Advantages of 5 MHz RF Channel Spacing

Advantages of 5 MHz RF Channel Spacing

We have already highlighted the advantages of wider RF channel spacing in terms of
relaxed frequency stability and RF filtering. Additionally, the fading envelope becomes
less severe as the bandwidth increases, relative to the operating frequency.
This is known as the coherence bandwidth�"the bandwidth over which all the signal
is affected by multipath fading. As the bandwidth increases, parts of the signal remain
unaffected. Fading is a phase cancellation effect and as such is frequency-specific. As
the fading depth reduces it becomes progressively easier to follow the multipath fading
with a power control loop.
In practice, both slow fading and fast fading (at least for slow mobility users) in a
relatively wideband channel can be compensated for by decreasing and increasing the
RF power of the handset in sympathy with the fade. In GSM, power control is done
every 500 ms (twice a second), in IMT2000MC it is done 800 times a second, and in
IMT2000DS it is done 1500 times a second.
This highlights some of the key differences between 3G handsets and GPRS/EDGE
handset hardware. 2G air interfaces are designed to support, more or less, constant rate
channels�"an 8 or 13 kbps codec, for example. The channels themselves tend to be of
variable quality; the result of the slow and fast fading experienced by mobile users. 2G
can in effect be characterized as being designed to work with constant rate variable
quality channels.
Attempts to deliver variable bandwidth (bandwidth on demand) have met with
some success in 2G as we will document in the next chapter, but intrinsically the bandwidth
limitations of a 200 kHz or 30 kHz channel limit the dynamic range that can be
delivered for users with highly variable, bursty data rates. Moving to a wider RF channel
makes it easier to deliver variable-rate constant-quality channel bandwidth. As we
will see in later chapters, this is more or less essential for the movement and management
of time-sensitive multimedia files and is the key performance differentiator
between 2G and 3G air interface propositions.