A Note about Radio Channel Quality

We also mentioned in passing the Rayleigh fading experienced on the radio channel
and the mechanisms we need to adopt to average out these channel impairments.
These include interleaving, frequency hopping (a GSM handset must be capable of
hopping every frame to one of 64 new frequencies), and equalization. Equalization is
needed to correct for the time shifts introduced by the multiple radio paths that may
exist between a base station and handset.
Given that radio waves travel at 300,000 km per second, in a millisecond they will
have covered 300 km, or looking at it another way, 1 km of flight path equates to 3.33 μs
of delay. In TDMA handsets, there needs to be a mechanism for managing multiple
paths that may be 4 or 5 km longer than the direct path.
Asymbol period in GSM is 3.69 μs. Therefore, a 5 km multipath will create a delayed
image of the modulated bit stream 4 bits behind the direct-path component. Multipath
is managed in GSM (and US TDMA) by using a training sequence embedded in the bit
burst, which effectively models and allows the handset to correct for a 4- or 5-bit time
shift. Given that the handset can be up to 35 km away from the base station, the handset
needs to adjust for the round-trip timing delay (a round-trip delay of 70 km is
equivalent to 63 symbol periods).
The timing advance in GSM is up to 64 symbols (the Tx slot is moved closer to the
RX slot in the frame). As we will see in the next chapter, this can be problematic when
implementing multislot handsets.
In CDMA the unique and unchanging properties of the pilot signal give the receiver
an accurate knowledge of the phase and time delay of the various multipath signals. It
is the task of the RAKE receiver to extract the phase and delay of the signals and to use
this information to synchronize the correlator in order to align the path signals prior to
combining them. This process is detailed in Chapter 3. In CDMA, the pilot channel
(IS95 CDMA/IMT2000MC) or pilot symbols (W-CDMA/IMT2000DS) provide the
information needed for the receiver to gain knowledge of both the phase and amplitude
components of the radio signal received.

The wider the dynamic range of operation required from a handset, the harder it is
to deliver channel quality. In GSM, for example, it was decided in the 1980s to support
35 km macrocells (later extended to 70 km for Australia) down to 50-meter picocells.
This requires substantial dynamic range. It was also decided to support high-mobility
users (up to 250 kmph). This high-mobility requirement makes it necessary to track
and correct for Doppler effects in the receiver and requires substantial signaling overhead
to manage handovers from cell to cell.
In GSM, 62 percent of the bandwidth available is used for channel coding and signaling
overhead; only 38 percent of the allocated bandwidth is actually used to carry
user data. Similarly, many decisions in 3G handset design—RAKE receiver implementation,
for example—depend on the dynamic range of the operational requirement: the
minimum and maximum cell radius and the mobility of the user. Channel quality (and
hence bandwidth quality) is dependent on a very large number of variables.
The job of the RF and DSP designer is to make sure handsets can continue to deliver
acceptable performance in all operating conditions. As we will see with GPRS, this can
be difficult to achieve.