Planning of LTE Radio Networks in WinProp AWE Communications GmbH Otto-Lilienthal-Str. 36 D-71034 Böblingen
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Date
Changes
V1.0
Nov. 2010
First version of document
V2.0
Jan. 2011
Second version of document with modified example
V3.0
Feb. 2011
Third version of document with updates for GUI and sample data
V4.0
Nov. 2011
Fourth version of document with updates on multiple access scheme
LTE Planning in WinProp
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1 Motivation 3GPP Long Term Evolution (LTE), is the latest standard in the mobile network technology tree that produced the GSM/EDGE and UMTS/HSxPA network technologies. The LTE specification provides downlink peak rates of at least 100 Mbit/s, an uplink of at least 50 Mbit/s and RAN round-trip times of less than 10 ms. LTE supports scalable carrier bandwidths, from 1.4 MHz to 20 MHz and supports both frequency division duplexing (FDD) and time division duplexing (TDD).
Peak user throughput
Latency F a c t o r 2 - 3
0 1 r o t c a F
HSPA R6
LTE
HSPA R6
LTE
Spectral efficiency 4 2 r o t c a F
HSPA R6
LTE
Figure 1: Main advantages of LTE compared to HSPA
The main advantages within LTE are high throughput, low latency, higher spectral efficiency, plug and play, FDD and TDD in the same platform, an improved end-user experience and a simple architecture resulting in low operating costs. LTE will also support seamless passing to cell towers with older network technology such as GSM, cdmaOne, UMTS, and CDMA2000. The next step for LTE evolution is LTE Advanced and is currently being standardized in 3GPP Release 10. E-UTRA is the air interface of 3GPP's Long Term Evolution (LTE) upgrade path for mobile networks. It is the abbreviation for evolved UMTS Terrestrial Radio Access Network and provides a single evolution path for GSM/EDGE o UMTS/HSPA o CDMA2000/EV-DO o TD-SCDMA o The LTE air interface will provide the following features: Peak download rates of more than 300 Mbps for 4x4 antennas, more than 170 Mbps for o 2x2 antennas (20 MHz), Peak upload rates of more than 75 Mbps for every 20 MHz of spectrum using a single o antenna, Increased spectrum flexibility supports slices as small as 1.4 MHz and as large as 20 MHz, o Supporting an optimal cell size of 5 km (rural areas), and up to 100 km cell sizes with o acceptable performance, in urban areas cell sizes less than 1 km, Good support for mobility, i.e. high performance mobile data is possible at speeds of up to o 350 km/h, Support for MBSFN (Multicast Broadcast Single Frequency Network) for provision of o Mobile TV
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2 LTE Air Interface Downlink: The physical layer of E-UTRA uses orthogonal frequency-division multiplexing (OFDM) for the downlink with the following properties: Cyclic prefix of 4.7µs to compensate multipath (extended cyclic prefix of 16.6µs) o Radio frame in time domain 10 ms long and consists of 10 sub frames of 1 ms each o Every sub frame consists of 2 slots where each slot is 0.5 ms o The sub-carrier spacing in the frequency domain is 15 kHz o 12 sub-carriers together (per slot) form a resource block, i.e. one resource block is 180 kHz o 6 Resource blocks fit in a carrier of 1.4 MHz o 50 (25, 100) Resource blocks fit in a carrier of 10 MHz (5 MHz, 20 MHz, respectively) o In the downlink there are three main physical channels: Physical Downlink Shared Channel (PDSCH) is used for all the data transmission o Physical Multicast Channel (PMCH) is used for broadcast transmission using a SFN o Physical Broadcast Channel (PBCH) is used to send most important system information o Supported modulation formats are QPSK, 16QAM and 64QAM. For MIMO operation in single user MIMO (higher data rate) 2x2 MIMO and 4x4 MIMO systems are supported.
Up to 20 MHz
SC-FDMA
Uplink User 1
User 2
User 3
Downlink
OFDMA Frequency
Figure 2: Frequency assignment in LTE for downlink and uplink
Uplink: In the uplink LTE uses a pre-coded OFDM called Single Carrier Frequency Division Multiple Access (SC-FDMA) in order to compensate the high peak-to-average power ratio (PAPR) of OFDM. This reduces the need for linearity of power amplifier, and so the power consumption. In the uplink there are three physical channels: Physical Random Access Channel (PRACH) used for initial access o Physical Uplink Shared Channel (PUSCH) carries the data o Physical Uplink Control Channel (PUCCH) carries control information o The same modulation formats as in downlink are used: QPSK, 16QAM and 64QAM.
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3 Modelling in WinProp For considering the LTE air interface in the radio network planning project the corresponding WinProp LTE wst-file (wst stands for wireless standard) has to be selected at the project definition. Accordingly the parameters on the air interface page (see Figure 3) for multiple access, duplex separation, bandwidth, carriers, transmission modes (modulation and coding schemes), cell assignment and mobile station are correctly defined. There are various wst-files available depending on the frequency band and the bandwidth. MIMO technology can be activated optionally if spatial multiplexing shall be considered (see the specific MIMO application note for further details).
Figure 3: Air interface definition for LTE
Further settings for the multiple access scheme OFDM/SOFDMA can be defined on the corresponding page (see Figure 4).
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Figure 4: Air interface settings for LTE
Tx Power Settings (Downlink) The Tx power is split among the given sub-carriers (by default each sub-carrier gets the same power). In case the sub-carriers with reference (pilot) or control signals should get more power an additional back-off can be defined (negative value in case of more power, positive value in case of less power). The value defined for the reference (pilot) signal power back-off influences the available signal power for the cell assignment. The resulting shares of the Tx power for the transmission of reference (pilot), control and data signals are displayed here, which are computed by evaluation of the settings defined in the field "Symbols". Cell Load The cell load can be either used to control the "Tx power in downlink" or the "number of used subcarriers". If the option "cell load controls Tx power in downlink" is selected the power is adapted on all data sub-carriers in the same way which leads to the same interference situation on all data sub-carriers (e.g. in case of 50% load every data sub-carrier gets 3 dB less than the max. power per sub-carrier). If the option "cell load controls number of used sub-carriers" is selected, the subcarriers are transmitted either with the max. power per sub-carrier or the sub-carriers are not transmitted at all (e.g. in case of 50% load only every second data sub-carrier is used in the considered cell, which means that the other 50% of the sub-carriers can be used without interference in the neighboring cells).
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Sub-carriers Depending on the bandwidth the FFT order is automatically selected and the number of guard, sub-carriers is predefined. The values for the sub-carrier spacing and the symbol duration are also predefined. Symbols This field allows to define the split of the resources (symbols and sub-carriers) among the different signal types (reference/pilot, control, and data). If "identical settings for each symbol" is selected the split can be only done in the frequency domain by defining a corresponding number of subcarriers for pilot and reference signals (the remaining sub-carriers are used for data transmission). If the option "individual sub-carrier settings for symbols (pilot./ref.)" is selected a more detailed assignment of the resources in both the time and frequency domain is possible, which is especially important for LTE networks as it effects the power and interference situation for the reference signals (RSRP, RSRQ, RSSI). The following figure shows the split among data, control, and pilot/reference signals in an LTE physical resource block (for the symbols which carry the reference signal). This resource assignment for LTE is defined in the table of the Symbols section in the above shown dialogue.
Figure 5: Air interface settings for LTE
Resource blocks In this field the number of sub-carriers in one resource block can be defined. The option "fractional load allowed" is only relevant if the option "cell load controls number of used sub-carriers" is selected. In case the transmission modes include the transmission of a certain number of resource blocks in parallel (e.g. 25 in case of 5 MHz bandwidth) the activation of this option evaluates the situation on higher granularity (resource block level), i.e. how many resource blocks can be transmitted in parallel (especially if below the defined number in the transmission mode).
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Frequency Reuse The Fractional Frequency Reuse is an important feature in LTE in order to improve the performance at the cell border (as often a frequency reuse of 1 is applied). By using only a part of the bandwidth at the cell border it is possible to use other sub-carriers in the different cells (at the cell border only). In order to activate this feature the corresponding fractional frequency reuse factor has to be selected (default disabled). Based on that the defined transmission modes are also analyzed with reduced number of resource blocks (according to the reduced bandwidth) considering the interference reduction due to the fractional frequency reuse.
Transmission Modes On the air interface page (see Figure 3) the possible transmission modes are listed with modulation type, code rate and data rate. The transmission modes are also predefined in the wstfile. The settings for an individual transmission mode are presented in Figure 6.
Figure 6: Transmission mode definition for LTE
Here the properties for downlink (left) and uplink (right) are defined individually, however by default in a symmetrical way (regarding transmission parameters, required SNIR and Tx power back-off). Furthermore it is possible to switch from this bi-directional mode to the individual analysis of downlink only or uplink only. The first block defines the parameters for the data transmission with modulation, code rate, number of resource blocks and overhead ratio. These parameters result in a feasible data rate for this transmission mode. The number of resource blocks can be defined according to the bandwidth (25 for 5 MHz, 50 for 10 MHz and 100 for 20 MHz) or by using just one resource block (in this case the number of streams is determined as result which indicates how many resource blocks can be
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received in parallel for this mode, thus the throughput is the given data rate multiplied with the number of streams).
Figure 7: SNIR targets depending on the modulation and coding scheme for LTE
In the second block the reception thresholds for the individual transmission mode, i.e. SNIR target (as example see the values defined in Figure 7) and Rx power (optional) can be defined. The Tx power back-off shall be defined with respect to the used modulation. In case of QAM modulation a back-off of 3 dB is recommended. The Tx power back-off reduces the SNIR for the corresponding transmission mode, which might lead to the situation that this transmission mode is no longer available.
LTE Network Simulation The performance of the LTE network (in terms of possible throughput) is derived from the computed SNIR map. Besides the available signal power the most significant impact is given due to the interference coming from neighboring cells as within a cell the different users are separated in the frequency and/or time domain. The interference level from the neighboring cells can be influenced by the definition of a corresponding load factor on the simulation page (see Figure 8). This value represents the assumed mean Tx power in downlink for the neighboring cells and is defined relative to the max. available Tx power in the corresponding cell. A value of 100% means that all the neighboring cells transmit the full power (i.e. 100% load), which clearly represents the worst case for the available throughput. In contrary a value of 0% means that no traffic is given in the neighboring cells and therefore only the noise power limits the performance. If the option "cell load controls number of used sub-carriers" is selected in the multiple access dialogue, the sub-carriers are transmitted either with the max. power per sub-carrier or the subcarriers are not transmitted at all. Accordingly in case of x% load in the considered cell, (1-x)% of the sub-carriers can be used without interference in the neighboring cells. In case of different loads (x, y) within two cells, (1-max(x, y))% of the sub-carriers can be used without interference in the neighboring cells. For min(x, y)% of the sub-carriers both cells produce interference and for the remaining part of the bandwidth only one cell is interfering.
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For the uplink direction instead of the mean Tx power percentage a corresponding noise rise can be defined, which is used for the uplink interference computation (default 3 dB).
Figure 8: Definition of simulation parameters
The load factor ("assumed mean Tx power in downlink") can be either defined globally on the simulation page (as shown in Figure 6), i.e. a homogeneous cell load for all cells in the scenario, or this parameter ("assumed mean Tx power in downlink") can be defined for each cell individually on the carrier settings for the individual antenna/cell (see Figure 9). If no individual cell load is defined for a cell, the global value defined on the simulation page is used.
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Figure 9: Definition of individual cell load
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4 Examples This section presents an example for the better understanding of the LTE capabilities in WinProp. Figure 10 shows an office scenario with three antennas. Two of the deployed antennas use the same carrier (sites 1 and 2) - otherwise there would be no co-channel interference in the scenario.
Site 3 Carrier 1
Site 1 Carrier 0
Site 2 Carrier 0
Figure 10: Office scenario with 3 antennas (2 different carriers)
The main parameters of the network are shown in the following table: Parameter
Value
Frequency
2110 MHz
System bandwidth
5 MHz
Transmit power
10 dBm Output power of PA
Antenna height
2.5 m
Min. required SNIR (depending on MCS)
Between –5.4 and 17.2 dB
Air interface
LTE
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The computed radio network planning results can be distinguished in 5 categories: •
•
•
•
•
Common results considering cell assignment and all transmission modes: Cell Area o Site Area o Best Server o Max. Data Rate (DL and UL) o Max. Throughput (DL and UL) o Cell Assignment: Received Power o Received Sites o Received Cells o Received Carriers o Max SNIR (DL) o Pilot: Interference o Pilot: Total Received o Reference Signals: RSRP (DL) o RSRQ (DL) o RSSI (DL) o Reference Signals: Received Power o Max SNIR (DL) o For each transmission mode (e.g. 16 QAM code rate ½): DL: Min Tx Power BS how much BS power is required to reach reception level o DL: Max Rx Power MS how much MS power is received on max. (full power BS) o DL: Max SNIR which SNIR is received on maximum (full power BS) o DL: Reception Probability o DL: Max Data Rate per user o DL: Nr Streams o DL: Max Throughput o UL: Min Tx Power MS how much MS power is required to reach reception level o UL: Max Rx Power BS how much BS power is received on max. (full power MS) o UL: Max SNIR which SNIR is received on maximum (full power MS) o UL: Max Data Rate per user o UL: Nr Streams o UL: Max Throughput o
Accordingly the result maps consider two different settings on the transmitting side. Either Min. required power on the transmitter in order to reach the reception level for the “Min Tx Power BS” and the “Min Tx Power MS” maps or full power transmission for all the other maps. Figures 11-13 show the max. throughput map evaluating all transmission modes and the site area map and best server map for this network configuration. Here the influence of the carrier assignment is clearly visible. The two antennas operating on carrier 0 are interfering each other (assumed load of 100%) while the site 3 on carrier 1 has less interference and provides therefore a higher throughout in the corresponding cell area. As the cell assignment is based on the highest received power in downlink (DL) the throughout is limited in the region where the signals from site 1 and site 2 arrive with similar levels (South of site 1). Here the interference limits the SNIR. © by AWE Communications GmbH
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Figure 11: Max. data rate (DL) for indoor network
Figure 12: Site Area map for indoor network
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Figure 13: Best Server map for indoor network
Figures 14 shows the RSRP map (reference signal received power) and figure 15 the max. SNIR map for the cell assignment. While the RSRP map is influenced by the location of the sites and the propagation condition, the SNIR map is clearly impacted by the carrier assignment. Despite there are high received powers around sites 1 and 2, the SNIR is limited due to the frequency reuse (same carrier).
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Figure 14: Reference signal received power RSRP (DL)
Figure 15: Max. SNIR (DL) map in cell assignment
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Figures 16 and 17 show two result maps for an individual transmission mode, first the min. required BS power map and then the max. SNIR map.
Figure 16: Min. required BS power map for transmission mode
Figure 17: Max. SNIR map for transmission mode
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In order to show the impact on the load the Figure 18 shows the number of streams for a higher transmission mode and the Figure 19 indicates the improved throughput due to the reduced load.
Figure 18: Max. SNIR map for transmission mode (30% load)
Figure 19: Max. data rate (DL) for indoor network (30% load)
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The following Figures show an example for an urban LTE deployment. The Figure 20 shows the received power in cell assignment for the deployed six 3-sector sites.
Figure 20: Received power (DL) in cell assignment for urban network
In the Figure 21 the max. available data rate (throughput) for the urban network is shown assuming a load of 30%. In the main beams of the individual sectors the max. data rate is available depending on the interference situation.
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LTE Planning in WinProp
Figure 21: Max. data rate (DL) for urban network (30% load)
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