CDMA2000 1xEV-DO Basics
- 1x EV-DO, the data optimised 3G cellular telecommunications (cell phone)
system for CDMA2000
CDMA2000 1x EV-DO cell phone system is a standard that has
evolved from the CDMA2000 mobile phone system and it is now firmly established
in many areas of the world. The letters EV-DO stand for Evolution Data Only or
Data Optimised. From the title it can be seen that it is a data only mobile
telecommunications standard that can be run on CDMA2000 networks.
The EV-DO cell phone system is capable of providing the full
3G data rates up to 3.1 Mbps now that release A of the standard has been issued.
The first commercial CDMA2000 1xEV-DO network was deployed by SK Telecom (Korea)
in January 2002. Now operators in Brazil Ecuador, Indonesia, Jamaica, Puerto
Rico, Taiwan and the USA to mention but a few have all launched networks and
more are to follow.
CDMA2000 1xEV-DO Basics
The CDMA2000 1xEV-DO cell phone system is defined under
IS-856 rather than IS-2000 that defines the other CDMA2000 standards, and as the
name indicates it only carries data. In Release 0 of the standard the maximum
data rate was 2400 Mbps in the forward (downlink) with 153 kbps in the reverse
(uplink) direction, the same as CDMA2000 1X. However in the later release of the
standard, Release A, the forward data rate rises to 3.1 Mbps, and 1.2 Mbps in
the reverse direction.
Data rates for CDMA2000 1xEV-DO
The forward channel forms a dedicated variable-rate, packet
data channel with signalling and control time multiplexed into it. The channel
is itself time-divided and allocated to each user on a demand and opportunity
driven basis. A data only format was adopted to enable the standard to be
optimised for data applications. If voice is required then a dual mode phone
using separate 1X channel for the voice call is needed. In fact the "phones"
used for data only applications are referred to as Access Terminals or ATs.
Air interface
The EV-DO RF transmission is very similar to that of a
CDMA2000 1X transmission. It has the same final spread rate of 1.228 Mcps and it
has the same modulation bandwidth because the same digital filter is used.
Although 1xEV-DO has many similarities with 1X transmissions, it cannot occupy
the same channels simultaneously, and therefore requires dedicated paired
channels for its operation. Accordingly new bands, often in the new 3G
allocations are being dedicated for EV-DO in some areas.
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Note on CDMA:
CDMA, Code Division Multiple Access, is a multiple
access scheme used by many 3G cellular technologies, and other forms of
wireless technology. It uses a process called Direct Sequence Spread
Spectrum where spreading codes are used to spread a signal out over a
given bandwidth and then reconstituting the data in the receiver by
using the same spreading code. By supplying different spreading codes to
different users, several users are able to utilises the same frequency
without mutual interference.
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Forward link
The forward link possesses many features that are specific to
EV-DO, having been optimised for data transmission, particularly in the downlink
direction. Average continuous rates of 600 kbps per sector are possible. This is
a six fold increase over CDMA2000 1X and is provided largely by the ability of
1xEV-DO to negotiate increased data rates for individual ATs because only one
user is served at a time.
The forward link is always transmitted at full power and uses
a data rate control scheme rather than the power control scheme used with 1X,
and the data is time division multiplexed so that only one AT is served at a
time.
In order to be able to receive data, each EV-DO AT measures
signal-to-noise ratio (S/N) on the forward link pilot every slot, i.e. 1.667 ms.
Based on the information this provides the AT sends a data rate request to the
base station. The AN receives requests from a variety of ATs, and decisions have
to be made regarding which ATs are to be served next. The AN endeavours to
achieve the best data transfer, and this is done by serving those ATs offering a
good signal to noise ratio. This is achieved at the expense of users at some
distance from the AN's antenna.
Accurate time synchronisation is required between the EV-DO
Access Nodes. To achieve this time information is taken from the Global
Positioning System as this is able to provide an exceedingly accurate time
signal.
Forward link channels
A number of channels are transmitted in the forward direction
to enable signalling, data and other capabilities to be handled. These channels
include the Traffic channel, MAC channel, Control channel and Pilot. These are
time division multiplexed.
Traffic Channel - This channels uses Quadrature Phase
Shift Keying (QPSK) modulation for data rates up to 1.2288 Mbps. For higher data
rates, higher order modulation techniques are used in the form of 8PSK with 3
bits per symbol or 16QAM with 4 bits per symbol. The levels of the I and Q
symbols are chosen so that the average power becomes 1.
The Incoming data to be used as the modulation comes from the
from the turbo coder and is scrambled by mixing it with a Pseudo Random Number
(PN) sequence. The initial state of the PN is derived from known parameters, and
is unique for each user. Every packet starts at the same initial value of the PN
sequence.
At the beginning of the transmission to each user, there is a
preamble that contains the user ID for the data. Its repeat rate is determined
by the data rate because lower data rates require higher repeat values. However
even at its largest, the preamble will fill no more than half the first slot.
Control Channel - This channel carries the signalling
and overhead messages.
Pilot - The differentiator between the cell and the
sector is still the PN offset of the pilot channel and the pilot signal is only
gated on for 192 chips per slot.
Medium Access Control (MAC) Channel - This channel
carries a number of controls including the Reverse Power Control (RPC), the Data
Rate Control (DRC) Lock, and the reverse activity (RA) channels.
Reverse Link
The reverse link for 1xEV-DO has a structure similar to that
for CDMA2000. In EV-DO all signalling is performed on the data channel and this
means that there is no Dedicated Control Channel. The data channel can support 5
data rates which are separated in powers of 2 from 9.6 to 153.6 kbps. These
rates are achieved by varying the repeat factor. The highest rate uses a Turbo
coder with lower gain. The following channels are transmitted in addition to
those used with 1X:
Reverse Rate Indicator (RRI) Channel - This indicates
the data rate of the Reverse Data Channel.
Acknowledgement (Ack) Channel - This channel is
transmitted after the AT detects a frame with the preamble detailing it to be
the recipient of the data.
Data Rate Control (DRC) Channel - This channel
contains a four bit word in each slot to allow the choice of 12 different
transmission rates.
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