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How to combat multipath problem

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mesfet

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diversity to combact multipath

Hi All,

Anyone have experience in tackling the multipath problem. I find that my receiver works well at open area but bit error rate increase when people walk passed. The receiver operates at 2.4GHz with 2MHz datarate. Are there any existing hardware or software solution to combat the problem. Any suggestions are welcome.


mesfet+
 

antennas

It would probably be easier to change the antenna than to change the protocol. circularly polarized antennas are good for multipath since the rotation direction changes on reflection. (two reflections gets it back to the original) This should reduce your problem. There are some suppliers of vertical antennas that are circularly polarized.

If you are point to point, directional antennas will help.

A crude software solution is to send the data several times at 10 second intervals.
 

There are many kind of diversity. the easiest oone is selection combining. just pickup the stongest signal at antennas. Maximum ration goves a better performance but the delays and gain are needed to be estimated. Equal gain combining is good but , again, the delays are needed to be estimated. If you can only use only one antenna, RAKE receiver is a good choice. all methods above assume that the system is narrowband respected to coherent bandwidth. if your system is wideband, you need an equalizer or pre-distortion.
 

Hi,
I guess you are using a spread spectrum technique. If that is the case and assuming you have ASIC/FPGA to correlate, you can find the direct path reducing the correlator spacing. This shall increase the processing required. If you are not working at the design level but at integration level, best is to use antenna techniques.

BR
Madhukar
 

Selection diversity with two antennas works fine for me. I use 2.4GHz frequency band and additional uC that monitors the received signal strength and switches on stronger antenna signal.
 

Don't send the packet 10 times as suggested but use interleaving together with FEC (RS) this will solve together with antenna diversity your multipath problem.

mesfet said:
Hi All,

Anyone have experience in tackling the multipath problem. I find that my receiver works well at open area but bit error rate increase when people walk passed. The receiver operates at 2.4GHz with 2MHz datarate. Are there any existing hardware or software solution to combat the problem. Any suggestions are welcome.


mesfet+
Click to expand...
 

If you have control, try frequency hoping (in addition to space divesity, as said above)..

The fading dip is frequecy dependent, so if you hop, the dip will go away except at the dip freq (which is tolerable if you use error correction). This way you do not lose all data, but some data, and this can be recovered using error correction coding ...etc.

Also interleaving is powerful.. if fading is sever, you may try using all those techniques: freq hoping, interleaving and space diversity (two antennas)..

Cheers...
 

Attached is data rate measurement comparing with and without diversity. Its a Bluetooth system (frequency hopping and FEC included) that communicates via serial port and uses two diversity antennas combined in two ortoghonal beams, each beam in own channel. In indoor application, this action is very similar to selection diversity. The measurement was taken in heavy crowded factory environment (full of equipment, moving parts, people, etc.). Method of work is very simple: prior to data in each packet, uC monitors signal strength in each channel and switches on better.
 

Hi All,

Thank you for the suggestions. After hours of meeting, we are going to increase the FEC and check if it helps removing the problem.

mesfet+
 

both users need diversity?

I met similar problem at 2.4G band project. After reading the above discussion, we find that there are several means to overcome multipath fading effect, such as hopping frequecy, FEC, interleaving, circularly polarity antenna, spacing diversity antenna. But circularly polarity antenna is difficult to be realised and designed.
1. We usually use wired antenna with mobile end, which is linearity polarity. Microstrip antenna is circularly polarity antenna, however, it occupys large area.

2. Should both communication ends (base & mobile) use diversity technique? My now project situation is this probaly realised within base, difficult within mobile(handset) as the spacing limitation. Any suggestions?
thks.
 

Hi cybq,

We have had similar problems with a (narrowband) 868MHz system. An antenna diversity has solved the problem. It is sufficient if only one station (the base) has two or more antennas. Our mobile is attached to a wire antenna (1/4 lambda) the base has a 1/4 lambda and a patch antenna.
In my opinion a very broadband transmission (direct sequence spread spectrum with high chip rate) should solve this problem with only one antenna. The problem is the low resulting datarate...

Bye
 

Hi,

You can use antenna diversity technique. Just place two antenna in orthogonal direction.
 

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