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# What's the differencce between fast flat fading and slow flat fading?

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#### elb10yl

##### Newbie level 3
Hi all,

I am doing my project recently.. I need to simulate DS-CDMA system over fast flat fading and slow flat fading by packet data. I'm not quite sure what's the difference between them. And what's the single tap flat fading? Is it fast fading or slow fading? Can anyone help me?

Fast vs. slow is in the time domain, whereas flat vs. selective is in the frequency domain.

1- If the channel changes withing a symbol time it is fast. Otherwise it is not fast.
2- If the channel frequency response is constant over the signal bandwidth then it is flat. Otherwise, it is frequency selective.

Single tap channel means it is flat because fading will be in the form hx+n. Frequency selective channels will have more than one tap, and hence it causes ISI.

elb10yl

### elb10yl

Points: 2
Fast vs. slow is in the time domain, whereas flat vs. selective is in the frequency domain.

1- If the channel changes withing a symbol time it is fast. Otherwise it is not fast.
2- If the channel frequency response is constant over the signal bandwidth then it is flat. Otherwise, it is frequency selective.

Single tap channel means it is flat because fading will be in the form hx+n. Frequency selective channels will have more than one tap, and hence it causes ISI.

Thanks for your reply! But I still have something confused. What's the tap mean? Does it represent the number of path? So, the singe tap means one path fading. Fast flat fading and slow flat fading are multipath fading. Is that right?

Thanks for your reply! But I still have something confused. What's the tap mean? Does it represent the number of path? So, the singe tap means one path fading. Fast flat fading and slow flat fading are multipath fading. Is that right?

One tap means the channel in the discrete-time base-band representation it is one complex multiplicative scalar. Mutipath fading is a general term means that a signal transmitted over these channels propagates over multiple paths that can or can not be resolved in time, but usually it is used in the literature to mean frequency selective channels that cause ISI, i.e.: when the multiple copies can be resolved in time (resolvable paths)

elb10yl

### elb10yl

Points: 2
Thanks for your reply! Do you know how to simulate the fading channels by packet data? I used long data length in AWGN channel, that's ok, but my teacher told me if I use packet data to transform in fading channel, the faidng of each packet can get a good performance than the long data length. Do you know why? Or can you give me some examples.