[V2X] Comparison of IEEE 802.11p and LTE-V2X: An Evaluation With Periodic and Aperiodic Messages of Constant and Variable Size

네트워크/논문 분석·리뷰

메릴린 2023. 3. 7. 13:51

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Introduction

IEEE 802.11p	3GPP (The Third Generation Partnership Project)
DSRC(Dedicated Shot Range Communication)	LTE-V2X (based on ‘PC5’)
ITS-G5(specified in Europe by ETSI)	Celluar V2X (C-V2X) (based on ‘sidelink LTE radio interface’)

link level : LTE-V2X can improve the link budget over IEEE 802.11p by around 7dB and increase the communication range and reliability at the link level
system level:
1. The model generating periodic message
2. The model generating aperiodic messages of variable size
⇒ IEEE 802.11p outperforms LTE-V2X

simpler and more flexible MAC
use an OFDM(Orthogonal Frequency Division Multiplexing)</span class="blue-highlight">-bsed PHY(physical) layer with a channel bandwidth of 10MHz.
DCF(Distributed Coordination Function) of IEEE 802.11
- CSMA/CA(Carrier Sense Multiple Access with Collision Avoidance)
- CCA(Clear Channel Assistant) threshold
capture effect
- decoding receiver
  - when receiver capture a sharp increase(e.g. by 10dB)
  - stop current decoding
  - start decoding the new packet
- prevent higher signal from being interferer
- strong impact on hidden terminal problem
- improve the packet reception probability at short distances.

$$
12 \cdot \text{OFDM sub-carriers} = \text{RB} \ \sum\text{RB} = \text{channel bandwidth}
$$

$1\cdot\text{RB} = 180\text{kHz}$
$1 \cdot \text{sub-carrier} = 15\text{kHz}$
data → TB로 encapsuled
control information → SCI 로 encapsuled
parameters
- the number of RBs per sub-channel
- the number of sub-channel per sub-frame
sensing based SPS(Semi-Persistent Scheduling) scheme
- sensing based
  - identify and select sub-channels that are not occupied by other vehicles
  - RRI (Resource Reservation Interval)
    - utilize sub-channel by $t+\text{RRI}$ and notify other vehicles it
- Reselection Counter
  - reselect sub-channel period
  - decemented by 1 after each transmission
  - When become 0
    - reserve new sub-channel with probability $(1-P)$
      - $P$ : augments
      - maintain current reservation with probability $P$
  - if current reservation of new-subchannel < latency deadline of new packet
  - set the RRI in the SCI equal to 0 when change into new sub-channel
- 3 steps of the sensing-based SPS scheme
  👉 new sub-channel의 후보가 될 CSRs List를 구성
  1. 같은 sub-frame의 이웃한 sub-channel들로 구성
  2. 지금부터 자신의 Reselection Counter 동안 다른 vehicle에 사용될 예정인 CSR 제외
  3. RSRP threshold보다 큰 RSRP를 가지는 CSR 제외 *(적어도 전체 20%의 CSR 포함)*
  4. 남은 CSRs 중 lowest average RSSI를 가진 CSRs 추리기 *(전체 20%의 CSR 포함)*
  5. 4번까지 남은 CSR 중에서 random하게 new sub-channel 고름

IEEE 802.11p MAC	LTE-V2X MAC
nodes can access the channel at any time if they sense the channel is free	they have to reserve the channel and notify it using RRI in SCI associated to a TB ⇒ pre-defined time-frequency structure
not really affected by the size of messages and the time interval between messages	be affected by message varibility

Reselection of vehicle A causes packet collisions
neighboring vehicles don’t know the fact that A reselect new sub-channel(s) until next TB is transmitted
If A and another neighboring vehicle select same sub-channel and their Selection Windows overlap, their transmissions can collide.

⇒ Reselection occurs although Reselection Counter of vehicle doesn’t reach 0.

a new message has bigger size than the previous message so it doesn’t fit in the reserved sub-channel(s).

the time interval between the previous message and a new message is smaller than RRI
And the latency deadline of the new mesaage is earlier than the next reservation time.
- $(T_{G2}+100\text{ms}) < T_{R2}$
the vehicle is forced to reselect new sub-channels to transmit TB before the latency deadline

⇒ When an additional reselection ouccrs

If a vehicle decides to select new sub-channel when it transmits the last TB that make Reselection Counter zero, the vehicle sets RRI to zero
A vehicle doesn’t set RRI to zero in its last transmission if the vehicle its new sub-channel because of additional reselection.
It cannot inform neighboring vehicles that it will not utilize the previously reserved sub-channels.

If a vehicle A wants to transmit a new message at $T_{R3}$ and this message is generated at $T_{G2}$.
the time interval between messages is larger than RRI
- $(T_{G2}-T_{G1}) > \text{RRI}$
After $\text{RRI}$, the vehicle cannot reserve its sub-channel(s) at $T_{R3}$.
- can’t announce its utilization
Other vehicles believe then the sub-channels at $T_{R3}$ are free.
- Other vehicles believe that the vehicle A reserve its sub-channel(s) until $T_{R2}$.

If another vehicle generate a new message before $T\_{R2}$, the sub-channel(s) reserved by the vehicle A is exclude as candidate sub-channels. (Fig. 5.b)
If another vehicle generate a new message after $T\_{R2}$, the vechicle believes that vehicle A does not use the previous reserved sub-channel(s) ⇒ The risk of collision exists ‼️

When new TB is smaller than the reserved sub-channels.
Then, some of reserved sub-channels will be left unused and other vehicles cannot utilize them.

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