TDMA Evolution

TDMA Evolution

By the end of the 1990s, the mix of deployed technologies included AMPS/TDMAnetworks
(in the United States and parts of Latin America and Asia), using 30-kHz RF
channel spacing, GSM networks using 200 kHz RF channel spacing, and CDMA networks
using 1.25 MHz channel spacing. The proposed migration route for AMPS/
TDMA network operators was to introduce 200 kHz channel rasters and a new 3, 6, 8,
16, 32, or 64 slot frame structure, the idea being to provide more flexible bandwidthon-
demand capability.
Part of the logic here is to take into account the likely dynamic range of the information
rate needing to be presented to the channel. For example, a simultaneously
encoded voice, image, video, and data stream could result in a composite information
rate varying from 15 kbps to 960 kbps, and the rate could change every frame, or every
10 ms. This would be a 64-to-1 ratio (18 dB), hence the choice of a slot structure that can
encompass a 64-slot frame where a single user can be allocated anything between 1 slot
(a 1/64 duty cycle) to 64 slots (a 64/64 duty cycle) or any value in between. The 16, 32,
and 64 slot frames are intended to be used with eight-level PSK, giving a maximum
user data rate of 384 kbps.
A second objective is to harmonize the IS54 AMPS/TDMA air interface and GSM.
Both air interfaces would have an eight-slot frame in common, both air interfaces
would have eight-level PSK in common for higher bit rate applications and the 16, 32,
and 64 slot frame structure for high dynamic range applications.
The eight-phase PSK implementation is known as Enhanced Data Rate for GSM
Evolution (EDGE) and would be implemented in an AMPS/TDMA network using
either 3 × 200 kHz channels (Compact EDGE) or 12 × 200 kHz channels (Classic EDGE).
A 50 kHz guard band is added on either side to provide protection to and from the
IS136 30 kHz channels.
Table 1.3 shows the combined proposal submitted to the ITU and called IMT2000SC
(single RF carrier with adaptive time-division multiplexing). The proposal is promoted
by the Universal Wireless Communications Consortium (UWCC), now known as
3G Americas.

The implementation of EDGE into an AMPS/TDMA network requires some care to
avoid disturbing existing reuse patterns, using either 1/3 reuse with Compact EDGE
or 4/12 reuse with Classic EDGE (see Tables 1.4 and 1.5).
The bandwidth negotiation protocols (multislot allocation and release) would be
common to GSM (part of the General Packet Radio Service protocol) and IS54/IS136
TDMA. IMT2000SC is one of the four air interface standards presently being promoted
for 3G networks, the others being IMT2000MC, IMT2000DS, and IMT2000TC.
IMT2000MC provides an evolution from the existing IS95 CDMA air interface and is
promoted by the CDMA Development Group (CDG) and 3GPP2�"the third-generation
partnership project standards group dedicated to air interface and network interface
standardization and IS95 CDMA backward compatibility (see Figure 1.8).
The original IS95 CDMA networks use a 1.2288 Mcps rate to occupy a 1.25 MHz RF
channel. The multichannel (MC) refers to using multiple, that is 3, 6, or 12 × 1.25 MHz
carriers to increase per user bit rates. For example 3 × 1.25 MHz carriers will occupy 5
MHz, equivalent to IMT2000DS.

In practice, there are three ways to increase data rates:
 Allocating PN multiple codes to single users
 Increasing the chip rate, for example, from 1.2288 Mcps to 3.6864 Mcps (or
higher multiples)
 Using higher-level modulation schemes such as 8 PSK or 16-level QAM�"a version
known as 1xEV and promoted by Qualcomm.
At time of writing, 1xEV, rather than the multicarrier IMT2000MCimplementation,
is the favored evolution route and is generically known as CDMA2000.
IS54TDMA/IMT2000SC and IS95 CDMA/CDMA2000 are supported by a network
standard known as IS41. GSM/IMT2000DS is supported by a network standard known
as GSM-MAP (Mobile Application Part).