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eRAN3.0 Extended Cell Range Solution
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6.1
Date
2012-07-15
HUAWEI TECHNOLOGIES CO., LTD.
Copyright © Huawei Technologies Co., Ltd. 2012. All rights reserved. No part of this document may be reproduced or transmitted in any form or by any means without prior written consent of Huawei Technologies Co., Ltd.
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About This Document
About This Document Author Prepared by
Zhang Han (employee ID: 00198370)
Date
Reviewed by
Date
Approved by
Date
2012-05-07
Change History Date
Issue
Description
Author
2012-05-19
1.00
Completed the draft.
Zhang Han (employee ID: 00198370)
2012-05-28
2.0
Modified the draft based on the review comments.
Zhang Han (employee ID: 00198370)
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eRAN3.0 Extended Cell Range Solution
Contents
Contents About This Document........ Document.............................. ............................................ ............................................ ........................................................... ..........................................ii .....ii 1 Overview....... Overview............................. ............................................ ............................................ ............................................ ............................................. .......................................... ...................1 1 1.1 Preface...............................................................................................................................................................1 1.2 Abbreviations and Acronyms............................................................................................................................. Acronyms............................................................................................................................. 1
2 Performanc Performance e Analysis........... Analysis................................. ............................................ ............................................ ............................................ ......................................2 ................2 2.1 Limitation of the GP in a Special Subframe on the Coverage...........................................................................2 2.2 Impact of the PRACH Preamble Format on the Coverage................................................................................5 2.3 Factors Affecting the Extended Cell Range and Enhanced Features................................................................. Features.................................................................7 7 2.3.1 Antenna Height and Selection..................................................................................................................7 2.3.2 Receive Diversity.....................................................................................................................................9 2.3.3 TTI Bundling..........................................................................................................................................10 2.4 Summary..........................................................................................................................................................10
3 Solutions............ Solutions.................................. ............................................ ............................................ ............................................ ............................................... ................................... ..........11 11 3.1 Application Scenarios Where Extended Cell Range Is Applicable..................................................................11 3.2 Configurations for Extended Cell Range on the eNodeB................................................................................16 3.3 Case Analysis...................................................................................................... Analysis................................................................................................................................................... ............................................. 17
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2 Overview
1
Overview
1.1 Preface Long Term Evolution (LTE) products are mainly used for wireless access to metropolitan area networks (MANs) and are also used to cover wide areas, for example, the sea or desert. As a country with a large sea area, Greece demands the LTE extended cell range feature. This feature wide by someenables operator operators. s. coverage with limited investment of operators, and therefore, is favored This document is based on the eRAN3.0 LTE LTE product features. It provides references for other scenarios of LTE LTE extended cell coverage and guidance for the specifications of extended cell coverage. The advantages of the LTE extended cell range feature shown in tests can be emphasized in commercial promotion.
1.2 Abbreviations and Acronyms Abbreviations and Acronyms
Full Name
LTE
Long Team Revolution
TDD
Time Division Duplex
RTT
Round trip time
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Performance Analysis
Extended cell coverage has the following limitations:
The maximum transmission delay of radio signals is limited by the guard period (GP). Therefore, the extended cell coverage range is limited. An extended cell coverage range demands a specific GP of a special subframe.
The format of the preamble on the physical random access channel (PRACH) limits the transmission delay, and therefore limits the coverage radius.
The link transmission loss also limits the extended cell coverage.
2.1 Limitation of the GP in a Special Subframe on the Coverage An LTE LTE frame is 10 ms long and consists of 10 subframes, and each subframe is 1 ms long. Subframes are used for uplink (UL) and downlink (DL) alternatively alternatively.. Protocols have defined six UL-DL configurations, and Huawei products support UL-DL configurations configurations 0, 1, 2, and 5. Each UL-DL configuration configuration applies in a certain scenario. As shown in Table 1.1 1. 1, a frame contains two special subframes when the UL-DL configuration configuration 0, 1, 2, or 6 is used, and contains only one special subframe when the UL-DL configuration configuration 3, 4, or 5 is used. Table 1.1 UL-DL configuration defined by protocols
UL-DL
Subframe Number
Configuration
DL-to-UL SwitchPoint Periodicity
0
1
2
3
4
5
6
7
8
9
0
5 ms
D
S
U
U
U
D
S
U
U
U
1
5 ms
D
S
U
U
D
D
S
U
U
D
2
5 ms
D
S
U
D
D
D
S
U
D
D
3
10 ms
D
S
U
U
U
D
D
D
D
D
4
10 ms
D
S
U
U
D
D
D
D
D
D
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UL-DL
2 Performance Analysis
Subframe Number
Configuration
DL-to-UL SwitchPoint Periodicity
0
1
2
3
4
5
6
7
8
9
5
10 ms
D
S
U
D
D
D
D
D
D
D
6
5 ms
D
S
U
U
U
D
S
U
U
D
D indicates that the subframe is reserved for DL transmissions, U indicates that the subframe is reserved for UL transmissions, and S indicates a special subframe. As shown in Figure 1.2, 1.2, a special subframe consists of three fields: downlink pilot timeslot (DwPTS), GP, and uplink pilot timeslot (UpPTS). Figure 1.2 LTE TDD frame structure (with a switch-point periodicity of 5 ms) One radio frame, T f f = 307200T s = 10 ms One half-frame, 153600T s = 5 ms
One slot, 30720T s
T slot slot=15360T s
Subframe #0
Subframe #2
Subframe #3
Subframe #4
Subframe #5
Subframe #7
Subframe #8
Subframe #9
One subframe, 30720T s DwPTS
GP
DwPTS
UpPTS
GP
UpPTS
Table 2.1 Configuration of a special subframe
UL-DL Configuration
Normal CP
Extended CP
DwPTS
GP
UpPTS
DwPTS
GP
UpPTS
0
3
10
1
3
8
1
1
9
4
1
8
3
1
2
10
3
1
9
2
1
3
11
2
1
10
1
1
4
12
1
1
3
7
2
5
3
9
2
8
2
2
6
9
3
2
9
1
2
7
10
2
2
8
11
1
2
The yellow fields in Table 2.1 indicate 2.1 indicate the special subframe configurations used by eRAN3.0 products. Issue 3.0 (2012-07-15)
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Based on the preceding analysis, the following conclusions can be drawn:
If the cell radius is shorter than 40 km, the GP must occupy one to four symbols.
If the cell radius ranges from 40 to 104 km, the GP must occupy nine to ten symbols.
The GP must be longer than the UL-DL round trip delay (T RTT) and contain a UL-DL switch delay (TUD). Figure 1.3 Delay during DL and UL transmission
As shown in Figure 1.3, 1.3, the user equipment (UE) receives DL data TRTT/2 after the DwPTS and transmits UL data TRTT/2 before the UpPTS. There is a UL-DL switching delay for the transmission of the UE. The length of a symbol equals one fourteenth of 1 ms, that is, 7.14e-5 seconds. According According to the speed of light, the round trip time (RTT) between an E-UTRAN
NodeB (eNodeB) (eNodeB) to a cell edge UE (CEU) is seconds, that is, 0.09333 0.09333 x Rcell (km) symbols. E-UTRAN is short for evolved universal terrestrial radio access network. The UL-DL switching switching delay of the UE is 20 s, that is, 0.28 symbol (2e-5/7.14e). The UE timing deviation (1.56 s at most) must be taken into the calculation of cell radius. Table 3. 3.1 1 describes the mapping between the number of symbols occupied by a GP and the maximum cell radius that is supported. Table 3.1 Mapping between the number of symbols occupied by a GP and the maximum cell radius
Number of Symbols Occupied by a GP
Maximum Cell Radius That Is Supported (km)
Maximum Cell Radius with the Timing Deviation Taken into Calculation (km)
1
7.71
7.48
2
18.42
18.19
3
29.14
28.91
4
39.85
39.62
7
72
71.77
8
82.71
82.48
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Number of Symbols Occupied by a GP
Maximum Cell Radius That Is Supported (km)
Maximum Cell Radius with the Timing Deviation Taken into Calculation (km)
9
93.42
93.19
10
104.14
103.91
NOTE
In Table 3.1 3.1,, the number of symbols occupied by a GP is calculated based on the assumption that the length of a symbol is 2192 Ts. If the length of both symbol 0 and symbol 7 is 2208 Ts according to protocols,, the special subframe cconfiguratio protocols onfigurations ns are as fo follows: llows: If a normal cyclic prefix (CP) is used, the special subframe configurations are 3:10:1 or 3:9:2. If an expanded CP is used, the special subframe configurations are 3:8:1 or 3:7:2. Symbol 7 (2208 Ts) is occupied when either a normal CP or an expanded CP is used, increasing the GP length. The fourth or later decimal places are not taken into consideration due to little impact on the GP length.
2.2 Impact of the PRACH Preamble Format on the Coverage Figure 1.4 Preamble format
As shown in Figure 1.4, 1.4, CP is the cyclic prefix, and GT is the guard duration. No message can be sent during the guard guard duration (GT), which which is used to mitigate the the interference of the PRACH on the subsequent UL frames. When a UE randomly accesses the network on the UL, only the DL synchronization is realized. Therefore, the data carried on the PRACH received by the eNodeB with with a delay due to the transmission delay. delay. If no GT is configured, configured, the preamble of this UE conflicts with the preamble preamble transmitted on the same resource resource block (RB) for another UE after the subsequent transmission time interval (TTI). Other UEs are synchronous with the eNodeB.
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Figure 1.5 Relationship between the preamble format and the transmission delay
If the maximum transmission delay is Tmax, the GT must equal twice the Tmax. Otherwise, the UL access signals interfere with the signals of other UEs. Based on the coverage radius requirements and the available PRACH resources, protocols specify five preamble formats, as shown in Table 5.1. 5.1. This table also lists the cell radius determined by the GT used as the loopback delay. delay. The cell radius is calculated using the following formula:
T represents TGT. Table 5.1 Mapping between the preamble formats and the maximum cell radius
Preamble Format
Duration
TCP (Ts)
TSEQ (Ts)
TGT (Ts)
TGT (us)
Radius (km)
Frame Configuration
0
1 ms
3168
24576
2976
96.875
14.53125
0, 1, 2, or 5
1
2 ms
21024
24576
15840
515.625
77.34375
0, 1
2
2 ms
6240
49152
6048
196.875
29.53125
0, 1
3
3 ms
21024
49152
21984
715.625
107.34375
0
4
157.3 us
448
4096
288
9.375
1.40625
0, 1, 2, or 5
The length of valid preambles for format 1 is the same as that for format 0. Therefore, the requirement of format 1 for physical layer resources is similar to that of format 0. Formats 2 and 3 require twice the physical layer resources required by format 0. If the random access channel (RACH) specifications (such as the number of preambles and the channel access period) for formats 2 and 3 are the same as that for format 0, specifications of other channels deteriorate. For example, specifications of the UL traffic channel and control channel decrease to 3/5 of the original specifications. Theref Therefore, ore, formats 2 and 3 are not recommended. recommende d. Since extended cell range applies to a wide area with a light traffic, the RACH specifications can be decreased to ensure that physical layer resources used for formats 2 and 3 are the same as those used for format 0. In this way, spec specifications ifications of other channels are not affected. Format 0 supports a cell radius of 14.5 km at most theoretically. theoretically. If a larger cell radius is required, format 0 is not applicable. For example, China Mobile requires a cell radius longer Issue 3.0 (2012-07-15)
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than 14.5 km for wide area coverage. To To meet this requirement, the eNodeB must support formats 1, 2, and 3. Preamble formats 1 and 2 specify 2 ms preambles, which are allowed only for UL-DL configurations configurations 0 and 1. Preamble format 3 specifies 3 ms preambles, which is allowed only for UL-DL configuration 0. NOTE
In eRAN3.0, preamble formats 1, 2, and 3 are optional features, which are available only after being enabled by a license.
2.3 Factors Affecting the Extended Cell Range and Enhanced Features In addition to the maximum transmission delay, path loss is also important for the expanded cell range. Path loss is affected by the factors such as the transmit power of eNodeBs and UEs, antenna gains, multiple-antenna receive diversity, diversity, and minimum rate at the cell edge. Moreover,, the coverage radius is affected Moreover affected by the height of the eNodeB and UE antennas and the transmission model. Based on analysis of the feasibility of extended cell range, this section describes the crucial factors affecting the coverage range.
2.3.1 Antenna Height and Selection Increasing the eNodeB antenna height is the priority strategy used to enhance the coverage. A higher antenna has a larger coverage. In mountainous and hilly areas, a single eNodeB cannot cover a large area. Therefore, eNodeBs are usually installed on medium-height mountains and the antenna height ranges from 60 to 150 m. In flat areas such as grassland and deserts, the antenna height is about 35 m due to limitations of tower investment and engineering difficulties. Assume that the height of a UE is 3 m. Figure 1.6 shows 1.6 shows the increase rates of the coverage radius changed with the increased eNodeB antenna height. (The data is calculated using an estimation tool.) Figure 1.6 Increase rates of the coverage radius changed with eNodeB antenna height
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As shown in Figure 1.6, 1.6, the eNodeB with an initial height of 10 m is increased to 100 m with an increasing step of 10 m. When the eNodeB height is increased from 10 m to 20 m, the coverage is increased by 37.2%. When the eNodeB height is increased from 90 m to 100 m, the coverage increases only 4.1% compared with that when the eNodeB is 90 m, the coverage increases by 4.1%. That is, the increase of the coverage slows down with the increase of the antenna height. This shows that the antenna height increase is less helpful for the coverage expansion after the eNodeB is increased to a certain height. Therefore, the coverage cannot be expanded merely by increasing the eNodeB height. Increasing the UE antenna height is another strategy used to enhance the coverage. A height of 3 m or more is recommended for UE antennas. Assume that the height of a UE is 30 m. Figure 1.7 shows 1.7 shows the increase rates of the coverage radius changed with the increased UE antenna height. (The data is calculated using an estimation tool.) Figure 1.7 Increase rates of the coverage radius changed with UE antenna height
As shown in Figure 1.6, 1.6, the UE with an initial height of 1 m is increased to 10 m with an increasing step of 1 m. An increase of 1 m for the UE leads to an increase of 22.13% of the coverage. The coverage radius increases linearly with the increase of UE height. Therefore, the UE height is crucial for enhancing the coverage. Based on the preceding analysis, the following conclusions can be drawn: Both the eNodeB antenna height and the UE antenna height cannot exceed certain values. The increase of UE antenna height enhances the coverage more effective effectively ly than the increase of the eNodeB antenna height. Therefore, when the eNodeB antenna height is limited, increase the UE antenna height. Adopting high-gain antennas for eNodeBs also enhances the coverage, and adopting highgain directional antennas for UEs enhances the receive performance. performance. Assume that the height of an eNodeB is 30 m and the height of a UE is 3 m. Figure 1.8 shows 1.8 shows the increase rates of the coverage radius changed with the increased UE antenna gain. (The data is calculated using an estimation tool.)
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Figure 1.8 Increase rates of the coverage radius changed with UE antenna gain
As shown in Figure 1.8, 1.8, the initial UE antenna is increased from 0 dBi to 16 dBi with an increasing step of 2 dBi. An increase of 2 dBi for the UE leads to an increase of 14% of the coverage. Increase Increase of UE antenna gains leads to increased effective transmit power of the UE antenna and reduced minimum receive signal strength of the UE antenna, which benefits both the UL and DL transmission. The benefit of increasing the eNodeB antenna gain is the same as that of increasing the UE antenna gain.
2.3.2 Receive Diversity Multiple Input Multiple Output (MIMO) is implemented by using multiple antennas at the transmit or receive end. MIMO makes better use of spatial resources and improves the spectral efficiency, efficiency, to increase gains of multiplexing, diversities, arrays, and interference rejection combining (IRC). In this way, way, the system capacity, capacity, system coverage, and data rate are enhanced. The following receive diversities increase the coverage:
UL four-antenna receive diversity
UL eight-antenna receive diversity IRC
UL four-antenna four-antenna receive diversity and UL eight-antenna receive diversity achieve diversity gains by reducing the fading degree of the combined signals based on non-correlation between deep deep fading degrees degrees of different different antennas. The The fading degree is represented by signal signal to interference plus noise ratio (SINR) variance. The eNodeB uses the IRC or other multi-antenna interference reduction algorithms to obtain interferencee reduction gain when interferenc interferenc interferencee occurs. For the LTE system, UL coverage is much superior to DL coverage. Therefore, the UL coverage must be enhanced to implement extended cell range. Multiple-antenna receive diversity enhances the UL coverage by decreasing the UL demodulation threshold and improving the UL receive sensitivity. Four or eight receive antennas and IRC are recommended recommende d for the UEs in the extended cell range.
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Assume that the height of an eNodeB is 30 m and the height of a UE is 3 m. Figure 1.9 shows 1.9 shows the increase rates of the coverage radius when two, four, and eight receive antennas are adopted by the UE. (The data is calculated using an estimation tool.) Figure 1.9 Change of the coverage radius with multiple-antenna receive diversity
As shown in Figure 1.9, 1.9, the coverage radius increases by 19.96% after 1T4R replaces 1T2R for the UE. It increases by 19.54% after 1T8R replaces 1T2R for the UE. In conclusion, adopting multiple-antenna receive diversities improves the UL receive sensitivity and therefore enhances the coverage.
2.3.3 TTI Bundling A TTI TTI is the smallest time unit of scheduling, which equals 1 ms. By using TTI bundling, a resource block (RB) is transmitted over consecutive subframes, which are treated as one transmission unit. In this way, way, signaling overheads are reduced. During UL scheduling, TTI bundling helps improve transport quality in the following scenarios: The UE channel quality is poor poor.. Transmit Tra nsmit power is limited, for example, when a UE resides at the cell edge. If TTI bundling is enabled, more transmission opportunities are available for UEs within the delay budget for services on the air interface. The UL coverage is enhanced.
2.4 Summary During the planning for extended cell range, antenna height, multiple-antenna diversities, high-gain antennas, and TTI bundling can be used to enhance the cell coverage.
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3
Solutions
This chapter describes three scenarios where extended cell range is applicable, and provides configurations for extended cell range on the eNodeB and a configuration case.
3.1 Application Scenarios Where Extended Cell Range Is Applicable Three scenarios where extended cell range is applicable are described as follows: 2.
Sea
There are few obstacles over the sea to block radio signals. When signals between eNodeBs are transmitted within the line of sight (LOS) distance, the free space propagation model can be used and the signal attenuation can can be effectively effectively reduced. Due Due to earth curvature, curvature, the offshore area propagation propagation model is used during link planning for extended cell range. Based on parameters listed in Table 1.1, 1.1, the maximum coverage radius is 31.74 km when the DL rate at the cell edge is 1024 kbit/s and the UL rate at the cell edge is 128 kbit/s, as shown in Figure 2.2. 2.2. Note that the eNodeB has a high altitude in this scenario. Table 1.1 Basic parameters of the eNodeB and UE
Parameter
eNodeB
UE
Maximum transmit power
46 dBm
23 dBm
Antenna gain
18 dBi
0 dBi
Noise figure
3.5 dB
7 dB
Height
50 m
3m
Bandwidth
20 MHz
DL/UL configuration
SA1 (2:2)
Special subframe configuration
10:2:2
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Figure 2.2 Simulation results based on the propagation model in the offshore area in TDD systems
3.
Flat Rural Areas
In flat rural areas, there are few obstacles and therefore a long propagation distance is available. Within non line of sight (NLOS) distance in rural areas, Huawei COST231-HATA model can be used. It is recommended recommended that extended extended cell range is within the LOS distance. distance. If the LOS Issue 3.0 (2012-07-15)
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environment is available between the UE and the eNodeB, the free space propagation model can be used. The extended cell range feature allows eNodeBs to cover a wide rural area, which greatly reduces operators' investment. The extended cell range feature applies to scenarios such as rural areas, deserts, and the sea where a wide coverage radius instead of high UL and DL rates is required. Whether the coverage is within the LOS distance is crucial for the extended cell range. If the coverage is within the LOS distance, the propagation distance can be calculated using the propagation model. If the coverage is out of the LOS distance, the propagation distance cannot be predicted exactly exactly.. In most cases, the propagation propagation on the sea sea or rural areas areas is LOS propagation. propagation. Project O in country G is used as an example. Table 1. 1.1 1 lists basic parameters of the eNodeB and outdoor CPE in project O. Table 1.1 Basic parameters of the eNodeB and outdoor CPE
Parameter
eNodeB
Outdoor CPE
Maximum transmit power
46 dBm
23 dBm
Antenna gain
18 dBi
10 dBi
Noise figure
3.5 dB
7 dB
Height
35 m
3m
Bandwidth
20 MHz
DL/UL configuration
SA1 (2:2)
Special subframe configuration
10:2:2
Figure 3.2 lists 3.2 lists the predicted coverage radius in project O when the UL and the DL rates at the cell edge are 1024 kbit/s and 256 kbit/s, respectively. respectively.
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Figure 3.2 Predicted results of project O in rural areas
As shown in Figure 3.2, 3.2, the maximum UL coverage radius is 11.22 km due to the coverage limitation on the UL and the maximum DL coverage radius is 18.21 km. 4.
Sky
In sky scenarios, the sky propagation model is used. This model is based on the test results in Civil Aviation Aviation Administration of China (CAAC) Zhongtian airport and This the test frequency 1053the MHz. The propagation is sky, without anyproject obstacles. model is usediswhen following conditionsenvironment are met: Issue 3.0 (2012-07-15)
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The eNodeB height is at least 40 m. m.
The cell bandwidth is 1 GHz. If the cell bandwidth is not 1 GHz, the calculation result must be converted.
There is no obstacle between the eNodeB and UE and the earth curvature is the only factor that affects the propagation.
The calculation is based on the main antenna lobe. The calculation on other antenna directions is adjusted based on the radiation. If these conditions are not met, the calculation must be adjusted due to path loss. If these conditions are met, the sky propagation model can be used.
Figure 4.1 Sky propagation model
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3.2 Configurations for Extended Cell Range on the eNodeB Figure 4.2 Configurations for extended cell range on the eNodeB
The following configurations are recommended:
Preamble formats 0 to 4 are available for a time division duplex (TDD) cell with a radius equal to longer than 1400 m. Preamble format 4 is recommended.
Preamble formats 0 to 3 are available for a TDD cell with the radius longer than 1400 m but shorter than or equal equal to 14,500 m. Preamble Preamble format 0 is recommended. recommended.
Preamble formats 0 to 3 are available for a TDD cell with the radius longer than 14,500 m but shorter than or equal to 29,500 m, and the preamble format 2 is recommended.
Preamble formats 1 and 3 are available for a TDD cell with the radius longer than 29,500 m but shorter than or equal to 77,300 m, and the preamble format 1 is recommended.
Preamble recommended but shorterformat than or3 equal eisqual to 100,000 for m. a TDD cell with the radius longer than 77,300 m Issue 3.0 (2012-07-15)
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Preamble formats 1 and 2 are available only when UL-DL configuration 0 or 1 is used. Preamble format 3 is available only when UL-DL configuratio configuration n 0 is used.
3.3 Case Analysis Site U in Jordan There is no large obstacle in this rural area scenario. The required coverage radius is 8.3 km. Analysis:
The UL-DL configuration depends on the preamble format. Format 0 supports a coverage range of 14.5 km, which meets the coverage radius requirement.
UL-DL configurations configurations 0, 1, 2, or 5 can be used when format 0 is configured.
The length of the GP in a special subframe affects the selection of special subframes. A minimum of two symbols are required to meet the coverage requirement. Therefore, Therefore, ULDL configurations configurations 5, 6, or 7 can be used.
Based on the preceding analysis, special subframe configurations 5, 6, or 7 and UL-DL configurations 0, 1, 2, or 5 can be used for the required coverage radius.
Parameterss such as site altitude and site location must be configured based on scenarios. Parameter
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