siae dynamic modulation wp

7/30/2019 Siae Dynamic Modulation WP

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Dynamic/Adaptive Modulation Page 1 of 8

Dynamic / Adaptive Modulation

______________________

White Paper

Siae Microeletttronica SpA

February 2007


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INDEX

Dynamic / Adaptive modulation...................................................................................... 3

General Concepts .......................................................................................................... 3

Conditions for Dynamic/Adaptive modulation. ......................................................... 4Traffic classification ................................................................................................... 4

Link Quality measurement.......................................................................................... 4

Modulation change...................................................................................................... 5

SIAE implementation of dynamic/adaptive modulation........................................... 5Dynamic modulation................................................................................................... 5

Adaptive modulation................................................................................................... 6


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Dynamic / Adaptive modulation

General Concepts

A radio communication systems have to be designed with a nominal received level well

above its threshold. This allows received signal reduction in bad weather conditions.

This procedure has a drawback: when propagation conditions are favourable the systemcould support higher data traffic but it cannot increase its throughput unless it change

modulation scheme.

Modern radio systems can do that. In SIAE implementation this is named Adaptive or

Dynamic modulation. The differences between those terms will be clarified in the

following.In order to understand what Dynamic/Adaptive means, we should recall some

information from signals theory:

BER)specified(atdensityspectralbit/NoiseperEnergy0

figurenoiseReceiver

rateBit

10BERatfieldReceived

0)(10114

3

3

3

3

10

3

10

10

1010

=

=

=

==

+++=

N

Eb

NF

Br

Ps

N

EbNFBrLOGPs

-

We can discover that minimum received signal, able to guarantee certain performances

(BER), depends on some fixed figures, such as required bit rate(Br) and demodulator

implementation

0N

Eb.

If a receiver is working at such a level that cannot allow the required quality, somecountermeasures can be activated.

- Increasing TX power on remote terminal. This can obviously be done, iftransmitter is not jet emitting its maximum power.

- Reducing user bit rate

- Changing modulation scheme and FEC redundancy

SIAE implementation does them all.


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In addition, SIAE maintain channel width, constant with no effect on other links, without

any need to reconfigure entire network.In our implementation, if the selected modulation requires less back off, we are able to

give to our customers, some extra dB of system gain.

The following table summarizes performances of different modulations schemes.All data have been normalized to 4QAM.

User Bit rate

3100

N

Eb

Tx power System gain

4QAM 1 0 0 0

16QAM 2 +4 -4 -10

32QAM 2.5 +6 -5 -11

128QAM 3.5 +10 -6 -16

A system using, for example 128QAM will increase its system gain by 16 dB if it

switches to 4QAM, at the same time its transport capability will be reduced 3.5 times.

Conditions for Dynamic/Adaptive modulation.In order to allow a radio system to manage its quality in a successful way some

conditions have to be verified.

Traffic classification

In the previous example, traffic is reduced by a factor of 3.5 when 128QAM to 4QAM

switch takes place.All traffic exceeding 4QAM capacity cannot be carried anymore.

This means that traffic must be classified in at least two classes. One , high priority class,

that will be transferred anyway and a low priority class that will be carried only if

enough bandwidth is available.

Siae implementation differs for PDH and ETH applications.

- PDH E1 ports are manually configured for being high or low priority.

- ETH packets are usually classified according to 802.1q, Siae systems are also

able to classify them, according to IP TOS. This allows real time traffic to be

always transmitted.

Link Quality measurement

In order to trigger a modulation change some switch criteria must be considered.

- Mean Square Error. Noise level detected at demodulator input. This will allow the

system to counteract reduction of receive field, as well as an interferer.


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- Receive signal level. It is the simplest criterion, but it is not able to react to

interference.

- BER . It is a classical criterion and reacts to rx field as well as to interferers. Usingthis criterion only it is very difficult achieving error free switch. Errors tend to

propagate outside equipments before modulation switch.

Modulation change.

Modulator and demodulators must be able to switch configurations, in a very short time.

This will allow to have small traffic interruptions that anyhow have to be checked in the

application network.

Siae implements on its actual radios two kinds of Adaptive/Dynamic modulation

schemes.

SIAE implementation of dynamic/adaptive modulation

Dynamic modulation

It is available as SW upgrade on existing AL radios and for PDH traffic only.

It implements a fast modulation switch with very short traffic interruptions (150 mS

Typical) on surviving E1s.

This limit is usually considered to be well below the maximum link interruption that a

GSM network can tolerate.

It is based on Receive signal level that via the RTPC loop enables a second criterion, that

is based on estimated BER.It involves both directions at the same time.

SIAE Dynamic Modulation implementation figures are here below specified:

High order modulation 16QAM

Low order modulation 4QAM

Channel BW 3.5 , 7, 14, 28 MHz

BER Threshold 10-9

Minimum modulation transition Time 1..256 Sec (Configurable)

Traffic interruption 150mS typical, 200mS guaranteed

Jitter Hits on TDM interfaces 30S

PDH traffic classification Manual

Number of managed priority classes 2


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The following picture explains how modulation switches take place.

In order to clarify the process, let suppose that the system is operating using 16QAM withvery good receive level (point 0).

As received level decreases the operating point starts moving rightward .

When operating point reaches mark 3, transmitter is at maximum power level, this will

enable modulation downshift.

If the received signal continues to decrease the system will reach point 4 where

modulation downshift takes place. After that, the transmitter will increase again its power

(4QAM requires less back off) and will reach working point 6.

Adaptive modulation

It is available now on current ALFO product line (Full Outdoor Radio). It will beavailable on next releases of AL (Split Mount) family.

It is based on MSE measurements that allows the system to react to any source of

degradation, well before errors are detected by the FEC.

It involves obviously one direction at a time and the process is completely error free for

surviving traffic.


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In current ALFO series, traffic classification uses IP TOS, whilst PDH traffic, if

configured, is always considered as high priority. New AL family also allows

customized E1 priority/ classification.

SIAE Adaptive Modulation implementation figures are here below specified:

High order modulation 16QAM or 32QAM or 128QAM(*)

Low order modulation 4QAM

Channel BW 3.5 7 14 28 MHz

MSE Threshold Configurable

Minimum modulation transition Time 200 mS

Traffic interruption 0 (Hitless)

Jitter Hits on TDM interfaces According to G823

Delay variation on TDM interfaces 0

PDH traffic classification Fixed (highest priority)ETH traffic classification 802.1q or IP TOS

Number of managed priority classes 4

(*) 128QAM available on next AL series


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The receiver is continuously monitoring MSE, if it falls below a configured limit, a

modulation switch request is transmitted backward.

Modulator will react to the request, changing to the required modulation and sending

back a modulation switch signal.

The remote receiver will detect the ongoing change and will preset itself for the new

modulation. This will allow it to change (that takes place some microseconds after the

alerting signal) on the fly.

siae dynamic modulation wp

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