Metric temporal logic


Metric temporal logic is a special case of temporal logic. It is an extension of temporal logic in which temporal operators are replaced by time-constrained versions like until, next, since and previous operators. It is linear-time logic that assumes both the interleaving and fictitious-clock abstractions. It is defined over a point-based weakly-monotonic integer-time semantics. For MTL, the exact complexity of the satisfiability problems is known and independent of interval-based or point-based, synchronous or asynchronous interpretation: EXPSPACE-complete.
MTL has been described as a prominent specification formalism for real-time systems. Full MTL over infinite timed words is undecidable.

Syntax

The full metric temporal logic is defined similarly to linear temporal logic, where a set of non-negative real number is added to temporal modal operators U and S. Formally, MTL is built up from:
When the subscript is omitted, it is implicitly equal to.
Note that the next operator N is not considered to be a part of MTL syntax. It will instead be defined from other operators.

Past and Future

The past fragment of metric temporal logic, denoted as past-MTL is defined as the restriction of the full metric temporal logic without the until operator. Similarly, the future fragment of metric temporal logic, denoted as future-MTL is defined as the restriction of the full metric temporal logic without the since operator.
Depending on the authors, MTL is either defined as the future fragment of MTL, in which case full-MTL is called MTL+Past. Or MTL is defined as full-MTL.
In order to avoid ambiguity, this article uses the names full-MTL, past-MTL and future-MTL. When the statements holds for the three logic, MTL will simply be used.

Model

Let intuitively represent a set
of points in time. Let a function which associates a letter to
each moment. A model of a MTL formula is
such a function. Usually, is
either a timed word or a signal. In
those cases, is either a discrete subset or an interval
containing 0.

Semantics

Let and as above and let some fixed time. We are now going to explain what it means
that a MTL formula holds at time,
which is denoted.
Let and. We first consider the formula. We say
that if and only if
there exists some time such that:
We now consider the formula We say
that if and only if
there exists some time such that:
The definitions of for the values
of not considered above is similar as the definition
in the LTL case.

Operators defined from basic MTL operators

Some formulas are so often used that a new operator is introduced for
them. These operators are usually not considered to belong to the
definition of MTL, but are syntactic sugar which denote more
complex MTL formula. We first consider operators which also exists in LTL. In
this section, we fix MTL formulas
and.

Operators similar to the ones of LTL

Release and Back to

We denote by
the
formula. This formula holds
at time if either:
The name "release" come from the LTL case, where this formula simply
means that should always hold,
unless releases it.
The past counterpart of release is denote by
and is equal to the
formula.

Finally and Eventually

We denote by or the formula. Intuitively, this formula holds at time
if there is some time such
that holds.
We denote by or the
formula. Intuitively, this formula
holds at time if for all time, holds.
We denote by
and the formula similar
to and,
where is replaced by. Both formula has intuitively the same meaning, but when we
consider the past instead of the future.

Next and previous

This case is slightly different from the previous ones, because the intuitive meaning of the "Next" and "Previously" formulas differs depending on the kind of function considered.
We denote by or
the
formula. Similarly, we denote by the formula. The following discussion about the Next operator also holds for the Previously operator, by reversing the past and the future.
When this formula is evaluated over a timed word, this formula
means that both:
When this formula is evaluated over a signal, the notion of next time does
not makes sense. Instead, "next" means "immediately after". More
precisely means:
We now consider operators which are not similar to any standard LTL operators.

Fall and Rise

We denote by , a formula which holds when becomes true. More precisely, either did not hold in the immediate past, and holds at this time, or it does not hold and it holds in the immediate future. Formally is defined as.
Over timed words, this formula always hold. Indeed and always hold. Thus the formula is equivalent to, hence is true.
By symmetry, we denote by , a formula which holds when becomes false. Thus, it is defined as.

History and Prophecy

We now introduce the prophecy operator, denoted by. We denote by the formula. This formula asserts that there exists a first moment in the future such that holds, and the time to wait for this first moment belongs to.
We now consider this formula over timed words and over signals. We consider timed words first. Assume that where and represents either open or closed bounds. Let a timed word and in its domain of definition.
Over timed words, the formula holds if and only if also holds. That is, this formula simply assert that, in the future, until the interval is met, should not hold. Furthermore, should hold sometime in the interval. Indeed, given any time such that hold, there exists only a finite number of time with and. Thus, there exists necessarily a smaller such.
Let us now consider signal. The equivalence mentioned above does not hold anymore over signal. This is due to the fact that, using the variables introduced above, there may exists an infinite number of correct values for, due to the fact that the domain of definition of a signal is continuous. Thus, the formula also ensures that the first interval in which holds is closed on the left.
By temporal symmetry, we define the history operator, denoted by. We define as. This formula asserts that there exists a last moment in the past such that holded. And the time since this first moment belongs to.

Non-strict operator

The semantic of operators until and since introduced do not consider the current time. That is, in order for to holds at some time, neither nor has to hold at time. This is not always wanted, for example in the sentence "there is no bug until the system is turned-off", it may actually be wanted that there are no bug at current time. Thus, we introduce another until operator, called non-strict until, denoted by, which consider the current time.
We denote by and either:
For any of the operators introduced above, we denote the formula in which non-strict untils and sinces are used. For example is an abbreviation for.
Strict operator can not be defined using non-strict operator. That is, there is no formula equivalent to which uses only non-strict operator. This formula is defined as. This formula can never hold at a time if it is required that holds at time.

Example

We now give examples of MTL formulas. Some more example can be found on article of fragments of MITL, such as metric interval temporal logic.
A standard infinite word is a
function from to. We can
consider such a word using the set of time,
and the function. In this case,
for an arbitrary LTL
formula, if and only
if, where is
considered as a MTL formula with non-strict operator and subscript. In this sense, MTL is an extension of LTL.
For this reason, a formula using only non-strict operator with subscript is called an LTL formula. This is because the

Algorithmic complexity

The satisfiability of ECL over signals is EXPSPACE-complete.

Fragments of MTL

We now consider some fragments of MTL.

MITL

An important subset of MTL is the Metric Interval Temporal
Logic. This is defined similarly to MTL, with the
restriction that the sets, used in and, are intervals which are not
singletons, and whose bounds are natural numbers or infinity.
Some other subsets of MITL are defined in the article MITL.

Future Fragments

Future-MTL was already introduced above. Both over timed-words and over signals, it is less expressive than Full-MTL.

Event-Clock Temporal Logic

The fragment Event-Clock Temporal Logic of MTL, denoted EventClockTL or ECL, allows only the following operators:
Over signals, ECL is as expressive as MITL and as MITL0. The equivalence between the two last logics is explained in the article MITL0. We sketch the equivalence of those logics with ECL.
If is not a singleton and is a MITL formula, is defined as a MITL formula. If is a singleton, then is equivalent to which is a MITL-formula. Reciprocally, for an ECL-formula, and an interval whose lower bound is 0, is equivalent to the ECL-formula.
The satisfiability of ECL over signals is PSPACE-complete.

Positive normal form

A MTL-formula in positive normal form is defined almost as any MTL formula, with the two following change:
Any MTL formula is equivalent to formula in normal form. This can be shown by an easy induction on formulas. For example, the formula is equivalent to the formula. Similarly, conjunctions and disjunctions can be considered using De Morgan's laws.
Strictly speaking, the set of formulas in positive normal form is not a fragment of MTL.