# Equalizers with low data rate systems

1. In case of using equalizers with ZigBee systems, what type of fading does the channel show, flat or frequency selective, fast or slow?

2. Is the inter-symbol interference (ISI) problem encountered in ZigBee systems?

I am asking this question because I konw that ZigBee Systems transmit data at low rate (250 kb/sec), and ISI problem and hence frequency selective fading appear with high data rate applications. I found some papers employing equalizers with ZigBee under Rayleigh fading conditions, but the type of fading was not indicated.

In the $2.4 \textrm{ GHz}$ band, the data rate is indeed just $250\textrm{ kbit/s}$ but the direct-sequence spread spectrum (DSSS) coding results in a bandwidth of $2\textrm{ MHz}$. This means that there will be flat fading only if the channel's coherence bandwidth is significantly greater than $2\textrm{ MHz}$, say around $10\textrm{ MHz}$ (this is just a fuzzy estimate). This corresponds to a maximum delay spread of approximately $1/(10\textrm{ MHz})=100\textrm{ ns}$.
Consequently, any environment with a delay spread greater than approximately $100\textrm{ ns}$ will result in frequency-selective fading. So in relatively small indoor areas the fading will be mostly flat, but in large office spaces or outdoors you can expect frequency-selective fading.