Design Issues for a Multislot Phone

The idea of a multislot phone is that we can give a user more than one channel. For
instance, one slot could be supporting a voice channel, other slots could be supporting
separate but simultaneous data channels, and we can give a user a variable-rate channel.
This means one 9.6 kbps channel (one slot) could be expanded to eight 9.6 kbps
channels (76.8 kbps), or if less coding overhead was applied, one 14.4 kbps channel
could be expanded to eight 14.4 kbps channels (115 kbps). Either option is generically
described as bandwidth on demand.

In practice, the GSM interface was designed to work with a 1/8 duty cycle. Increasing
the duty cycle increases the power budget (battery drain) and increases the need
for heat dissipation. Additionally, multislotting may reduce the sensitivity of the handset,
which effectively reduces the amount of downlink capacity available from the base
station, and selectivity—a handset working on an 8-over-8 duty cycle creates more
interference than a handset working on a 1-over-8 duty cycle).
The loss of sensitivity is because the time-division duplexing (time offset between
transmit and receive) reduces or disappears as a consequence of the handset using
multiple transmit slots. For certain types of GPRS phone this requires reinsertion of a
duplex filter, with typically a 3 dB insertion loss, to separate transmit signal power at,
say, +30 dBm from a receive signal at -102 dBm or below.
Revisiting the time slot arrangement for GSM shows how this happens (see Figure
2.1). The handset is active in one time slot—for example, time slot 2 will be used for
transmit and receive. The transmit and receive frames are, however, offset by three
time slots, resulting in a two time slot separation between transmit and receive. The
time offset disappears when multiple transmit slots are used by the handset. An additional
complication is that the handset has to measure the signal strength from its serving
base station and up to five adjacent base stations. This is done on a frame-by-frame
basis by using the six spare time slots (one per frame) to track round the beacon channels
sent out by each of the six base stations. Multislotting results in rules that have
hardware implications.

There are three classes of GPRS handset. Class A supports simultaneous GPRS and
circuit-switched services—as well as SMS on the signaling channel—using a minimum
of one time slot for each service. Class B does not support simultaneous GPRS and
circuit-switched traffic. You can make or receive calls on either of the two services
sequentially but not simultaneously. Class C is predefined at manufacture to be either
GPRS or circuit-switched. There are then 29 (!) multislot classes.
For the sake of clarity we will use as examples just a few of the GPRS multislot
options (see Table 2.1): Class 2 (two receive slots, one transmit), Class 4 (three receive
slots, one transmit), Class 8 (four receive slots, one transmit), Class 10 (four receive
slots, two transmit), Class 12 (four receive slots, four transmit), and as a possible longterm
option, Class 18 (up to eight slots in both directions).
The maximum number of time slots Rx/Tx is fairly self-explanatory. AClass 4 handset
can receive three time slots and transmit one. A Class 10 handset can receive up to
four time slots and transmit up to two time slots, as long as the sum of uplink and
downlink time slots does not exceed five.
The minimum number of time slots, shown in the right-hand column, describes the
number of time slots needed by the handset to get ready to transmit after a receive slot
or to get ready to receive after a transmit slot. This depends on whether or not the
handset needs to do adjacent cell measurements. For example, Tta assumes adjacent cell
measurements are needed. For Class 4, it takes a minimum of three time slots to do the
measurement and get ready to transmit. If no adjacent cell measurements are needed
(Ttb), one time slot will do. Tra is the number of time slots needed to do adjacent cell
measurements and get ready to receive. For Class 4, this is three time slots. If no adjacent
cell measurements are needed (Trb), one slot will do.
The type column refers to the need for a duplex filter; Type 1 does not need a duplex
filter, Type 2 does. In a Class 18 handset, you cannot do adjacent cell measurements,
since you are transmitting and receiving in all time slots, and you do not have any time
separation between transmit and receive slots—(hence, the need for the RF duplexer).

The uplink and downlink asymmetry can be implemented in various ways. For example,
a one-slot or two-slot separation may be maintained between transmit and receive.
Rx and Tx slots may overlap, resulting in loss of sensitivity. In addition, traffic channels
may be required to follow a hop sequence, across up to 64 RF 200 kHz channels
but more often over 6 or 24 channels.
Figure 2.2 shows the traffic channels hopping from frame to frame and the adjacent
cell monitoring being done in one spare time slot per frame. The base station beacon
channel frequencies—that is, the monitor channels—do not hop. The transmit slot on
the uplink may be moved closer to the Rx slot to take into account round-trip delay.
From a hardware perspective, the design issues of multislotting can therefore be
summarized as:
 How to deal with the loss of sensitivity and selectivity introduced by multislotting
(that is, improve Tx filtering for all handsets, add duplex filtering for Type 2)
 How to manage the increase in duty cycle (power budget and heat dissipation)
 How to manage the different power levels (slot by slot)
A user may be using different time slots for different services and the base station
may require the handset to transmit at different power levels from slot to slot depending
on the fading experienced on the channel.

The design brief for a multislot phone is therefore:
 To find a way of maintaining or increasing sensitivity and selectivity without
increasing cost or component count
 To find a way of improving power amplifier (PA) efficiency (to decrease the
multislot power budget, decrease the amount of heat generated by the PA, or
improve heat dissipation, or any combination of these)
 To provide a mechanism for increasing and decreasing power levels from slot
to slot