Subject: Re: [manet] A question in multiuser MIMO/SIMO

From: Joe Carey <[email-masked]>
Date: Thu, 01 Jun 2006 08:59:40 -0600

Yihu Li wrote:
> Hi Joe,
>
> Thank you for your explaination. It helps a lot.
>
> As you pointed out, "The key to making this work is the coding.  Pick 
> the right spreading /  timing / synchronization / channel sounding 
> codes" and "timing recovery is more comlicated". So, do you know any 
> work on this, that is, coding and timing recovery to make this work or 
> discuss this? Any reference will be great helpful.
The best reference I have for synchronization in general is:
"Telecommunications Systems Engineering" by Lindsey & Simon.  It's a 
classic reference by a couple of Bell Labs guys that I keep coming back 
to it because its got more detail than anything more recent that I've 
found.  It gets into a lot of parametric effects (i.e. accuracy & 
stability in the presence of noise and interference).

It's tougher to pick a single reference for code selection.  The key 
concept to understand in depth is correlation of signals.  For a quick 
introduction, I'd start with Bob Dixon's book, "Spread Spectrum 
Systems."  For something more in-depth I'd look to Bernard Sklar's 
"Digital Communications".  I'd also get a copy of Andy Viterbi's book on 
CDMA.  Then I'd do literature searches on "space-time codes", 
"pseudo-noise codes", and gps. 

Yihu, the way you originally formulated the problem with the two 
transmitting stations being completely unsynchronized; therefore I 
suggest you put particular emphasis on pseudo-noise codes.  I would also 
look to GPS to provide some hints. 

In the case you described, the signals can be received by a station with 
a single antenna.  Two antennas at the receiver will improve the 
performance of the system by providing spatial filtering.

Here's a link to a paper by Lei Poo at Stanford titled: "Space-Time 
Coding for Wireless Communications: A Survey."
http://www.ehb.itu.edu.tr/~ozguroruc/pdf/Space-Time%20Coding%20for%20Wireless%20Communication-%20A%20survey.pdf

While there may be better papers, he has done an excellent job of 
pulling together a bibliography.

joe
>
> Regards,
>
> Yihu
> ----- Original Message ----- From: "Joe Carey" 
> <[email-masked]>
> To: "Yihu Li" <[email-masked]>
> Cc: "manet mailing list" <[email-masked]>
> Sent: Wednesday, May 31, 2006 4:13 PM
> Subject: Re: [manet] A question in multiuser MIMO/SIMO
>
>
>> Hi Yihu,
>>
>> Yes, its still possible when the systems are unsynchronized, but 
>> timing recovery is more complicated.  This is the CDMA reverse link 
>> problem, from the handset to the base station.  You might be tempted 
>> to argue that the handsets synchronize off the same forward link, but 
>> I would make the counterpoint that in a CDMA system the two handsets 
>> could be traveling and therefore have different doppler shifts and so 
>> are effectively unsynchronized.
>>
>> The key to making this work is the coding.  Pick the right spreading 
>> / timing / synchronization / channel sounding codes and it can be 
>> made to work.
>>
>> Another significant problem in the situation you describe is the 
>> difference in amplitude between signals received from S1 and S2.  If 
>> one station is closer than the other, coding gain alone will not be 
>> sufficient.  In that case you need to implement power control in 
>> place to keep the two signals received at R at approximately the same 
>> level, so that coding gain can effectively separate the two signals.
>>
>> Ramanathan at BBN wrote an interesting paper recently on future 
>> network topologies that explored variations on this idea for ad-hoc 
>> networks.
>>
>> joe
>>
>> Yihu Li wrote:
>>> Hi Joe,
>>>
>>> Thanks a lot.
>>>
>>> I just want to double confirm the problem: nodes S1 and S2 are not 
>>> synchronized in transmission (not as the conventional single-user 
>>> MIMO in which multiple antennas in a transmitter transmit at the 
>>> same time slot or MIMO coorperative communication in which the nodes 
>>> with single antennas are coordinated to transmit at the same time 
>>> slot). Instead, S1 and S2 start their transmissions in different 
>>> time slots. So, you think node R can still receive the packets from 
>>> both stations correctly in this case, right? If so, what is the 
>>> differences/comlexities of this case to the one where transmissions 
>>> through different antennas are synchronized?
>>>
>>> Thanks again.
>>>
>>> Yihu
>>> ----- Original Message ----- From: "Joe Carey" 
>>> <[email-masked]>
>>> To: "Yihu Li" <[email-masked]>
>>> Cc: "manet mailing list" <[email-masked]>
>>> Sent: Wednesday, May 31, 2006 1:40 PM
>>> Subject: Re: [manet] A question in multiuser MIMO/SIMO
>>>
>>>
>>>> Yihu,
>>>>
>>>> Yes, it is possible.  In fact, CDMA cell phones work this way 
>>>> (except R usually has only one antenna).  Typically we describe the 
>>>> problem with S1 and S2 being the same station, but it could be made 
>>>> to work with them being different stations.
>>>>
>>>> It requires some fancy waveform coding.  You are correct, 
>>>> synchronization is one of the big issues.  Determining the 
>>>> propagation channel characteristics so you can sort it out on the 
>>>> far end is another big issue.
>>>>
>>>> Here is a link to the Enhanced Wireless Consortium.  Download the 
>>>> 802.11n draft spec, which gives some detail on a particular MIMO 
>>>> implementation:
>>>> http://www.enhancedwirelessconsortium.org/home
>>>> It'll cure any insomnia you might have.
>>>>
>>>> You can also google or do a literature search on "space time coding."
>>>>
>>>> joe
>>>>
>>>> Yihu Li wrote:
>>>>> Hi all,
>>>>>
>>>>> I have a question about the synchronization problem in a SIMO system.
>>>>>
>>>>>    S1(one antenna)-------->R (two antennas)<----------S2(one antenna)
>>>>>
>>>>> In the scenario above, two transmitters S1 and S2 each have one 
>>>>> antenna, and the receiver R has two antennas. Assuming spatial 
>>>>> multiplexing mode is used, S1 first sends a packet to R, and then 
>>>>> S2 also starts to send another packet to R while the transmission 
>>>>> between S1 to R is still ongoing. Could R receive the two packets 
>>>>> successfully or the two packets will be collided? If R can receive 
>>>>> both, it has to first sychronize to S1 for reception, how can it 
>>>>> sychronize to S2 thereafter? Is it possible?
>>>>>
>>>>> Thanks.
>>>>>
>>>>> Yihu
>>>>>
>>>>> _______________________________________________
>>>>> manet mailing list
>>>>> [email-masked]
>>>>> https://www1.ietf.org/mailman/listinfo/manet
>>>>>
>>>>
>>>
>>
>

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