FPGA Central

Hi,
In the case of Single Frequency Network, several
transmitters simultaneously send the same signal
over the same frequency channel.
The transmitters and receivers are usually synchronized
with the others, using GPS or a signal from the main station.
But, How is the problem of sending the data always at
the precise/accurate frequency handled between the
user equipment and the main stations ?
Any threshold limit is present ? What is the correction
mechanism/equalizer available for it ?

Thx in advans,
Karthik Balaguru

karthikbalaguru wrote:

> Hi,
> In the case of Single Frequency Network, several
> transmitters simultaneously send the same signal
> over the same frequency channel.

It depends.

> The transmitters and receivers are usually synchronized
> with the others,

It depends.

> using GPS or a signal from the main station.

It depends.

> But, How is the problem of sending the data always at
> the precise/accurate frequency handled between the
> user equipment and the main stations ?

It depends )))
What is your question in particular?

> Any threshold limit is present ?

Threshold of what?

> What is the correction
> mechanism/equalizer available for it ?

Available for what? Is it rake receiver that you are asking about?


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

On Dec 2, 10:24*am, Vladimir Vassilevsky <nos...@nowhere.com> wrote:
> karthikbalaguru wrote:
> > Hi,
> > In the case of Single Frequency Network, several
> > transmitters simultaneously send the same signal
> > over the same frequency channel.
>
> It depends.
>
> > The transmitters and receivers are usually synchronized
> > with the others,
>
> It depends.
>
> > using GPS or a signal from the main station.
>
> It depends.
>
> > But, How is the problem of sending the data always at
> > the precise/accurate frequency handled between the
> > user equipment and the main stations ?
>
> It depends )))
> What is your question in particular?
>

Managing the SFN under conditions
that might result in shift of frequencies.

> > Any threshold limit is present ?
>
> Threshold of what?
>

Threshold limit for the deviation in the
precise frequency .

> > What is the correction
> > mechanism/equalizer available for it ?
>
> Available for what? Is it rake receiver that you are asking about?
>

Equalizers to take care of the frequency
variations.

Thx in advans,
Karthik Balaguru

On 12/2/2009 7:28 AM, karthikbalaguru wrote:
> On Dec 2, 10:24 am, Vladimir Vassilevsky<nos...@nowhere.com> wrote:
>> karthikbalaguru wrote:
>>> Hi,
>>> In the case of Single Frequency Network, several
>>> transmitters simultaneously send the same signal
>>> over the same frequency channel.
>> It depends.
>>
>>> The transmitters and receivers are usually synchronized
>>> with the others,
>> It depends.
>>
>>> using GPS or a signal from the main station.
>> It depends.
>>
>>> But, How is the problem of sending the data always at
>>> the precise/accurate frequency handled between the
>>> user equipment and the main stations ?
>> It depends )))
>> What is your question in particular?
>>
>
> Managing the SFN under conditions
> that might result in shift of frequencies.
>
>>> Any threshold limit is present ?
>> Threshold of what?
>>
>
> Threshold limit for the deviation in the
> precise frequency .
>
>>> What is the correction
>>> mechanism/equalizer available for it ?
>> Available for what? Is it rake receiver that you are asking about?
>>
>
> Equalizers to take care of the frequency
> variations.
>
> Thx in advans,
> Karthik Balaguru
>

What is the context of your question? DVB-T was architected to support
SFN, and the answer in that context may be different than the context of
some other signal. Do it's difficult to address your questions without
a tighter context.

For the DVB-T case, the general idea is that the additional signals just
look like more multipath reflections. The demodulator then doesn't have
to do anything special as long as the total effective delay spread is
within the window of the cyclic prefix. Since DVB-T is a broadcast
system and it is assumed that the antennas on each end of the link are
fixed, the synchronization issues are not affected very much.


--
Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.abineau.com

Eric Jacobsen wrote:


> What is the context of your question? DVB-T was architected to support
> SFN, and the answer in that context may be different than the context of
> some other signal. Do it's difficult to address your questions without
> a tighter context.

I thought of simulcast paging systems.

> For the DVB-T case, the general idea is that the additional signals just
> look like more multipath reflections.

It is worse then multipath as there is significant overlap zone where
the fields of several transmitters have comparable strength. That
results in deep narrowband fading pattern.

> The demodulator then doesn't have
> to do anything special as long as the total effective delay spread is
> within the window of the cyclic prefix.

This is not quite right. Cyclic preffix solves ISI problem from one
source, however several overlapping sources is a different picture.
Also, there is not much of cyclic preffix in DVB-T.

> Since DVB-T is a broadcast
> system and it is assumed that the antennas on each end of the link are
> fixed, the synchronization issues are not affected very much.

There are two separate issues with different effects on SFN:
synchronization of the modulation and synchronization of carrier frequency.

Anyway, SFN is very unoptimal and better be avoided if possible.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com