,2,0,2) 2 (0,1,1,2) (0,1,1,1) (0,1,1,0) (0,1,0,1) (0,1,0,0) N/A (0,1,1,1) 3 (0,0,0,2) (0,0,0,1) (0,0,0,0) (0,0,0,2) (0,0,0,1) (0,0,0,0) (0,0,0,2) 4 (0,0,1,2) (0,0,1,1) (0,0,1,0) (0,0,0,1) (0,0,0,0) N/A (0,0,1,1) 5 (0,0,0,1) (0,0,0,0) N/A (0,0,0,0) N/A
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