Subject: [NEXTEL-1] RE: 800 MHz interference - Public Safety's 'Silent Danger

From: Marcelrf <[email-masked]>
Date: Tue, 30 Mar 2004 20:35:04 -0500

Yes, we did and we (I thought) agreed to disagree. With the many systems
migrating to digital it will become more and more critical to have
another measurement methodology beside RSSI . The Cellular world has
moved forward why can't Public Safety?  Bernie made it very clear in his
document to TIA how important it was. RSSI alone is an useless barometer
to measure interference in a many systems. The majority of interference
imposed on public Safety Systems is digital it would be best to be able
to speak their language. I guess this will be another loop hole in the
interference scenario. Remember your position a few years from now
because I will.

Cheers,
Marcel

"Buchanan, David" wrote:

> Marcel - We went over that - few systems are digital and the method in
> appendix F will establish the signal level to trigger action
> regardless of analog or digital systems operated by public safety.BER
> adds little value for this process.
>
> -----Original Message-----
> From: Marcelrf [mailto:[email-masked]]
> Sent: Tuesday, March 30, 2004 1:55 PM
> To: [email-masked]; WFHSG
> Subject: [WFHSG] RE: 800 MHz interference - Public Safety's 'Silent
> Danger
>
> [email-masked] wrote:
>
> One of the most important parts of the Consensus Plan is Appendix F,
> which specifically rewrites the interference rules to create a
> specific level of interference protection for systems from the effects
> being experienced.
>
> But Fails to take into account BER measurements.......
>

---

From: Rick Parrish <[email-masked]>
Date: Tue, 30 Mar 2004 19:56:34 -0600

Marcelrf wrote:

> Yes, we did and we (I thought) agreed to disagree. With the many 
> systems migrating to digital it will become more and more critical to 
> have another measurement methodology beside RSSI .

On 800, it's probably a trunked radio system in which case one channel 
per site is digital - the control channel. This makes BER is a valid 
metric even on an otherwise analog system  (just like it was for AMPS).

-rick

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From: Marcelrf <[email-masked]>
Date: Tue, 30 Mar 2004 21:51:36 -0500

It is a valid metric in many arenas of wireless. A most important figure of
merit for a data / voice digital radio product then is the error rate produced
by an adjacent user employing a similar transmission modulation method to that
employed. This information is normally conveyed by a Bit Error Rate signature
curve which shows the ratio of unwanted interfering signal level, to the
wanted data signal, for a given error rate . The BER signature shows the
measured signal power ratios for the error rate at a number of frequencies
offset from our own nominal channel frequency. The BER signature can give an
appreciation of the interference protection from adjacent channel users for
any frequency offset (or channel separation).  Using the BER signature curve
for a product and a propagation loss curve, you can see how far away a user on
the adjacent channel must be to produce a BER equal to that for which the BER
signature curve was based on.  Measurement of BER degradation due to adjacent
channel data modulation is carried out in the same manner as adjacent channel
desensitisation for voice radio equipment.
This is nothing new and is in fact an industry standard. Ignoring this just
makes the task of identifying interference more arduous.
As I have said many times the 1st sign of interference on an iDEN network is
HIGH RSSI and a low SQE (BER,C/I, ect.).

 
Yahoo! Groups Links

---

From: "Sean O'Hara" <[email-masked]>
Date: Wed, 31 Mar 2004 08:59:09 -0500

C'mon here...

For NARROWBAND communications in a frequency-flat fading channel, BER is a
direct function of  S/(sum(I)+N).  Public safety narrowband (25 kHz or less)
communications can be considered to be affected by only flat fading
conditions.  Therefore BER measurements bring little to the picture for
narrowband cases, and nothing to the picture for analog cases.  Delay spread
is not an issue here.

We all discussed BER degradation at length (this was our first thought
actually), and settled on signal strength measurements because they are (1)
easy to do and (2) cheap, requiring relatively simple equipment.  These
factors combine to help get interference situations resolved quickly.

The procedure, for those that do not know it is pretty simple.  If you have
enough signal to communicate within the area that you are receiving
interference (and enough is defined VERY generously for PS), then it is up
to the interferer(s) to remedy the situation.  Simple.  Easy.  Effective.
Fair.

Sean

----- Original Message -----
From: "Marcelrf" <[email-masked]>
To: <[email-masked]>; "WFHSG" <[email-masked]>
Sent: Tuesday, March 30, 2004 9:51 PM
Subject: Re: [NEXTEL-1] RE: 800 MHz interference - Public Safety's
'SilentDanger

 
Yahoo! Groups Links

---

From: Marcelrf <[email-masked]>
Date: Wed, 31 Mar 2004 15:30:51 -0500

Sean,

I disagree with your statement:

" Public safety narrowband (25 kHz or less) communications can be considered to
be affected by only flat fading conditions".

Here is why:

If the signal bandwidth is smaller than the channel coherent bandwidth, that is,
Bs < Bc, then the signal is faded in a similar manner for all the frequency, and
the fading is said to be frequency non-selective (aka  flat fading). When Bs >
Bc, the fading is said to be frequency-selective, which is related to multiple
paths. Multiple paths can be a source of interference (except in MIMO) for
non-diversity receivers such as those employed in public safety today.

Regarding your comment

"(1) easy to do and
(2) cheap, requiring relatively simple equipment."

 Bernie points out below

"The static BER% of the test receiver can be measured using normal test
equipment."

Let me add that using the criteria's "(1) easy to do and (2) cheap, requiring
relatively simple equipment." should not be a factor here. If a task is
difficult and requires complicated equipment it should be deferred to a more
qualified individual with the proper equipment.

So again I must disagree with you not only based on the statement in Bernie's
document but also from my past experiences and experiences of others.

=======================================================================

From: Determination of the Local Median Document

Bernie said:

Alternate 1) Including BER% measured values.

If the system is digital, both (C+I+N) RSSI and BER% can simultaneously be
captured.  This then provides addition data as to what the median C/(I+N) really
is.  This is more complex.  Different receivers of the same model and
manufacturer may have slightly different characteristics.

The static BER% of the test receiver can be measured using normal test
equipment.  The difference between the static C/N and faded C/N for the
necessary faded BER% can be used.  For example, if the 5% static BER is -120 dBm
and the 2% static BER% is 2.2 dB higher (-117.8 dBm) then the C/N for 2% static
BER is 2.2 dB greater than the value in TSB88A, Table A-1.  If the modulation is
C4FM that would be 7.6 dB + 2.2 dB = 9.8 dB.  The value of faded 2% BER is 17.7
dB, 7.9 dB above the static value.  Thus the C for 17.7 dB requires a desired of
-117.8 dBm + 7.9 = -109.9 dBm.

If the criterion is a 20 dB C/(I+N) The 1% faded BER can be used for C4FM.

The process of determining if 1% BER can be achieved in the local area will
determine if the desired C/(I+N) is being achieved.

This test provides a better understanding of the interference mechanism.  If
(C+I+N) is large, but the BER% increases, the (I+N) component is increasing.
===============================================================================

 As I said to David in my previous email  "I think what we have here is a
typical case of "Old School v. New School"

So there its is.......................

Cheers,

Marcel