122 IEEE TRANSACTIONS ON INTELLIGENT VEHICLES, VOL. 9, NO. 1, JANUARY 2024
TABLE III
L
ITERATURE OVERVIEW OF FL FOR CAV ALGORITHMS
This training process typically adopts a simple Stochastic
Gradient Descent (SGD) algorithm. The computational
infrastructure is usually limited.
3) Local Update Upload: After training the model, each ve-
hicle applies privacy-preserving techniques such as differ-
ential privacy (introduces artificial noise to the parameters)
and then uploads/communicates the model parameters to
the selected central server (Centralized Federated Learn-
ing, i.e., CFL) or other vehicles (Decentralized Federated
Learning, i.e., DFL).
4) Aggregation of Vehicle Updates: The server securely ag-
gregates the parameters uploaded from K vehicles to
obtain the global model. Furthermore, it tests the model’s
performance.
A. Centralized Federated Learning
In this section, we review two major aggregation methods in
the centralized framework, namely averaging and a more recent
technique called knowledge distillation.
1) Averaging: Most of the existing literature uses the Feder-
ated Averaging (FedAvg) algorithm [25] for the FL aggregation
process on the server—see Table III. FedAvg applies SGD op-
timization to local vehicles and performs a weighted averaging
of the weights of the vehicles on the central server. FedAvg
performs multiple local gradient updates before s ending the
parameters to the server, reducing the number of communication
rounds. For FL4CAV, data on each CAV are dynamically updated
at each communication round.
A typical FL setup has K vehicles that have their own local
data sets and the ability to perform simple local optimization. At
the central server, the optimization problem can be represented
as
min
x∈R
d
f(x)=
1
K
K
i=1
f
i
(x
i
)
, (1)
where f
i
: R
d
→ R for i ∈{1,...,K} is the local objective
function of the ith vehicle. The local objective function of the
ith vehicle can have the form,
f
i
(x
i
)=E
ξ
i
∼D
i
[(x
i
,ξ
i
)], (2)
where ξ
i
represents the data that have been sampled from the
local vehicle data D
i
for the i
th
vehicle. The expectation oper-
ator, E, is acting on the local objective function, (x
i
,ξ
i
),with
respect to a data sample, ξ
i
, drawn from the vehicle data, D
i
.
The function (x
i
,ξ
i
) is the loss function evaluated for each
vehicle, x
i
, and data sample, ξ
i
. Here, x
i
∈ R
d
represents the
model parameters of vehicle i, and X ∈ R
d×K
is the matrix
formed using these parameter vectors. The learning process
is performed to find a minimizer of the objective function,
x
i
= x
∗
=argmin
x∈R
d
f(x).
The data obtained from CAVs are typically non-independent
and non-identically distributed (non-IID). FedAvg faces chal-
lenges in realistic heterogeneous data settings, as a single global
model may not perform well for individual vehicles, and mul-
tiple local updates can cause the updates to deviate from the
global objective [44]. Several variants of FedAvg have been
proposed to address the challenges encountered by FL, such
as data heterogeneity, client drift, local vehicle data imbalance,
communication latency, and computation capabilities. FedProx
algorithm, FedAvg with a proximal term, has been proposed to
improve the convergence and reduce communication cost [45].
Dynamic Federated Proximal [46] algorithm (DFP) is an exten-
sion of FedProx that could effectively deal with non-IID data
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