Adaptive Transmit Power Control Algorithm for Sensing-Based Semi-Persistent Scheduling in C-V2X Mode 4 Communication
Abstract
:1. Introduction
- To be scheduled via Mode 3, the vehicles must stay connected to the LTE network. This imparts a significant burden on mobility management, especially for scenarios where vehicles have high mobility, such as highway scenarios.
- In the event of lost coverage, Mode 3 can no longer perform resource allocation, thus completely blocking V2X transmissions.
- The formulation for the A-TPC algorithm with SB-SPS for C-V2X Mode 4.
- Performance comparison of the proposed algorithm with the power adaptation algorithm presented in [30].
- Performance evaluation of the proposed A-TPC algorithm by employing 3GPP Standard Mode 4 settings in three realistic traffic scenarios, which are:
2. 3GPP C-V2X Mode 4 Algorithm
- Step 1. Channel sensing: To sense the channel, each vehicle continuously monitors the channel by measuring the sidelink received signal strength indication (SL-RSSI) for each subchannel to determine the interference level for every subframe. These sensing measurements are collected for a defined sensing time Tsense.
- Step 2. Listing available CAM resources: Following the sensing measurements, each vehicle enlists its available CAM resources, termed LCAM-R. The selection of the CAM resources follows the set of rules given below:
- ○
- The SL-RSSI of the selected CAM resource must be lower than a certain threshold PTH set by the vehicle.
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- The CAM resource must not be selected if it is not sensed during Tsense.
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- Out of all available CAM resources, already reserved resources must not be selected.
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- Following half-duplex transmission, a vehicle cannot sense the channel while it transmits.
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- If the remaining candidate CAM resources are less than 20% of the total available CAM resources, the threshold PTH is increased by 3 dB and step 2 is repeated.
- Step 3. CAM resource selection: After identifying the best 20% of CAM resources from LCAM-R, the vehicle randomly selects transmission CAM resources from among them. The vehicle can reserve the same CAM resources for a random number of subsequent transmissions with the same transmission interval TCAM. The minimum and maximum number of subsequent transmissions (nmin and nmax) before reallocation are defined by the MAC layer [17]. The 3GPP mandated values for nmin and nmax are depicted in Table 1.
- Step 4. CAM resources reselection: After transmitting each packet, the SB-SPS counter decreases, indicating the number of remaining consecutive packets. If the vehicle encounters zero SB-SPS counters, it can either keep the resources with a probability pk or reselect CAM resources with a probability of 1-pk.
3. Proposed System
3.1. Simulation Environment and Setup
- Step 1. System initialization: In the first step, the initialization procedure with the parameters defined in the configuration files is executed. The number of available CAM resources in each CAM period is evaluated. All the parameters have pre-defined values for each traffic scenario in the configuration files to facilitate the initialization process.
- Step 2. First CAM resources allocation: To reduce the initialization overhead, the first CAM resource allocation is performed for all the vehicles in each traffic scenario with the initialized parameters to mimic autonomous scheduling and perfect position estimation. Using this approach, collisions in resource allocation due to the appearance of many vehicles in a traffic scenario at a specific time interval can be avoided.
- Step 3. Simulation Cycle: After system initialization, the main part of the simulation, termed the simulation cycle, is executed and this is performed repeatedly for a defined simulation length Tsim. The process consists of updating the position of the vehicle, followed by the core step of quality assessment by calculating the received signal-to-interference-plus-noise ratio (SINR). The error estimation can be performed by determining the success or failure of CAM transmission using a defined minimum received SINR level for each traffic scenario. For a CAM transmission from vehicle x to vehicle y, the received SINR formulation from [33] can be given as:
- Step 4. Performance evaluation: This is the last step performed after the simulation cycle is completed, when the output performance metrics for each traffic scenario is evaluated. In this case, we focus on the packet reception ratio (PRR) for the CAM transmission defined as:
3.2. Adaptive Transmit Power Control Algorithm
- Step 1. Compute SL-RSSI matrix: In the first step, the vehicles performing reallocation of the CAM resources are listed according to their vehicle ID in the ID list (LID). To determine the number of vehicles performing reallocation, the index of LID is computed. Afterward, the SL_RSSI matrix for each vehicle in LID is computed by summing the SL-RSSI values in each subframe.
- Step 2: Interference calculation: In this step, the interference experienced by each vehicle over the sidelink is calculated by summing the SL-RSSI values from each neighbouring vehicle in the same subframe with the CAM resources. This interference is termed as ISL_RSSI.
- Step 3. Transmit power allocation: In this step, the transmit power is allocated to each vehicle during the process of CAM resources reallocation based on the interference and the density of vehicles in each scenario. The density of vehicles ρ is defined as the number of vehicles per km in each scenario. The maximum vehicle density ρTH for each scenario is defined in the simulation. For a vehicle density lower then ρTH, our proposed algorithm allocates the maximum transmit power of 23 dBm for CAM transmission. However, with higher vehicle density, the chances of failed CAM transmission increases due to a significant increase in interference. Following the ETSI ITS recommendation [18], the transmit power is initialized by adopting the 3GPP Mode 4 at 10 dBm, and adaptive power control is performed based on the amount of interference in the system. Depending on the MCS adopted in each scenario, the interference threshold ITH can be obtained by using Equation (1) with the minimum required SINR for each MCS defined in Table 3. Interference threshold ITH is defined as the maximum allowable interference level for achieving the minimum required SINR. By comparing the ISL_RSSI to an interference threshold ITH, the proposed algorithm increases the initialized transmit power by ∆P at every CAM resource reallocation, up to the maximum transmit power limit Pmax = 23 dBm.
Algorithm 1: Pseudo-code for the Proposed A-TPC Algorithm |
1. Procedure TRANSMIT POWER ALLOCATON |
2. Compute SL-RSSI matrix for all vehicles performing resource reallocation |
3. LID ← List containing ID of each vehicles |
4. VIndex ← Number of vehicles in LID |
5. SenseMAT ← SL-RSSI matrix for each vehicle in LID |
6. ρTH ← Threshold for vehicle density in each traffic scenario |
7. ITH ← Interference threshold for MCS in each traffic scenario |
8. ∆p ← Power increment of 3.25 dBm |
9. x ← 1 |
10. while x < VIndex + 1 do |
11. For all reselected CAM resources sum SL-RSSI values in same subframe |
12. NCR ← new CAM resources for VIndex (x) |
13. S_NCR ← SL-RSSI value in same subframe with NCR |
14. |
15. Adaptive transmit power control with SL-RSSI values |
16. if Vehicle density > ρTH |
17. Initialize PTx (x) ← 10 dBm |
18. if ISL-RSSI < ITH |
19. PTx (x) ← min (23, PTx (x) + ∆p) dBm |
20. else PTx (x) ← 23 dBm |
21. x ← x+1 |
22. return PTx |
4. Simulation Results
4.1. Performance Comparison with Existing Scheme
4.2. Performance Evaluation of Proposed Algorithm
5. Conclusions
Author Contributions
Funding
Conflicts of Interest
References
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Parameters | Definition | 3GPP constraints |
---|---|---|
Physical layer | ||
Tsense | Channel sensing time | 1 sec (mandated) |
PTH | Minimum threshold for the power level | ∈[−128, −2] dBm |
Rsel | Portion of CAM resources selected for transmission | 0.2 (mandated) |
T1 | First subframe for a new allocation | ≤4 |
T2 | Last subframe for a new allocation | ≥20, ≤100 |
MAC layer | ||
nmin | Minimum number of CAM periods before reallocation | 5 (mandated) |
nmax | Maximum number of CAM periods before reallocation | 15 (mandated) |
pk | Probability of maintaining same CAM resources | ∈[0, 0.8] |
Parameters | Values |
---|---|
Simulation inputs | |
Traffic scenario | 4-km highway (three lanes per direction) |
Vehicle density (ρ) | 100, 200 |
Vehicles speed | 114 km/h |
Awareness range | 150 m |
CAM periodicity (fCAM) | 10 Hz |
CAM size (BCAM) | 300 bytes |
Propagation model | WINNER+ |
Shadowing variance | LOS 3 dB, NLOS 4 dB |
MCS | 4 |
Minimum SINR | 2.76 dB |
Physical and MAC layer settings | |
Tsense | 1 s |
PTH | −110 dBm |
Rsel | 0.2 |
T1 | 1 |
T2 | 100 |
nmin | 5 |
nmax | 15 |
pk | 0 |
Parameters | Values | |||
---|---|---|---|---|
Traffic Scenario | ||||
Traffic models | Urban | Highway | ||
Vehicle density (ρ) | Medium | Congested | High | |
Awareness range | 100 m | 100 m | 200 m | |
Simulation inputs | ||||
Simulation duration (Tsim) | 90 s | |||
Bandwidth | 10 MHz | |||
CAM frequency (fCAM) | 10 Hz | |||
CAM size (BCAM) | 300 bytes | |||
MCS | MCS-4 | MCS-7 | MCS-7 | |
Tx/Rx antenna gain | 3 dB | |||
Duplexing type | Half duplex | |||
Transmit power (PTX) | 23 dBm, 10 dBm, variable | |||
Propagation model | WINNER+, Scenario B1 | |||
Shadowing variance | LOS 3 dB, NLOS 4 dB | |||
Minimum SINR | 2.76 dB | 7.30 dB | ||
Physical and MAC layer settings | ||||
Tsense | 1 s | |||
PTH | −110 dB | |||
Rsel | 0.2 | |||
T1 | 1 | |||
T2 | 100 | |||
nmin | 5 | |||
nmax | 15 | |||
pk | 0.4 |
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Haider, A.; Hwang, S.-H. Adaptive Transmit Power Control Algorithm for Sensing-Based Semi-Persistent Scheduling in C-V2X Mode 4 Communication. Electronics 2019, 8, 846. https://doi.org/10.3390/electronics8080846
Haider A, Hwang S-H. Adaptive Transmit Power Control Algorithm for Sensing-Based Semi-Persistent Scheduling in C-V2X Mode 4 Communication. Electronics. 2019; 8(8):846. https://doi.org/10.3390/electronics8080846
Chicago/Turabian StyleHaider, Amir, and Seung-Hoon Hwang. 2019. "Adaptive Transmit Power Control Algorithm for Sensing-Based Semi-Persistent Scheduling in C-V2X Mode 4 Communication" Electronics 8, no. 8: 846. https://doi.org/10.3390/electronics8080846