6. Click OK.
For TDD networks, you can determine the maximum coverage range that the sectors of your LTE network should have from the cyclic prefix duration and use this range as the Max Range parameter. You can calculate the maximum system range from the cyclic prefix as follows:
Max Range (m) = Cyclic Prefix (in ms) x 300000/2 Bearer Selection Thresholds
The default values of the bearer selection thresholds, the BLER quality graphs, and the bearer efficiency values in Atoll have been extracted from the graphs in the Figure 0.61 on page 85.
In the above graphs, the spectral efficiency is the number of useful data bits that can be transmitted using any modulation and coding scheme per Hz, the transition points between any two modulation and coding schemes give the default bearer selection thresholds in Atoll, and the normalised values from the slopes of the graphs, that represent the reduction in the spectral efficiency, give the block error rate.
If you want to model the bearer selection for a given level of BLER, instead of the default bearer selection which occurs at the transition points (i.e., as soon as a bearer becomes better than the lower one, it is selected), then you can use the following values of C/(I+N):
Calculating Bearer Selection Thresholds From Receiver Sensitivity Values
You can convert the receiver sensitivity values, from the specifications of your equipment, into bearer selection thresholds using the following conversion method:
Figure 0.61: Link Adaptation in LTE
BLER
Bearer
1 % 2 % 5 % 10 %
1 1.4 1 0.8 0.5
2 4.7 4.3 3.9 3.7
3 7.9 7.8 7.2 6.9
4 10.3 9.9 9.6 9.1
5 14.7 14.2 13.9 13.6
6 18.9 18.5 17.8 17.3
7 18.7 18.3 17.7 17.2
8 20.2 19.9 19.3 18.7
9 23.8 23.7 23.2 22.5
CNR RS 114–NF 10 Log SF×NUsed NTotal
---⎝ ⎠
⎛ ⎞
× – +
=
Where RS is the receiver sensitivity in dBm, NF is the noise figure of the receiver in dB, SF is the sampling frequency in MHz, is the number of subcarriers corresponding to the number of used resource blocks, and is the number of subcarriers corresponding to the total number of resource blocks.
In the above explanation, the term receiver refers to the base station in uplink and to the mobile/user equipment in the downlink.
Relation Between Bearer Efficiency And Spectral Efficiency
Spectral efficiency of a modulation and coding scheme is defined as the number of useful bits that can be transmitted each second over a channel of 1 Hz bandwidth. Spectral efficiency is hence given in terms of bps/Hz.
In Atoll, the efficiency of bearers (modulation and coding schemes) are defined in the Bearers table. The bearer efficiency is given in terms of bits/symbol. Remember that in Atoll a symbol refers to the data transmission unit which is 1 symbol duration long and 1 subcarrier width wide, as shown in Figure 0.62.
The concept of bearer efficiency is similar to spectral efficiency. The only difference is in the units used to define the two entities. Here is a simple example that compares spectral efficiency and bearer efficiency, and shows that the two are the same.
Spectral efficiency is given by:
Where BLER is the Block Error Rate, r is the coding rate for the bearer, and M is the number of modulation states. For simplification, we set BLER = 0, and use QPSK1/2, i.e., four modulation states and r = 0.5. With these values, we get a spectral efficiency of 1 bps/Hz for QPSK1/2. In other words, a communication channel using QPSK1/2 modulation and coding scheme can send 1 bps of useful data per unit bandwidth.
In order to compare the bearer efficiency and spectral efficiency of QPSK1/2, let’s say that QPSK1/2 has a bearer effi-ciency of 1 bits/symbol. Here as well, the number of bits refers to useful data bits. The width of a symbol in LTE is , from which we can calculate the useful symbol duration as well: . In one second, there can be symbol durations. If 15000 symbols are transmitted using QPSK1/2,
this gives us a data rate of , which is the data rate achievable using
one subcarrier of 15 kHz. We can find the spectral efficiency by normalizing the data rate to unit bandwidth. This gives:
In order to compare similar quantities, we have ignored the system parameters such as the cyclic prefix, TTG, RTG, and have considered that the entire frame is transmitted in one direction, uplink or downlink.
Displaying Coverage by Bearer Names
In addition to calculating and displaying coverage predictions by LTE bearers, i.e., bearer indexes, as explained in "Making a Coverage by Best Bearer" on page 43, you can also calculate and display coverage predictions by bearer names.
To create a coverage prediction by bearer name:
• Create a coverage prediction by best LTE bearer as explained in "Making a Coverage by Best Bearer" on page 43, except for the following differences:
- Click the Display tab (see Figure 0.63).
- Edit the Legend column of the display thresholds and enter the bearer names instead of the bearer indexes.
Figure 0.62: Symbol
NUsed NTotal
SE = (1–BLER) r Log× × 2( )M bps Hz⁄
∆F= 15 kHz TU 1
∆F--- 66.67 µ sec
= =
1 sec 66.67 µ sec⁄ = 15000
15000 Symbols/sec×1 bits/Symbol = 15000 bps
15000 bps/subcarrier 15 kHz/subcarrier⁄ = 1 bps/Hz
You can also save these modified coverage predictions as templates so that you do not have to modify the display options every time you want to create a similar coverage prediction.
0.6 Glossary of LTE Terms
Understanding the following terms and there use in Atoll is very helpful in understanding the LTE module:
• User: A general term that can also designate a subscriber, mobile, and receiver.
• Subscriber: Users with fixed geographical coordinates.
• Mobile: Users generated and distributed during simulations. These users have, among other parameters, defined services, terminal types, and mobility types assigned for the duration of the simulations.
• Receiver: A probe mobile, with the minimum required parameters needed for the calculation of path loss, used for propagation loss and raster coverage predictions.
• Bearer: A Modulation and Coding Scheme (MCS) used to carry data over the channel.
• Peak RLC Throughput: The maximum RLC layer throughput (user or channel) that can be achieved at a given location using the highest LTE bearer. This throughput is the raw data rate without considering the effects of retransmission due to errors and higher layer coding and encryption.
• Effective RLC Throughput: The net RLC layer throughput (user or channel) that can be achieved at a given loca-tion using the highest LTE bearer computed taking into account the reducloca-tion of throughput due to retransmission due to errors.
• Application Throughput: The application layer throughput (user or channel) that can be achieved at a given loca-tion using the highest LTE bearer computed taking into account the reducloca-tion of throughput due to PDU/SDU header information, padding, encryption, coding, and other types of overhead.
• Channel Throughputs: Peak RLC, effective RLC or application level throughputs achieved at a given location using the highest LTE bearer with the entire channel resources.
• User Throughputs: Peak RLC, effective RLC or application level throughputs achieved at a given location using the highest LTE bearer with the amount of resources allocated to a user by the scheduler.
• Traffic Loads: The uplink and downlink traffic loads are the percentages of the uplink and the downlink subframes in use (allocated) to the traffic (mobiles) in the uplink and in the downlink, respectively.
• Uplink Noise Rise: Uplink noise rise is the measurement of uplink interference with respect to the uplink noise.
• Resources: In Atoll, the term "resource" is used to refer to the average number of resource units, which is expressed in % (as traffic loads, when the average is performed over a considerably long duration) of the total number of resource units in a superframe of 1 sec.
Figure 0.63: Coverage by Best Bearer – Display tab
NRUL IUL+NUL NUL
---=