power spectral density as well as all finite-
order joint moments.
ergodicity
stochastic processes for which
ensemble averages can be replaced by tem-
poral averages over a single realization are
said to be ergodic.
For a stochastic pro-
cess to be ergodic, a single realization must
in the course of time take on configurations
closely resembling the entire ensemble of
processes. Stationary filtered white noise is
considered ergodic, while the sinusoidal pro-
cess
A cos(wt + φ) with random variables A
and
φ is not.
Erlang B
a formula (or mathematical
model) used to calculate call blocking prob-
ability in a telephone network and in partic-
ular in cellular networks. This formula was
initially derived by A. K. Erlang in 1917, a
Danish pioneer of the mathematical model-
ing of telephone traffic, and is based on the
assumption that blocked calls are forever lost
to the network. See also
Erlang C
.
Erlang C
similar to Erlang B, the for-
mula is based on a traffic model where the
call arrival process is modeled as a Poisson
process, the call duration is of variable length
and modeled as having an exponential dis-
tribution. The system is assumed to have a
queue with infinite size that buffers arriving
calls when all the channels in the switch are
occupied. The model is based on the assump-
tion that blocked calls are placed in the queue.
Erlang capacity
maximum number of
users in the system which leads to the maxi-
mum allowable blocking probability (for ex-
ample, 2%).
erosion
an important basic operation in
mathematical morphology. Given a structur-
ing element
B, the erosion by B is the oper-
ator transforming
X into the Minkowski dif-
ference
X B, which is defined as follows:
1. If both
X and B are subsets of a space E,
X B = {z ∈ E | ∀b ∈ B, z + b ∈ X}
2. If
X is a gray-level image on a space E and
B is a subset of E, for every p ∈ E we have
(X B)(p) = inf
b∈B
X(p + b)
3. If both
X and B are gray-level images on
a space
E, for every p ∈ E we have
(X B)(p) = inf
h∈E
X(p + h) − B(h)
with the convention
∞ − ∞ = +∞ when
X(p + h), B(h) = ±∞. (In the two items
above,
X(q) designates the gray-level of the
point
q ∈ E in the gray-level image X.) See
dilation
,
structuring element
.
ERP
See
effective radiated power
.
error
(1) manifestation of a fault at log-
ical level. For example, a physical short or
break may result in logical error of stuck-at-0
or stuck-at-1 state of some signal in the con-
sidered circuit.
(2) a discrepancy between a computed,
observed, or measured value or condition and
the true, specified, or theoretically correct
value or condition. See
bug
,
exception
.
error control coding
See
channel coding
.
error-correcting code (ECC)
code used
when communication data information in and
between computer systems to ensure correct
data transfer. An error correcting code has
enough redundancy (i.e., extra information
bits) in it to allow for the reconstruction of
the original data, after some of its bits have
been the subject of error in the transmission.
The number of erroneous bits that can be re-
constructed by the receiver using this code
depends on the Hamming distance between
the transmitted codewords. See also
error
detecting code
.
error correction capability
of a code is
bounded by the minimum distance and for an
(n, k) block code, it is given by t = [(d
min
−
c
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