correlation WCDMA with multipath diversity

[Zay_S]

Hi everybody,

In WCDMA, within using wide carrier bandwidth, it has certain performance benefits, such as increased multipath diversity. But I'm still confuse with this statement.

as i know, diversity is a phenomena that used for overcome fading due to multipath channels. But, what the correlation between wide carrier with increased multipath diversity??

thanks so much for helping me.

 

[m_pourfathi]

When a wide-band signal is sent through a channel it smears in time domain. by wide-band. by using diversity techniques like time diversity or maximal ration combining which is implemented by using rake receivers, we add the received signal with one or some delayed version of the signal depending on the diversity order we have implemented. for example if we are using a diversity order of 3 we delay the received signal twice and sum up the received signal with 2 delayed version. By using these delays we make sure that the signals summing up together are correlated. because when signal smears in time, depending on the channel the by the passage of time signal becomes less correlated in time. when we delay it we compensate the smearing delay of the signal. for more info search rake receiver in google or maximal ration combining.

 

[nalino]

Adding a few comments to "m_pourfathi"

In (W)CDMA the smallest signal resolution is wellknow "chip" and it's duration is approximately the inverse of bandwidth. Hence, higer the bandwidth the better the resolution for signal samples at receiver. When chip duration (Tc) is smallerm we have resolve more multipath at receiver and later combine using a MRC RAKE receiver to collectively adds up each paths signal (so the name RAKE)....

Wish this helps..

 

[Zay_S]

thank u so much to m_pourfathi and nalino. its really good explanation.

from that explanation, i can conclude that with used wide bandwidth we can reduce the entire time for combining signal for the same number of multipath signal in narrowband.or with the same whole time in narrowband, we can increase multipath signal number. and the peformance more better.

otherwise, is it consume more cost if we implemented more diversity order?? if yes, which is used in the practice, adding more diversity order or stay with existing?

thanks in advance

 

[m_pourfathi]

It depends on some things. First the environment. If it is a place like inside a city you have many multipath components, so adding diversity order will help. You can define a threshold for you components, let's say for example 20 dB below the strongest component which is usually the first one, so you can ignore all components below 20 dB and afterwards. That would be the maximum effective diversity order and not more. For other places like plains and open areas where you do not have many multipath components you do not need a large diversity order even 2 sometimes is enough. In this case adding more diversity order results in adding more complication to your receiver, making it more expensive. Also it degrades your Signal to noise ratio because each diversity component also receives noise form the environment.
So there should be a trade off between these two. For 3G mobile system a Rake receiver with diversity order of 3 is used and it'll do good. So adding your diversity order without considering the trade off will eventually degrades your system performance and complexity.