Newer Model Instrumentation Amp
in [Design]
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From: krw on 24 Jun 2010 19:36 On Thu, 24 Jun 2010 07:59:55 +0300, Paul Keinanen <keinanen(a)sci.fi> wrote: >On Wed, 23 Jun 2010 17:59:39 -0500, "krw(a)att.bizzzzzzzzzzzz" ><krw(a)att.bizzzzzzzzzzzz> wrote: > >> >>>A helmet is sufficiently large for spatial diversity or even MIMO >>>systems down to lower UHF and possibly even upper VHF. >> >>Spacial diversity doesn't help. > >If you have severe multipath problems, one of the worst modulations is >narrow band AM/FM, since it is quite likely that a single antenna is >at the multipath null, where the multipath signals cancel each other >completely. The time difference off the ceiling and the direct route is almost the same as our symbol rate. Things get ugly fast. Without the ceiling things are great. ;-) >The area at which the signals cancel each other is quite small and >gets smaller with higher frequencies. Having two or more antennas at >some distance from each other (e.g. on different part of your body), >it is unlikely that _all_ those antennas would be in multipath nulls. >With multiple antennas and multiple receivers, just sum the audio >outputs, but squelch those receivers that have a weak (noisy) signal >at the moment. It's not a matter of nulls, rather the receivers can't lock onto the signal. >Of course some more sensible modulation method, such as DSSS, TDMA or >OFDM would make more sense in an environment with heavy multipath >even with a single antenna. I think the radios are DSSS, with TDMA networking protocol. The receivers can't stay locked to the transmitters so they constantly drop off line. Doesn't happen at 900MHz.
From: Paul Keinanen on 25 Jun 2010 02:38 On Thu, 24 Jun 2010 18:36:09 -0500, "krw(a)att.bizzzzzzzzzzzz" <krw(a)att.bizzzzzzzzzzzz> wrote: >On Thu, 24 Jun 2010 07:59:55 +0300, Paul Keinanen <keinanen(a)sci.fi> wrote: > >>On Wed, 23 Jun 2010 17:59:39 -0500, "krw(a)att.bizzzzzzzzzzzz" >><krw(a)att.bizzzzzzzzzzzz> wrote: >> >>> >>>>A helmet is sufficiently large for spatial diversity or even MIMO >>>>systems down to lower UHF and possibly even upper VHF. >>> >>>Spacial diversity doesn't help. >> >>If you have severe multipath problems, one of the worst modulations is >>narrow band AM/FM, since it is quite likely that a single antenna is >>at the multipath null, where the multipath signals cancel each other >>completely. > >The time difference off the ceiling and the direct route is almost the same as >our symbol rate. Things get ugly fast. Without the ceiling things are great. >;-) So we are talking about 1-3 MHz, is this the actual data rate or the spread spectrum chip rate ? At 2.45 GHz ? In a SS system, the spreading chip rate should be several percent of the absolute frequency to be effective against multipath. Regardless modulation method, some general precautions against multipath: Is it possible to install the base station antenna on the inside of the dome itself ? This would generate a strong direct (dome, field) signal compared to reflected (dome, field, dome, field). If the base station antenna can not be installed in the dome itself but it has to be installed at ground level or somewhere in the audience, at least make sure that the radiation pattern above the horizontal plane is heavily attenuated, reducing power radiated into the dome (or received from the dome). A metallic shield (or mesh) above the antenna might be usable, but of course, it creates early reflections from the shield, which may be (or not) harmful depending on the modulation method. > >>The area at which the signals cancel each other is quite small and >>gets smaller with higher frequencies. Having two or more antennas at >>some distance from each other (e.g. on different part of your body), >>it is unlikely that _all_ those antennas would be in multipath nulls. >>With multiple antennas and multiple receivers, just sum the audio >>outputs, but squelch those receivers that have a weak (noisy) signal >>at the moment. > >It's not a matter of nulls, rather the receivers can't lock onto the signal. Don't lock at all or sometimes drops out of lock ? How long does it take to relock ? Is this time longer than an other antenna with separate receiver typically maintain lock with a moving receiver ? There are several ways to implement spatial diversity, e.g. on the base band level switching between full receivers or simply RF antenna switching when the received power drops on current antenna. To speed up relocking one might also notice that since at least one receiver usually remains locked, this information can be used to limit search options on the unlocked receiver.
From: krw on 25 Jun 2010 19:08 On Fri, 25 Jun 2010 09:38:54 +0300, Paul Keinanen <keinanen(a)sci.fi> wrote: >On Thu, 24 Jun 2010 18:36:09 -0500, "krw(a)att.bizzzzzzzzzzzz" ><krw(a)att.bizzzzzzzzzzzz> wrote: > >>On Thu, 24 Jun 2010 07:59:55 +0300, Paul Keinanen <keinanen(a)sci.fi> wrote: >> >>>On Wed, 23 Jun 2010 17:59:39 -0500, "krw(a)att.bizzzzzzzzzzzz" >>><krw(a)att.bizzzzzzzzzzzz> wrote: >>> >>>> >>>>>A helmet is sufficiently large for spatial diversity or even MIMO >>>>>systems down to lower UHF and possibly even upper VHF. >>>> >>>>Spacial diversity doesn't help. >>> >>>If you have severe multipath problems, one of the worst modulations is >>>narrow band AM/FM, since it is quite likely that a single antenna is >>>at the multipath null, where the multipath signals cancel each other >>>completely. >> >>The time difference off the ceiling and the direct route is almost the same as >>our symbol rate. Things get ugly fast. Without the ceiling things are great. >>;-) > >So we are talking about 1-3 MHz, is this the actual data rate or the >spread spectrum chip rate ? At 2.45 GHz ? The data rate is just under 1Mbps. We're using ~480Kbps of it, TDMA, IIRC. >In a SS system, the spreading chip rate should be several percent of >the absolute frequency to be effective against multipath. > >Regardless modulation method, some general precautions against >multipath: > >Is it possible to install the base station antenna on the inside of >the dome itself ? This would generate a strong direct (dome, field) >signal compared to reflected (dome, field, dome, field). It is (it's certainly not outside the dome, if that's what you mean). It's been tried in the ceiling, on the field, in the stands... We've tried directionals, omnis, amplified, padded, just about everything. One thing we haven't done is separating the transmit and receive antennas (so they can be padded differently). The solution, so far, is to abandon 2.4G, in favor or 900M, which works reasonably well (except for other system limitations - e.g. bandwidth). >If the base station antenna can not be installed in the dome itself >but it has to be installed at ground level or somewhere in the >audience, at least make sure that the radiation pattern above the >horizontal plane is heavily attenuated, reducing power radiated into >the dome (or received from the dome). A metallic shield (or mesh) >above the antenna might be usable, but of course, it creates early >reflections from the shield, which may be (or not) harmful depending >on the modulation method. ....been tried. We can get dome time just about anytime we need it. >>>The area at which the signals cancel each other is quite small and >>>gets smaller with higher frequencies. Having two or more antennas at >>>some distance from each other (e.g. on different part of your body), >>>it is unlikely that _all_ those antennas would be in multipath nulls. >>>With multiple antennas and multiple receivers, just sum the audio >>>outputs, but squelch those receivers that have a weak (noisy) signal >>>at the moment. >> >>It's not a matter of nulls, rather the receivers can't lock onto the signal. > >Don't lock at all or sometimes drops out of lock ? Drops, *frequently*. >How long does it take to relock ? Is this time longer than an other >antenna with separate receiver typically maintain lock with a moving >receiver ? Lock and negotiation takes longer than a single hop. I don't understand the second question. We are double-sending data, once for each antenna. The radios take care of that overhead (something we lose at 900M; see above). >There are several ways to implement spatial diversity, e.g. on the >base band level switching between full receivers or simply RF antenna >switching when the received power drops on current antenna. We do a dumb switch, once on each TDMA slot, currently. Actually, that would only help on the base. The mobile unit doesn't have any diversity. >To speed up relocking one might also notice that since at least one >receiver usually remains locked, this information can be used to limit >search options on the unlocked receiver. Don't think that's going to help us, at least directly.
From: Paul Keinanen on 26 Jun 2010 01:33 On Fri, 25 Jun 2010 18:08:27 -0500, "krw(a)att.bizzzzzzzzzzzz" <krw(a)att.bizzzzzzzzzzzz> wrote: >On Fri, 25 Jun 2010 09:38:54 +0300, Paul Keinanen <keinanen(a)sci.fi> wrote: > >>On Thu, 24 Jun 2010 18:36:09 -0500, "krw(a)att.bizzzzzzzzzzzz" >><krw(a)att.bizzzzzzzzzzzz> wrote: >> >>>On Thu, 24 Jun 2010 07:59:55 +0300, Paul Keinanen <keinanen(a)sci.fi> wrote: >>> >>>>On Wed, 23 Jun 2010 17:59:39 -0500, "krw(a)att.bizzzzzzzzzzzz" >>>><krw(a)att.bizzzzzzzzzzzz> wrote: >>>> >>>>> >>>>>>A helmet is sufficiently large for spatial diversity or even MIMO >>>>>>systems down to lower UHF and possibly even upper VHF. >>>>> >>>>>Spacial diversity doesn't help. >>>> >>>>If you have severe multipath problems, one of the worst modulations is >>>>narrow band AM/FM, since it is quite likely that a single antenna is >>>>at the multipath null, where the multipath signals cancel each other >>>>completely. >>> >>>The time difference off the ceiling and the direct route is almost the same as >>>our symbol rate. Things get ugly fast. Without the ceiling things are great. >>>;-) >> >>So we are talking about 1-3 MHz, is this the actual data rate or the >>spread spectrum chip rate ? At 2.45 GHz ? > >The data rate is just under 1Mbps. We're using ~480Kbps of it, TDMA, IIRC. I am still trying to understand the RF-characteristics of the signal. Is this direct TDMA like GSM mobile phone in which up to 8 handsets share a common 200 kHz RF channel and each handheld sending the data as a burst within the allocated time slot. This works well for GSM at 900 MHz and reasonably well at 1800 MHz, so with a significantly larger bandwidth it should work well on 2G4. Or are those individual signal time multiplexed into a single baseband and the modulate a single spread spectrum "carrier". How wide is the actual emission ? Asking in a different way, how many (non-overlapping) RF channels can be selected ? For SS, the spread signal (chip rate) should be one or two orders of magnitude faster than the actual data. >>In a SS system, the spreading chip rate should be several percent of >>the absolute frequency to be effective against multipath. >> >>Regardless modulation method, some general precautions against >>multipath: >> >>Is it possible to install the base station antenna on the inside of >>the dome itself ? This would generate a strong direct (dome, field) >>signal compared to reflected (dome, field, dome, field). > >It is (it's certainly not outside the dome, if that's what you mean). It's >been tried in the ceiling, on the field, in the stands... We've tried >directionals, omnis, amplified, padded, just about everything. One thing we >haven't done is separating the transmit and receive antennas (so they can be >padded differently). The solution, so far, is to abandon 2.4G, in favor or >900M, which works reasonably well (except for other system limitations - e.g. >bandwidth). > >>If the base station antenna can not be installed in the dome itself >>but it has to be installed at ground level or somewhere in the >>audience, at least make sure that the radiation pattern above the >>horizontal plane is heavily attenuated, reducing power radiated into >>the dome (or received from the dome). A metallic shield (or mesh) >>above the antenna might be usable, but of course, it creates early >>reflections from the shield, which may be (or not) harmful depending >>on the modulation method. > >...been tried. We can get dome time just about anytime we need it. A sports dome would be absolutely the last place I would use any ISM band for any professional communication :-). Setting up your system before the event and everything seems to work OK and there are plenty of SNR etc. Then the huge public is admitted into the dome, each carrying one or more ISM devices (Bluetooth, WLAN) etc. While with spread spectrum emissions, you do not have a discrete frequency channel free/occupied situation, but adding more and more spread spectrum devices into the same frequency band will gradually increase the background noise level and the SNR after despreading drops gradually, until the SNR drops too low and the communication fails. In the dome, the RF emitted from each device is bouncing around and finally absorbed by some soft tissue (the spectators). >>>>The area at which the signals cancel each other is quite small and >>>>gets smaller with higher frequencies. Having two or more antennas at >>>>some distance from each other (e.g. on different part of your body), >>>>it is unlikely that _all_ those antennas would be in multipath nulls. >>>>With multiple antennas and multiple receivers, just sum the audio >>>>outputs, but squelch those receivers that have a weak (noisy) signal >>>>at the moment. >>> >>>It's not a matter of nulls, rather the receivers can't lock onto the signal. >> >>Don't lock at all or sometimes drops out of lock ? > >Drops, *frequently*. Sounds very much like NBFM (12.5/25 kHz BW) on 1.3 GHz (23 cm) that I used a few decades ago. As the theory predicts, a simple ground reflection will create a comb filter like spectrum and for a specific narrow band signal channel, there are deep nulls at very close distance from each other. Moving just a meter and the signal drops several times. Stopping a car with a fixed antenna at traffic light would almost always cause stopping at the multipath null :-). The only thing that helped, is moving the antenna a few centimeters (spatial) or changing channel (frequency diversity). At highway speeds, the dropouts were so frequent (and short) that it did not affect the readability of the speech (equivalent to interleaving and ECC in digital communication). It still sounds that you are suffering from a narrow band signal with the RF field strength punctured by multipath nulls distributed close to each other all over your area of interest. Of course, since some data is lost, you must use quite a lot of error correction bits. Since you are only using about half of the available capacity, why not allocate the rest for ECC bits ? Since the multipath nulls create burst errors, interleaving should be used to convert burst errors to random errors that can then be corrected by ECC. The problem with interleaving is that it adds delay, which is annoying in two way conversation. As you said that the dropouts are frequent (assuming several each second), this also means that they are short, thus reducing the interleave delay needed. >>How long does it take to relock ? Is this time longer than an other >>antenna with separate receiver typically maintain lock with a moving >>receiver ? > >Lock and negotiation takes longer than a single hop. Those might be your worst problem. I am still not sure what the emission is like. Do you first lock into some direct sequence spread spectrum signal, then lock into TDMA frame and finally lock to the individual signal ? Or are each handset sending individual spread spectrum sequence during short time slots in a TDMA way ? >I don't understand the >second question. In a typical moving multipath situation, the signal is 80-95 % above threshold and only 5-20 % below threshold. In an ideal receiver, the recovery would be immediate and at least more than 80 % of the time the signal would be good even with a single receiver. With diversity reception with receivers, this time would be much closer to 100 %. However, if a receiver takes more than 50 % of the good signal time to relock, even two diversity receivers would not be enough to reach even close to 100 % service times.
From: krw on 26 Jun 2010 12:05
On Sat, 26 Jun 2010 08:33:03 +0300, Paul Keinanen <keinanen(a)sci.fi> wrote: >On Fri, 25 Jun 2010 18:08:27 -0500, "krw(a)att.bizzzzzzzzzzzz" ><krw(a)att.bizzzzzzzzzzzz> wrote: > >>On Fri, 25 Jun 2010 09:38:54 +0300, Paul Keinanen <keinanen(a)sci.fi> wrote: >> >>>On Thu, 24 Jun 2010 18:36:09 -0500, "krw(a)att.bizzzzzzzzzzzz" >>><krw(a)att.bizzzzzzzzzzzz> wrote: >>> >>>>On Thu, 24 Jun 2010 07:59:55 +0300, Paul Keinanen <keinanen(a)sci.fi> wrote: >>>> >>>>>On Wed, 23 Jun 2010 17:59:39 -0500, "krw(a)att.bizzzzzzzzzzzz" >>>>><krw(a)att.bizzzzzzzzzzzz> wrote: >>>>> >>>>>> >>>>>>>A helmet is sufficiently large for spatial diversity or even MIMO >>>>>>>systems down to lower UHF and possibly even upper VHF. >>>>>> >>>>>>Spacial diversity doesn't help. >>>>> >>>>>If you have severe multipath problems, one of the worst modulations is >>>>>narrow band AM/FM, since it is quite likely that a single antenna is >>>>>at the multipath null, where the multipath signals cancel each other >>>>>completely. >>>> >>>>The time difference off the ceiling and the direct route is almost the same as >>>>our symbol rate. Things get ugly fast. Without the ceiling things are great. >>>>;-) >>> >>>So we are talking about 1-3 MHz, is this the actual data rate or the >>>spread spectrum chip rate ? At 2.45 GHz ? >> >>The data rate is just under 1Mbps. We're using ~480Kbps of it, TDMA, IIRC. > >I am still trying to understand the RF-characteristics of the signal. > >Is this direct TDMA like GSM mobile phone in which up to 8 handsets >share a common 200 kHz RF channel and each handheld sending the data >as a burst within the allocated time slot. This works well for GSM at >900 MHz and reasonably well at 1800 MHz, so with a significantly >larger bandwidth it should work well on 2G4. That's how it works (base gets five slots, each mobile gets one). It works well except in certain locations, mostly domes. >Or are those individual signal time multiplexed into a single baseband >and the modulate a single spread spectrum "carrier". No, you had the networking layer pretty much right, above. I don't have the modulation details in front of me (it's not our radio). >How wide is the actual emission ? Asking in a different way, how many >(non-overlapping) RF channels can be selected ? FHSS over 43 channels, IIRC. >For SS, the spread signal (chip rate) should be one or two orders of >magnitude faster than the actual data. > >>>In a SS system, the spreading chip rate should be several percent of >>>the absolute frequency to be effective against multipath. >>> >>>Regardless modulation method, some general precautions against >>>multipath: >>> >>>Is it possible to install the base station antenna on the inside of >>>the dome itself ? This would generate a strong direct (dome, field) >>>signal compared to reflected (dome, field, dome, field). >> >>It is (it's certainly not outside the dome, if that's what you mean). It's >>been tried in the ceiling, on the field, in the stands... We've tried >>directionals, omnis, amplified, padded, just about everything. One thing we >>haven't done is separating the transmit and receive antennas (so they can be >>padded differently). The solution, so far, is to abandon 2.4G, in favor or >>900M, which works reasonably well (except for other system limitations - e.g. >>bandwidth). >> >>>If the base station antenna can not be installed in the dome itself >>>but it has to be installed at ground level or somewhere in the >>>audience, at least make sure that the radiation pattern above the >>>horizontal plane is heavily attenuated, reducing power radiated into >>>the dome (or received from the dome). A metallic shield (or mesh) >>>above the antenna might be usable, but of course, it creates early >>>reflections from the shield, which may be (or not) harmful depending >>>on the modulation method. >> >>...been tried. We can get dome time just about anytime we need it. > >A sports dome would be absolutely the last place I would use any ISM >band for any professional communication :-). We don't, obviously. It works extremely well in open stadiums (>95% market share ;-). >Setting up your system before the event and everything seems to work >OK and there are plenty of SNR etc. Nope, that's the worst-case situation. >Then the huge public is admitted into the dome, each carrying one or >more ISM devices (Bluetooth, WLAN) etc. While with spread spectrum >emissions, you do not have a discrete frequency channel free/occupied >situation, but adding more and more spread spectrum devices into the >same frequency band will gradually increase the background noise level >and the SNR after despreading drops gradually, until the SNR drops too >low and the communication fails. Masses of people absorb a lot of energy. That actually helps. >In the dome, the RF emitted from each device is bouncing around and >finally absorbed by some soft tissue (the spectators). Right, but enough gets to the receiver. Spectators actually help without the dome. >>>>>The area at which the signals cancel each other is quite small and >>>>>gets smaller with higher frequencies. Having two or more antennas at >>>>>some distance from each other (e.g. on different part of your body), >>>>>it is unlikely that _all_ those antennas would be in multipath nulls. >>>>>With multiple antennas and multiple receivers, just sum the audio >>>>>outputs, but squelch those receivers that have a weak (noisy) signal >>>>>at the moment. >>>> >>>>It's not a matter of nulls, rather the receivers can't lock onto the signal. >>> >>>Don't lock at all or sometimes drops out of lock ? >> >>Drops, *frequently*. > >Sounds very much like NBFM (12.5/25 kHz BW) on 1.3 GHz (23 cm) that I >used a few decades ago. > >As the theory predicts, a simple ground reflection will create a comb >filter like spectrum and for a specific narrow band signal channel, >there are deep nulls at very close distance from each other. Moving >just a meter and the signal drops several times. Stopping a car with a >fixed antenna at traffic light would almost always cause stopping at >the multipath null :-). The only thing that helped, is moving the >antenna a few centimeters (spatial) or changing channel (frequency >diversity). We don't have that option, if it did work. The radios aren't necessarily moving, though. >At highway speeds, the dropouts were so frequent (and short) that it >did not affect the readability of the speech (equivalent to >interleaving and ECC in digital communication). > >It still sounds that you are suffering from a narrow band signal with >the RF field strength punctured by multipath nulls distributed close >to each other all over your area of interest. Of course, since some >data is lost, you must use quite a lot of error correction bits. Not really. We do a double-send and throw away faulty packets (CRC checked) but there aren't any correction bits, per se. Everything is G927 compressed so that hides a lot of uglies. >Since you are only using about half of the available capacity, why not >allocate the rest for ECC bits ? Since the multipath nulls create >burst errors, interleaving should be used to convert burst errors to >random errors that can then be corrected by ECC. Don't think we have access to the bits. I'll have to look more closely into that, though. If we had access to them I'm sure we'd be making use of them for more data. >The problem with interleaving is that it adds delay, which is annoying >in two way conversation. As you said that the dropouts are frequent >(assuming several each second), this also means that they are short, >thus reducing the interleave delay needed. Each slot is .5ms, ten slots (five transmit, five receive) per hop. The system latency is pushing the limit for intelligibility (80-100ms). There is a *lot* of echo-cancellation stuff going on to minimize these issues. >>>How long does it take to relock ? Is this time longer than an other >>>antenna with separate receiver typically maintain lock with a moving >>>receiver ? >> >>Lock and negotiation takes longer than a single hop. > >Those might be your worst problem. Pretty hard to eliminate. The radios have to know who they're talking to before they can respond appropriately. Then there is the data in flight... >I am still not sure what the emission is like. Do you first lock into >some direct sequence spread spectrum signal, then lock into TDMA frame >and finally lock to the individual signal ? >Or are each handset sending individual spread spectrum sequence during >short time slots in a TDMA way ? Yes. At least I think that's the way the radios work. They all know ahead of time what the sequence is. This is negotiated once during system setup while they're wired together (actually, downloaded from the base). >>I don't understand the >>second question. > >In a typical moving multipath situation, the signal is 80-95 % above >threshold and only 5-20 % below threshold. In an ideal receiver, the >recovery would be immediate and at least more than 80 % of the time >the signal would be good even with a single receiver. With diversity >reception with receivers, this time would be much closer to 100 %. > >However, if a receiver takes more than 50 % of the good signal time to >relock, even two diversity receivers would not be enough to reach even >close to 100 % service times. I'll have to think about that (SWMBO is yelling to leave). |