[转] What is Ec/Io (and Eb/No)?

PS:http://www.telecomhall.com/what-is-ecio-and-ebno.aspx

If someone asks you "Which Signal Level for good call quality: -80 dbm or -90 dBm?"

Beware, if you respond quickly, you might end up missing. This is because the correct answer is ... it depends! The Signal Strength is a very important and essential measure for any technology (GSM, CDMA, UMTS, LTE, etc.). However, it is not the only one: let's talk a little today about another magnitude, equally important: the Signal Noise Ratio.

Although this ratio is of fundamental importance to any cellular system, is not well understood by many professionals. On the opposite side, professionals with a good understanding of this ratio are able for example, to correctly assess the RF links, and also to perform more extensive optimizations, obtaining the best possible performance of the system.

So, let's see a little about it?

Eb and No

To begin, we define the basic concepts of Eb and No. They are basic for any digital communication system, and generally we talk about it when we deal with Bit Error Rate and also Modulation techniques.

Simply put:

• Eb: Bit Energy.
  • It represents the amount of energy per bit.
• No: Noise Spectral Density.
  • Unit: Watts/Hz (or mWatts/Hz)

Which brings us to the classic definition of Eb/No:

• Eb/No: Bit Energy on the Spectral Noise Density.
  • Unit: dB

It did not help much, does it?

Do not worry. Indeed, only with the theoretical definition is still very difficult to see how this ratio is used, or how it can be measured.
But okay, let's walk a little further.

Okay, so how is Eb/No measured?

To understand how this ratio can be measured, let's imagine a simple digital communication system.

The ratio Eb/No is measured at the receiver, and serves to indicate how strong the signal is.

Depending on the modulation technique used (BPSK, QPSK, etc.) we have different curves for Bit Error Rate x Eb/No.

These curves are used as follows: for a
certain RF signal, which is the bit errors rate that I have? Is this bit
error rate acceptable for my system?

Whereas the gain that digital has, then
we can set a minimum criterion of signal to noise ratio, in order to
have each service (Voice/Data) operating acceptably.

In other words, we can theoretically determine how the performance would be for the digital link.

Note: it is worth remembering here that
this is a very complex subject. As always, we try to introduce to you
the most simplified possible through the use of examples and simple
concepts. Okay?

For example, a concept that could be
explored here - since we are talking about digital communication system -
is the Noise Figure. But we do not want to repeat here all the theory
explained in the University. Nor was it to have mentioned the noise
figure here, but as we talked about it, just understand as a noise level
that every receiver has, and that it is due to the process of
amplification and processing of signal.

Concepts like this, and other even more
complex, can be studied, if you wish. But now, let's continue with our
signal to noise ratio.

Eb/No -> Ec/Io

The concept of Eb/No applies to any
digital communication system. But today we are talking specifically to
Ec/Io, which is a measure of evaluation and decisions of CDMA and UMTS.

Note: all the technology uses signal-interference ratio. For example, in GSM, we use C/I.

As we are speaking of codes, it becomes
easier to understand the concepts by observing a simplified diagram of
Spread Spectrum Modulation.

In red, in transmitter have a narrowband
signal with data or voice modulated. This signal is spread and
transmitted. And spreads through the middle (air). In the receiver, the
signal is despread - using the same sequence that was spread - and thus
recovering the base narrowband signal.

To proceed, we must know some more
definitions. However, this point is quite delicate, as we enter a
conceptual area where we have differences between authors, differences
in translations/countries, where differences in technologies are
applied, etc..

Let's try to define in a generic way, and only the main.

• No: Spectral Density of Noise;
  • Noise generated by the RF components of the system, the air, among others.
• Io: Interference is the Broadband; Interfering co-channel, including yourself setor.
• E: is the signal (average) energy - do not confuse it with the sinal (average) power.
• b, c, s. ..: Energy
  are the power points in time, therefore related to the measure or
  'length' of the time (the average power is independent of time ).
  • Hence it comes Eb, Ec and Es, respectively relating to Bit Chip and Symbol in different times.

Note: With these concepts, several
formulas can be derived with different numerators and denominators. For
example, Es = Eb * k, where k = number of bits per symbol. In QPSK
modulation, where k = 2, Es = 2 * Eb. And the derivations of formulas
can reach far more complex equations, such as the definitions of
capacity of an AWGN channel, and further deductions for equivalences
(Ec/No, Eb/Nt, etc. ...). Again, it is not our purpose here today. We
only mention a few concepts, related.

Then come back to the practical level -
noting that theoretical approaches can be done more easily later, after
the basics are understood.

So let's keep today in ratios most common: Eb/No and Ec/Io.

As we defined Eb/No is
the Average Energy of a bit signal, on the Spectral Density of Noise. It
is primarily a parameter related to the manufacturer for different
bearers (based on the channel model). But it can also vary with the
environment (urban, rural, suburban), speed, diversity, use of power
control, application type, etc..

And now we can begin to define Ec/Io, one of the most important systems in CDMA and UMTS.

Note: An important observation is that
often when we refer to Ec/Io, we are actually referring to Ec/(Io + No).
What happens is that for practical purposes, we only have Ec/Io,
because the interference is much stronger and the noise can be
neglected. Otherwise: for CDMA interference is like a noise, then both
can be considered the same thing.

Okay, let's stop with the issues and concepts, and talk a little about the values of these indicators and their use in practice.

Eb/No Positive and Ec/Io Negative?

In terms of values, and talking
logarithmicly, if any ratio is less than 1, then the value is negative.
If greater than 1, positive.

We have Ec/Io in the air, which is
spread across the spectrum: then we have negative value to the ratio of
energy on the total noise (the energy is lower than the Total
Interference). It is measured at the input of receiver (NodeB, UE, etc).

Regarding Eb/No, it is in the baseband
after despreading and decoded only for one user - then we have a
positive amount of energy over the total noise. It is measured at the
output of receiver (NodeB, UE, etc).

Why should we use Ec/Io?

A more natural question would be: why we
can not simply use the Signal Strength measured by the mobile as a
guide for operations such as handover?

The answer is simple: the measured signal level corresponds to the Total RF power - All cells that the mobile sees.

So we need another quick and simple measure that allows us to evaluate the contribution of each sector individually.

We used to measure the pilot channel
signal of each sector to assess the quality: if the level of the pilot
is good, then also are good levels for the traffic channels for our call
in this sector. Likewise, if the pilot channel is degraded, so will the
other channels (including traffic) be, and it is best to avoid using
the traffic channels in this sector.

UMTS and CDMA systems, we have a pilot channel, some other control channels such as paging, and traffic channels.

The Ec/Io varies with several factors, such as the Traffic Load and and RF Scenario.

Of course, the Ec/Io is the final
composition of all these factors simultaneously (Composite Ec/Io), but
it's easier to understand talking about each one separately.

Change in Ec/Io according to the Sector Traffic Load

Each sector transmits a certain power.
Suppose in our example we have a pilot channel power setting of 2 W, and
a power of other control channels also fixed at 2 W.

To make it easier to understand, we
calculate the Ec/Io (pilot channel power to total power) of this sector
in a situation where we have no busy traffic channel (0 W).

Thus we have:

Ec = 2 W

Io = 0 + 2 + 2 = 4 W

Ec/Io = (2/4) = 0.5 = -3 dB

Now assume that several traffic channels
are busy (eg use 6 W for traffic channels). This is a situation of
traffic load, we'll see how is Ec/Io.

Ec = 2 W

Io = 2 + 2 + 6 = 10 W

Ec/Io = (2/10) = 0.2 = -7 dB

Conclusion: As the traffic load in the sector increases, the Ec/Io worsens.

Change in Ec/Io according to the scenario RF

According to the RF scenario - a single
server sector, some or many servers sectors - we can also take various
measures to Ec/Io.

Considering first a situation without
external interference, with only one server sector (dominant), the ratio
Ec/Io is about the same initially transmitted.

Ec/Io = (2/8) = 0.25 = -6 dB

Whereas a signal coming from this sector
in the mobile at level of -90 dBm (Io = -90 dBm), we have Ec = -90 dBm +
(- 6 db) = -96 dBm.

Let us now consider another situation. Instead of one, we have five
sectors signal arriving at the mobile (for simplicity, all with the same
level of -90 dBm).

Now have Io = -83 dBm (which is the sum
of five signals of -90 dBm). And the power of our pilot channel remains
the same (Ec = -96 dBm).

Thus: Ec/Io = -96 - (-83) = -13 dB

Conclusion: As many more sectors serves the mobile, the Ec/Io worsens.

This situation where we have many
overlapping sectors, and with the same level of signal is known as Pilot
Pollution - the mobile sees them all at once - each acting as
interferer to each other.

The solution in such cases is to
eliminate unwanted signals, by setting power parameters or physical
adjustments (tilt, azimuth), leaving just dominant signals which should
exist at this problematic place.

Okay, and what are typical values?

We have seen that for CDMA and UMTS
systems, the measurement of Ec/Io which is very important in the
analysis, especially in handover decisions.

And now also understand the measure Ec/Io as the ratio of 'good' energy over 'bad' energy, or 'cleaness' of signal.

But what are the practical values?

The value of Ec/Io fluctuates (varies),
as well as any wireless signal. If the value starts to get too low, you
start to have dropped calls, or can not connect. But what then is a good
range of Ec/Io for a sign?

In practical terms, values of Ec/Io for a
good evaluation of the network (in terms of this indicator) are shown
in the diagram below.

A composite Ec/Io ~ - 10 db is a reasonable value to consider as good.

Note: See we are talking about negative
values, and considering them 'good'. In other words, we are saying that
energy is below the Noise (and still have a good situation).

This is a characteristic of the system
itself, and Ec/Io 'most negative' or 'less negative' is going to allow
assessment of the communication.

In situations where Ec/Io is very low
(high negative number), and the signal level too (also high negative
number), first we need to worry in enhancing the weak signal.

Another typical situation: if the
measured Ec/Io is very low, even if you have a good signal level, you
can not connect, or the call will drop constantly.

I hope you've managed to understand how
the Ec/Io is important for CDMA and UMTS. Note, however, that this
matter is very complex, and supplementary reading - books and internet -
can further help you become an expert on the subject.

Anyway, the content displayed serves as
an excellent reference, especially if you're not familiar with the
concept of signal over noise for CDMA and UMTS.

And the Signal to Noise Ratio for other technologies?

The ratio Ec/Io is the most commonly
used to assess the condition of energy over interference, but applies
only in technologies that use codes (Ec).

But the concepts understood here to CDMA and UMTS are very similar - apply - for any technology, eg GSM, where we use the C/I.

Anyway, this is a topic for another tutorial, we saw today Ec/Io.

Conclusion

Today we had a brief introduction on the
Ec/Io ratio, a measurement for decisions in CDMA and UMTS, and used
togheter with the measured Signal Strength.

We have seen that it represents the
ratio of signal energy within the duration of a chip of the pilot
channel, on the Spectral Density of Noise + Interference.

This is a very important measure, which
somehow ignores the overall strength of the signal, and focuses on how
best to evaluate the pilot channel signal is desired, in relation to
noise that interferes with it.

Returning to our original question: A
strong signal level does not necessarily indicate an strong Ec/Io: it
depends on the level of interference.