The general idea of a 3G air interface—IMT2000DS, TC, or MC—is to move the process
of delivering sensitivity, selectivity, and stability from RF to baseband, saving on RF
component count, RF component complexity, and cost, and increasing the channel
selectivity available to individual users. You could, for example, support multiple
channel streams by having multiple RF transceivers in a handset, but this would be
expensive and tricky to implement, because too many RF frequencies would be mixing
together in too small a space.
Our starting point is to review how the IMT2000DS air interface delivers sensitivity,
selectivity, and stability, along with the associated handset hardware requirements.
At radio frequencies, sensitivity is achieved by providing RF separation (duplex spacing)
between send and receive, and selectivity is achieved by the spacing between RF
channels—for example, 25 kHz (PMR), 30 kHz (AMPS or TDMA), 200 kHz (GSM), or
5 MHz (IMT2000DS).
At baseband, the same results can be achieved by using digital filtering; instead of
RF channel spacing, we have coding distance, the measure of how separate—that is,
how far apart—we can make our 0s and 1s. The greater the distance between a 0 and a
1, the more certain we are that the demodulated digital value is correct. An increase in
coding distance equates to an increase in sensitivity.
Likewise, if we take two coded digital bit streams, the number of bit positions in
which the two streams differ determines the difference or distance between the two
code streams. The greater the distance between code streams, the better the selectivity.
The selectivity includes the separation of channels, the separation of users one from
another, and the separation of users from any other interfering signal. The distance
between the two codes (shown in Figure 3.1) is the number of bits in which the two
codes differ (11!).
As code length increases, the opportunity for greater distance (that is, selectivity)
increases. An increase in selectivity either requires an increase in RF bandwidth, or a
lower bit rate.
Stability between two communicating devices can be achieved by locking two codes
together (see Figure 3.2). This is used in TDMA systems to provide synchronization
(the S burst in GSM is an example), with the base station providing a time reference to
In IMT2000DS, the code structure can be used to transfer a time reference from a
Node B to a handset. In addition, a handset can obtain a time reference from a macro
or micro site and transfer the reference to a simple, low-cost indoor picocell. 59
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