Let $\boldsymbol{G}$ be a reductive group over a finite field $\mathbb{F}\_q$, $G = \boldsymbol{G}(\mathbb{F}\_q)$, $W = \mathrm{W}(\mathbb{F}\_q)$ the Witt vectors over $\mathbb{F}\_q$, and $K = \mathrm{Frac}(W)$ its fraction field. I'll abuse notation by also writing $\boldsymbol{G}$ for the corresponding (unramified) reductive group over $K$.


When $\boldsymbol{G}$ has connected center, Lusztig proves the following: 


**Theorem.** There is a canonical bijection between (isomorphism classes of) irreducible $\overline{K}$-representations of $G$ and special conjugacy classes in $\widehat{\boldsymbol{G}}(\overline{K})$ stable under $g \mapsto g^q$.


Here $\widehat{\boldsymbol{G}}$ is the dual group of $\boldsymbol{G}$. The condition of being special is a condition of Lusztig relating to special representations of Weyl groups through the Springer correspondence.


On the other hand, one could write a Langlands-type statement as follows: irreducible $\overline{K}$-representations of $G$ should correspond to $\widehat{\boldsymbol{G}}(\overline{K})$-conjugacy classes of $L$-parameters for $G$.  

One possible definition of an $L$-parameter is that of a homomorphism over $\mathrm{Gal}(\overline{\mathbb{F}\_q}/\mathbb{F}\_q)$ of the form
$$\varphi \colon \langle \mathrm{Frob}\_q \rangle \times \mathrm{SL}\_2(\overline{K}) \to {}^L \boldsymbol{G}(\overline{K}), $$
where ${}^L \boldsymbol{G} = \widehat{\boldsymbol{G}} \rtimes \mathrm{Gal}(\overline{\mathbb{F}\_q}/\mathbb{F}\_q)$ is the Langlands dual group of $\boldsymbol{G}$, and where we require $\mathrm{Frob}\_q$ to have semisimple image in $\widehat{\boldsymbol{G}}(\overline{K})$, and the restriction of $\varphi$ to $\mathrm{SL}\_2(\overline{K})$ to be algebraic.  

Of course, in this situation, the data of an $L$-parameter $\varphi$ is equivalent to that of a particular kind of $\widehat{\boldsymbol{G}}(\overline{K})$-conjugacy class in ${}^L \boldsymbol{G}(\overline{K})$. However, Lusztig's conditions on the conjugacy classes do not appear anywhere. For instance, when $\boldsymbol{G}$ is split, an $L$-parameter is simply a $\widehat{\boldsymbol{G}}(\overline{K})$-conjugacy class in $\widehat{\boldsymbol{G}}(\overline{K})$.  

Instead, we might want to add an extra condition which would pin down the image of $\mathrm{Frob}\_q$ to lie in $\widehat{\boldsymbol{G}}(K)$, in order to parallel the following result, pertaining to the semisimple part of the correspondence: 


**Theorem.** There is a canonical bijection between (isomorphism classes of) irreducible semisimple $\overline{K}$-representations of $G$ and semisimple conjugacy classes in $\widehat{\boldsymbol{G}}(\mathbb{F}\_q)$.


However, such a modification would not account for the unipotent part, nor would it account for the condition of being special.  

It seems, then, that this notion of $L$-parameter is the wrong one. What is the fix?



Reconciling Lusztig's results with the Langlands philosophy
The way I like to think about this is that a Langlands parameter for the group $G({\mathbb F}\_q)$ should be the "restriction to inertia" of a tame Langlands parameter for the group $G(K)$.


That is, a tame Langlands parameter (say, over ${\mathbb C}$) for $G(K)$ should be a
pair $(\rho,N)$, where $\rho$ is a map $W\_K \rightarrow \hat G({\mathbb C})$ that factors through the tame quotient of $W\_K$ and $N$ is a nilpotent "monodromy operator", that is, a nilpotent element of the Lie algebra of $\hat G$ that satisfies a certain commutation relation with $\rho$.


The tame quotient of $W\_K$ is generated by the tame inertia subgroup $I\_K$ and a Frobenius element $F$; local class field theory identifies $I\_K$ with the inductive limit of the groups
${\mathbb F}\_{q^n}^{\times}$, and conjugation by Frobenius acts on this by raising to $q$th powers.


I don't have the details in front of me, but if I recall correctly the Deligne-Lusztig parameterization involves several choices (for instance, an identification of $\overline{\mathbb F}\_q^{\times}$ with a suitable space of roots of unity in ${\mathbb C}$.) My understanding is that if one unwinds these choices, they amount to a choice of topological generator $\sigma$ for the inductive limit of the ${\mathbb F}\_{q^n}^{\times}$. 


Thus, if one starts with a Langlands parameter $(\rho,N)$ for $G(K)$ and restricts $\rho$
to inertia, this restriction is determined by $\rho(\sigma)$, which is a semisimple element of $\hat G({\mathbb C})$ that is conjugate to its $q$th power. The
pair $(\rho(\sigma),N)$ should be the Langlands parameter for the group $G({\mathbb F}\_q)$. 


There should then (roughly) be a compatibility between depth zero local Langlands and the Deligne-Lusztig parameterization, as follows: let $K$ be the kernel of the reduction map
$G(W) \rightarrow G({\mathbb F}\_q)$, and let $\pi$ be an irrep of $G(K)$ with
Langlands parameter $(\rho,N)$. Then the $K$-invariants $\pi^K$ of $\pi$ are naturally a
$G({\mathbb F}\_q)$-representation, and $\pi^K$ should contain the representation
of $G({\mathbb F}\_q)$ corresponding to $(\rho(\sigma),N)$ via Deligne-Lusztig. You should take this with a bit of a grain of salt, as I haven't thought the details through carefully. But it should be correct on a "moral" level, at least.


For $GL\_n$ this falls under the rubric of the so-called "inertial local Langlands correspondence". For more general groups the picture is more conjectural, but there
are ideas along these lines in the paper of DeBacker-Reeder on the depth zero local Langlands correspondence.


