5 MHz CDMA: IMT2000DS

5 MHz CDMA: IMT2000DS

IMT2000DS is the air interface standard promoted by Ericsson and the 3GPP1, the
third-generation partnership project standards group dedicated to promoting interworking
with other standards and, specifically, backward compatibility with GSM�"
an aspect of particular interest to existing GSM network operators. Harmonization
with GSM implied using a 13 or 26 MHz clock reference, rather than the 19 MHz clock
reference used in IMT2000MC, and a 200 kHz channel raster.

Although RF channel spacing is nominally 5 MHz, it is possible to bring lowerpower
microcells together with 4.4 MHz RF spacing (but still keeping to the 200 kHz
raster). This has the benefit of increasing the guard band between higher-power macrocells
and lower-power microcells.
The idea of maintaining a 200 kHz channel raster is to simplify synthesizer design for
dual-mode GSM/IMT2000DS phones. Additionally, GSM and IMT2000DS share the
same share frame structure from the multiframe upward, the multiframe length being
120 ms (see Figure 1.9). This simplifies the implementation of GSM to IMT2000DS and
IMT2000DS to GSM handovers, and could potentially allow for the use of GSM F bursts
and S bursts to provide frequency and synchronization for IMT2000.
The IMT2000DS measurement frame and equivalent GSM control channel align to
facilitate intersystem handover. Additionally, the code structure was chosen such that
frequency accuracy could be transferred from outdoor macrocells or microcells to
handsets, relaxing the frequency stability requirements of the handset. In turn, the
handsets can transfer the frequency reference to indoor picocells, thereby avoiding the
need for a GPS reference to be piped from outside a building to an indoor base station.
The code structure is termed asynchronous, for reasons we will explain later.
The advantage of the IMT2000MC (CDMA2000) code structure is that it supports very
resilient code channel acquisition. When a handset is first turned on, it can acquire the
wanted code channel very cleanly. The disadvantage is that timing accuracy within
the handset and base station needs to be within a fraction of the chip duration, hence the
relatively tight tolerance for IMT2000MC frequency stability.

Short-term stability has also to be tightly controlled (the jitter budget), since this will
compromise code correlation in IMT2000MC. This is why higher chip rates in
IMT2000MC tend to be avoided, and multiple RF carriers or higher level modulation
are used as an alternative method for delivering higher bit rates. Long-term stability is
also more critical for IMT2000MC. The relatively relaxed stability requirements for
IMT2000DS save component costs in the handset but increase the complexity of code
acquisition.
IMT2000TC shares a similar air interface to IMT2000DS�"along with the same
advantages and disadvantages, but it uses time-division duplexing (similar to a DECT
cordless phone). In IMT2000TC the 15 time slots in a frame are used to divide uplink
users from downlink users (see Figure 1.10). In effect, this is a CDMA air interface with
a time-division multiplex. Each time slot can be additionally subdivided into separate
code streams.
As with Digital Enhanced Cordless Telecommunications (DECT), the assumption
here is that the air interface will only be used in small cells and that low powers will be
used, easing the duplex requirement. The bandwidth can be increased on demand in
either direction with between 2 and 13 slots on the uplink and between 1 and 14 slots
on the downlink.