  One of the features characterizing almost every multiple access (MA)
communication system is the processing gain. Through the use of spreading
sequences, the processing gain of Random CDMA systems (RCDMA), is devoted to
both bandwidth expansion and orthogonalization of the signals transmitted by
different users. Another type of multiple access system is Impulse Radio (IR).
In many aspects, IR systems are similar to time division multiple access (TDMA)
systems, and the processing gain of IR systems represents the ratio between the
actual transmission time and the total time between two consecutive
ransmissions (on-plus-off to on ratio). While CDMA systems, which constantly
excite the channel, rely on spreading sequences to orthogonalize the signals
transmitted by different users, IR systems transmit a series of short pulses
and the orthogonalization between the signals transmitted by different users is
achieved by the fact that most of the pulses do not collide with each other at
the receiver.
  In this paper, a general class of MA communication systems that use both
types of processing gain is presented, and both IR and RCDMA systems are
demonstrated to be two special cases of this more general class of systems. The
bit error rate (BER) of several receivers as a function of the ratio between
the two types of processing gain is analyzed and compared under the constraint
that the total processing gain of the system is large and fixed. It is
demonstrated that in non inter-symbol interference (ISI) channels there is no
tradeoff between the two types of processing gain. However, in ISI channels a
tradeoff between the two types of processing gain exists. In addition, the
sub-optimality of RCDMA systems in frequency selective channels is established.
