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Wire and Wireless Communication Applications Chap. 8
The IS-54, IS-136 North American Digital Cellular (NADC) system, also called TDMA
and D-AMPS, was the first 2G standard adopted for use in the United States. It uses TDMA
to accommodate up to three users on each 30-kHz-wide channel. It is designed to replace
AMPS analog signaling on a channel-for-channel basis. Thus, an incumbent AMPS service
provider can gradually convert some channels to IS-54 digital service as needed, depending
on customer demand.
The IS-95 standard was developed by Qualcomm and adopted for use in the United
States 900-MHz cellular band in 1993. It uses CDMA to accommodate up to 35 users in a
1.2–MHz-wide channel. One IS-95 channel can be used to replace 41 30-kHz-wide AMPS
channels. The advantage of CDMA is that the same channel frequencies may be reused in
adjacent cell sites, because the users are distinguished by their different spreading codes.
(A different set of spreading codes is used in adjacent cell sites.) The CDMA approach has
many advantages. For example, the number of allowed users for each channel bandwidth is
not a hard number (as it is in TDMA, where there are a fixed number of available time slots),
since additional users with new spreading codes can be added. The additional users cause a
graceful increase in noise level. Users can be added until the errors become intolerable (due to
the increased noise level). It is expected that the CDMA approach will become more and more
popular and become the dominant type of system. Details of CDMA are given in books by
Rhee [Rhee, 1998] and Garg [Garg, 2000].
The Integrated Digital Enhanced Network (iDEN) was developed by Motorola. It
has been adopted by Nextel, now Sprint Nextel Corp., in their system, which uses specialized
mobile radio (SMR) frequencies in the 800 MHz band. The iDEN system uses TDMA
to accommodate up to 3 users in each 25 kHz-wide channel. Each channel uses four
16QAM carriers so that 4 4 = 16 data bits are sent in parallel during each symbol timeinterval.
This is similar to OFDM signaling, which was discussed in Sec. 5–12. Thus, the
time duration of a symbol is relatively long, so that the transmitted signal is resistant to
multipath fading. In addition to the usual cellular phone features, the iDEN protocol
allows for a virtual private network (VPN) to be established for business subscribers.
Thus, co-workers of the business can direct connect a private call or a group call to each
other over the VPN. Because of its poor ability to handle data, iDEN is scheduled to be
phased out in 2013.
As indicated in Table 8–11, all of these 2G systems use some sort of low-bit-rate
speech coding of the VF signal so that a larger number of users can be accommodated in the
limited RF bandwidth. See Sec. 3–8 for a discussion of speech coding. For example, the
vector sum excited linear prediction filter (VSELP) encoder used for the NADC and iDEN
systems is a modified version of the codebook excited linear prediction (CELP) technique
developed by B. Atal at the AT&T Bell Laboratories [Gerson and Jasiuk, 1990]. This technique
mimics the two main parts of the human speech system: the vocal cords and the vocal
tract. The vocal tract is simulated by a time-varying linear prediction filter, and the sounds
of the vocal cord used for the filter excitation are simulated with a database (i.e., a codebook)
of possible excitations. The VSELP encoder (at the transmitter) partitions the talker’s
VF signal into 20-ms segments. The encoder sequences through the possible codebook
excitation patterns and the possible values for the filter parameters to find the synthesized
speech segment that gives the best match to the VF segment. The encoder parameters that
specify this best match are then transmitted via digital data to the receiver, where the