CDMA2000 1X / 1XRTT basics tutorial
- a tutorial and overview of the basics of cdma20001x sometimes referred to as 1xRTT, giving details of the operation and evolution.
CDMA2000 is the evolution of the original IS-95 cdmaOne system. CDMA2000 has a number of evolutions of which the first was CDMA2000 1X, sometimes also called CDMA2000 1XRTT.
CDMA2000 1X which is also standardised as IS-2000 supports circuit-switched voice, and has the capability to provide up and sometimes beyond 35 simultaneous call per sector and as such it doubles the capacity of the original IS-95 networks. It also enables the transmission and reception of data at rates up to 153 kbps in both directions. It was recognized by the International Telecommunications Union (ITU) as an IMT-2000 standard in November 1999.
The aim of the CDMA2000 is to provide a migration path from the original cdmaOne / IS-95 system through the CDMA2000 1X format to further high speed formats. These different standards have all been standardised under the IS-format and a diagram of the migration path is given below:
The CDMA2000 1X format is the basic 3G standard, but in what is termed CDMA2000 1xEv, there are further developments. There are basically two routes for the evolution that were initially proposed, only one of which was deployed:
- CDMA2000 1X EV-DO: The first of these known as CDMA2000 1xEV-DO (EVolution Data Only or as is becoming more widely known Evolution Data Optimised) is something of a sideline from the main evolutionary development of the standard. It is defined under IS-856 rather than IS-2000, and as the name indicates it only carries data, but at speeds up to 3.1Mbps in the forward direction and 1.8 Mbps in the reverse direction, the speed in the reverse link being upgraded as part of Release A of the standard. The first commercial CDMA2000 1xEV-DO network was deployed by SK Telecom (Korea) in January 2002.
- CDMA2000 1X EV-DV: The second is CDMA2000 1X EV-DV (Evolution Data and Voice). The idea was that this system would carry both data and voice services. It was never deployed as the EV-DO system was deployed in preference and there was no requirement for a data and voice service as voice could be carried on DO as either VoIP or by falling back to the CDMA2000 1X format.
CDMA2000 1XRTT and 3XRTT
The CDMA2000 1XRTT and 3XRTT terms refer to what are termed "Radio Transmission Technologies". The original IS-95 and deployments of CDMA2000 utilised the 1.25 MHz channel spacing. This provided what is effectively the first phase of the 3G development and roll out. However to enhance the performance beyond that possible using the technologies such as 1xEV-DO and 1xEV-DV, the channel bandwidth of 1.25 MHz was deemed insufficient for even higher data rates. Accordingly by increasing the bandwidth, higher data rates were possible. The further evolution of the CDMA2000 system involves utilising channel bandwidths of 3 times the standard 1.25 MHz bandwidth under what was termed 3XRTT. Further bandwidth increases to 5X, 7X and so forth could in theory be contemplated.
For CDMA2000 1XRTT technology, a Spreading Rate 1 (SR1) was used where the signal was spread to occupy a bandwidth of 1.25 MHz. Here the spread rate was the same as that used for IS-95, i.e. 1.2288 Mcps. For 3XRTT technology, Spreading Rate 3 (SR3) was used. Here the spreading rate was 3.6864 Mcps. It was found that if the spreading rate remained the same but the data rate increased, as happens with video downloads and other 3G applications, the processing gain decreased. Accordingly the coverage and signal strength needed to be improved to match the new conditions. By increasing the spreading rate, the performance could be boosted without the need for improvements in coverage.
CDMA2000 1X overview
There are a number of updates and changes that were introduced to improve the performance of CDMA2000 1X, IS2000 over cdmaOne IS-95. However in all cases backward compatibility is maintained, allowing both IS-95 and CDMA2000 mobiles to access the same base stations. This provided a cost effective upgrade path for both users and operators.
For CDMA2000 1X, several new methods of coding and spreading were used and these enabled much higher capacities to be achieved.
- Walsh Codes: The first major change in CDMA2000 1X was that the Walsh Codes used were increased from 64 bits for IS-95 to 128 bits for CDMA2000 1X. In addition to this, CDMA2000 1X used more error coding functions as well and used turbo codes rather than the convolutional codes used for IS-95. This enabled higher speed data to be sent. In addition to this interleaving and symbol repetition were used to provide the various data rates.
- Turbo codes: Turbo codes were introduced into CDMA2000 1X. They were a new class of error correction codes that enabled transfer rates over a noisy channel to approach the "Shannon" limit. The turbo coding principle was first proposed in 1993 by Professors Claude Berrou and Alain Glaxieux. Originally their claims that the codes could double throughput for a given power were treated with scepticism, but their findings were eventually proved to be true. Turbo coders use powerful interleavers that reduce the susceptibility of a data stream to random and impulsive noise. By working on "soft" bits from a radio receiver, the Turbo codes enable the decoder to extract the maximum level of data from the noisy signals. Turbo codes require two encoders and two decoders per link. These blocks operate in parallel and work synergistically. They also used an iterative process to reduce the amount of processing required, but despite this they still require more processing power than previous coding systems such as convolutional codes.
- Spectrum efficiency: Apart from the improvements in the spreading and channel generation, there were also changes in the air interface itself. The IS-95 forward link used a form of QPSK where the data on both the I and Q channels are the same. However for CDMA2000 1X the I and Q channels were different, and this gave the advantage that half the bandwidth could be used for the same number of chips, or twice the number of chips can be sent in the same bandwidth. While this did make the reception more sensitive to phase errors, other improvements included an improved system of forward power control and forward transmit diversity.
- Reverse link upgrades: Similarly there were significant changes on the reverse link where several new channels were added. These included a pilot channel as well as supplemental data channels and a control channel for signalling. Additionally, similar to the forward link the reverse link used Walsh Codes to differentiate between the different channels. A further change was that the format of the carrier modulation was changed. With the reverse link now transmitting multiple channels the use of OQPSK would not prevent zero crossings. To achieve this, the modulation format was changed to a scheme known as Orthogonal Complex Quadrature Phase Shift Keying (OCQPSK). This form of modulation required a number of stages. First the channels to be transmitted were split so that some take the I path and others take the Q path. Next they were scrambled along with the Walsh code spreading. In the scrambling process the probability of zero crossings was identified and using a scheme known as Orthogonal Variable Spreading Function (OVSF) the probability of zero crossings was reduced. Accordingly the channels were spread with a Walsh Code sequence and summed with the correct gain to produce the I and Q sequences. These were then further spread by a long PN code with its mobile specific long mask to identify the mobile and these I and Q sequences were modulated onto the carrier. Although particularly complicated, this form of modulation did have fewer zero crossings and the power amplifier in the mobile did not have to be run in a linear mode, thereby saving battery power.
The CDMA2000 1X system gave many significant advantages over the original IS-95 scheme. Enabling higher data rates it also allowed improvements in performance as well as improvements in spectrum efficiency that enabled operators to gain a higher return on the spectrum. Also users saw improvements in performance.
By Ian Poole
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