Monday, 7 December 2015

LTE UE Power on Synchronize Development Study

Mainly summary experience of this document is about UE power on, UE uses PSS/SSS to have a synchronization with the system and give a development introduction to LTE synchronize principle.
 Now In LTE system, UE will search the available cell first once it is power on. Then UE will do cell synchronize, CP identity, and system model (FDD/TDD) identity according to PSS and SSS. And UE will know more cell information through demodulating PBCH. At last we may want to know how UE uses PSS/SSS to do a cell synchronize. How will UE know the cell ID and blind detection CP? In this document will mainly talk about these several questions.


Before we want to know synchronize theory, we acquaint the frame structure from time domain first. The frame structure is described as below in the protocol 36.211:
Every frame time length of FDD LTE (type 1) is 10ms. The frame is divided into 10 sub frames. Every sub frame has two time slot, so one frame has 20 time slots total. The number is from #0 to #19.
Every slot contained in some OFDM symbols. Every OFDM is separated by using Cyclic Prefix (CP) in order to improve the system to avoid time delay. According to different system coverage requirement we can use normal CP4.6875usand extend CP16.67us to add separately. Extend CP is used to support LTE big area cell coverage and multiple cell broadcast services. When we use extend CP, every slot only contains in 6 OFDM symbols and the normal CP contains in 7 OFDM symbols, the figure is shown as below.
FDD LTE frequency domain structure
After we general understanding FDD LTE frame structure, then we see the structure of FDD LTE frequency domain again. FDD LTE uses OFDM multiple access, which divide some sub carriers from frequency domain. Every sub carrier is 15 kHz. Frequency domain location 12 sub carriers and time domain one time slot (7 OFDM symbol) resource make up of one resource block (RB). The figure is shown as below.
PSS and SSS position in time and frequency domain
The protocol defines in the time domain, P-SCH is located in #0 and #10 slots of the last OFDM symbol(Yellow block),S-SCH is located in #0 and #10 slots of the second to last OFDM symbol(purple block). P-SCH is sent every 5ms. The content is the same in one frame. S-SCH is sent every 5ms, too. The content is different in one frame.
In frequency domain, however the bandwidth of the system, P-SCH and S-SCH are located in the total 1.08M bandwidth over the head and tail frequency. The bandwidth contains 6 RB and 72 sub carriers. In fact the synchronize channel just uses 62 sub carriers around DC, both sides leave 5 sub carrier to make a protection. The central frequency that contained sub carrier is named DC. Because DC will have serious interference when do modulation, that can cause SINR is very low. The result serious affects the data transmission efficiency. This means the power will be very low when DC sub carrier is deal with by the system and it will not take any available data.
P-SCH and S-SCH synchronize signal occur progress
LTE has 504 physical-layer cell identities. All the IDs are divided into 168 groups and every group has 3 ID. So the PCI formula is defined as. range is 0167,that means logo group. range is 0~2,that means three different sub ID.
The protocol 36.211 introduces PSS and SSS as:
The sequence used for the primary synchronization signal is generated from a frequency-domain Zadoff-Chu sequence according to
The relations between u andare shown as below.
Table 4-1  Root indices for the primary synchronization signal

Root index
0
25
1
29
2
34
Because of the good autocorrelation and low cross correlation of ZC sequence, UE just need 3 sequences to do calculation with PSS. The sequence root u that appears peak value can get the result of.
The SSS is made up of 31 bits length binary sequences interwoven cascade. Because it uses 2 sequences, it can bring two different 62 bit sequences, which can be used to separate 0# and 10#. Below figure shows the SSS modulation progress. According to the calculate progress, we can get,, then.
According to the description of protocol, we can knowrelations are shown as below.
Table 4-2  PCI group logo,,relations















0
0
1
34
4
6
68
9
12
102
15
19
136
22
27
1
1
2
35
5
7
69
10
13
103
16
20
137
23
28
2
2
3
36
6
8
70
11
14
104
17
21
138
24
29
3
3
4
37
7
9
71
12
15
105
18
22
139
25
30
4
4
5
38
8
10
72
13
16
106
19
23
140
0
6
5
5
6
39
9
11
73
14
17
107
20
24
141
1
7
6
6
7
40
10
12
74
15
18
108
21
25
142
2
8
7
7
8
41
11
13
75
16
19
109
22
26
143
3
9
8
8
9
42
12
14
76
17
20
110
23
27
144
4
10
9
9
10
43
13
15
77
18
21
111
24
28
145
5
11
10
10
11
44
14
16
78
19
22
112
25
29
146
6
12
11
11
12
45
15
17
79
20
23
113
26
30
147
7
13
12
12
13
46
16
18
80
21
24
114
0
5
148
8
14
13
13
14
47
17
19
81
22
25
115
1
6
149
9
15
14
14
15
48
18
20
82
23
26
116
2
7
150
10
16
15
15
16
49
19
21
83
24
27
117
3
8
151
11
17
16
16
17
50
20
22
84
25
28
118
4
9
152
12
18
17
17
18
51
21
23
85
26
29
119
5
10
153
13
19
18
18
19
52
22
24
86
27
30
120
6
11
154
14
20
19
19
20
53
23
25
87
0
4
121
7
12
155
15
21
20
20
21
54
24
26
88
1
5
122
8
13
156
16
22
21
21
22
55
25
27
89
2
6
123
9
14
157
17
23
22
22
23
56
26
28
90
3
7
124
10
15
158
18
24
23
23
24
57
27
29
91
4
8
125
11
16
159
19
25
24
24
25
58
28
30
92
5
9
126
12
17
160
20
26
25
25
26
59
0
3
93
6
10
127
13
18
161
21
27
26
26
27
60
1
4
94
7
11
128
14
19
162
22
28
27
27
28
61
2
5
95
8
12
129
15
20
163
23
29
28
28
29
62
3
6
96
9
13
130
16
21
164
24
30
29
29
30
63
4
7
97
10
14
131
17
22
165
0
7
30
0
2
64
5
8
98
11
15
132
18
23
166
1
8
31
1
3
65
6
9
99
12
16
133
19
24
167
2
9
32
2
4
66
7
10
100
13
17
134
20
25
-
-
-
33
3
5
67
8
11
101
14
18
135
21
26
-
-
-
 Solution
Through the knowledge of PSS/SSS time domain location, frequency location and the PSS/SSS occur way and modulation way we study above, we can know the cell search flow after UE power on easily.
Before UE access to the network, it should know clearly about such information: current network type, CP type (length), central frequency, time clock synchronize, current cell location. When UE power on, it can receive data from LTE several cells that may exist the central frequency and then calculate the RSSI bandwidth. According to the signal power to judge the frequency whether has cell that probably be used nearby. If UE can save the frequency and operator information before it was power off. It can attempt to stay in the original cell, or it will do whole network frequency service search.
Then UE attempts to receive PSS around the central frequency. According to the description of chapter 4.1, UE will use 3 type known sequences to correlate with PSS signal. Then it will know the system PSS information:
1)   Central frequency
2)   Cell group logo PCI012
3)   Sub frame synchronize information
Because P-SCH is located in #0 and #10 slots last OFDM symbol, however length of CP, we can confirm slot edge once we confirm PSS. But the contents of PSS are same in slot0 and slot 10. We can’t separate the two slots and can’t obtain the system frame information. That mean we don’t know it is the start of the frame or the middle of the frame.
Bcathe transmission model is different for FDD LTE or TDD LTE, the time slots of PSS and SSS are different. Extend CP and normal CP length will affect the SSS location, too.  (Assume PSS is known). So UE just need to do Max 4 times blind check, then it will know the SSS location. When the blind check for SSS is success, the length of CP and network type is confirmed, too. (Because we confirm the SSS and PSS time interval).
Aftafter we get SSS, according to the description of chapter 4.1, we can calculate by using modulated. According to the formula of chapter 4.1, we can calculate current cell PCI ID.  As the contents of two sub frames in one frame are different, then we can confirm that whether it is forward half frame or backward half frame to finish the frame synchronize. At this time, the UE check cell progress has been finished since UE power on to synchronize.
Cell broadcast signal modulate
Chapter 3.3 refers that PBCH channel is the same with synchronize signal that is broadcast in the DC 1.08MHz bandwidth. It is located in the time domain #1slot the forward 4 OFDM symbol. So when UE synchronize, it can read PBCH. PBCH information includes in several contents:
1)       System bandwidth information
2)       PHICH configuration
3)       System frame number
4)       System antenna configuration information
PBCH just take very limited information. More system information is taken by SIB. SIB information is delivered by PDSCH. UE need to read PDCCH control information, then it can modulate PDSCH data correctly. In order to read PDCCH, we must know the symbol number that PDCCH used in sub frame. It’s decided by PCFICH. If UE can know the whole network condition, it has a long way to walk.
 Summary and Notes
ynchronize signal is the door of UE access to the network. It plays a very important role in network structure. After we know the progress of UE synchronize, we can also know LTE frame structure, time domain structure, frequency domain structure, UE power on progress, even LTE’s whole physical channel. All of these help us to obtain LTE knowledge and skills.

Budi Prasetyo

About Budi Prasetyo

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