UMTS CDMA technology
- tutorial, or overview of the basics of CDMA, code division multiple access scheme used within UMTS, or Wideband CDMA, WCDMA.
The use of CDMA, code division multiple access, in the form of Wideband CDMA, WCDMA for use with the 3G UMTS telecommunications system marked a distinct change in the type of technology used for the multiple access scheme for a telecommunications system. However it offered many advantages for both users and operators and as a result, it has provided many benefits.
The use of CDMA for UMTS and other cellular formats was made possible by the fact that semiconductor technology had moved forward sufficiently. At the time the first cellular technologies were introduced sufficient processing power could be provided to enable the coding and decoding of CDMA to be accomplished within a mobile handset.
CDMA as a form of multiple access scheme was first used on the cdmaOne system that was first deployed in the USA in 1995, and has successors that were marketed under the CDMA2000 banner. The use of a CDMA based technology for UMTS represented a further step forward in the use of CDMA.
CDMA for UMTS
The choice of CDMA for use with the third generation, 3G UMTS telecommunications system arose from a variety of technical reasons. It offers significant advantages over the schemes used in the previous 2G systems that were predominantly TDMA based schemes.
The main benefits of the use of CDMA as a multiple access scheme are:
- Improved spectral efficiency: The use of CDMA as the multiple access technology, combined with the QPSK modulation format used provides significant improvements in terms of the spectral efficiency. Figures for the performance improvements gained vary considerably dependent upon the conditions, but the scheme gives some significant benefits. Some calculated estimates give figures as high as three or four times that of technologies such as GSM, although in reality the benefits may be a bit less.
- Adjacent cells may use the same channel frequency: As a result of the way in which spread spectrum signals such as CDMA operate.
- Improved handover: Within CDMA it is possible to do what is termed a "soft handover" where the UE communicates with two base stations at the same time. This significantly improves handover reliability.
- Enhanced security: The use of spread spectrum and the multiple spreading codes for CDMA significantly reduces the possibility of eavesdropping, although within GSM eavesdropping of the transmitted signal was not the problem it was for the original analogue systems where anyone with a scanner radio receiver could listen to telephone conversations.
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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UMTS CDMA format
The data to be transmitted is encoded using a spreading code particular to a given user. In this way only the desired recipient is able to correlate and decode the signal, all other signals appearing as noise. This allows the physical RF channel to be used by several users simultaneously.
The data of a CDMA signal is multiplied with a chip or spreading code to increase the bandwidth of the signal. For WCDMA, each physical channel is spread with a unique and variable spreading sequence. The overall degree of spreading varies to enable the final signal to fill the required channel bandwidth. As the input data rate may vary from one application to the next, so the degree of spreading needs to be varied accordingly.
For the downlink the transmitted symbol rate is 3.84 M symbols per second. As the form of modulation used is QPSK this enables two bits of information to be transmitted for every symbol, thereby enabling a maximum data rate of twice the symbol rate or 7.68 Mbps. Therefore if the actual rate of the data to be transmitted is 15 kbps then a spreading factor of 512 is required to bring the signal up to the required chip rate for transmission in the required bandwidth. If the data to be carried has a higher data rate then a lower spreading rate is required to balance this out. It is worth remembering that altering the chip rate does alter the processing gain of the overall system and this needs to be accommodated in the signal processing as well. Higher spreading factors are more easily correlated by the receiver and therefore a lower transmit power can be used for the same symbol error rate.
The codes required to spread the signal must be orthogonal if they are to enable multiple users and channels to operate without mutual interference. The codes used in W-CDMA are Orthogonal Variable Spreading Factor (OVSF) codes, and they must remain synchronous to operate. As it is not possible to retain exact synchronisation for this, a second set of scrambling codes is used to ensure that interference does not result. This scrambling code is a pseudo random number (PN) code. Thus there are two stages of spreading. The first using the OSVF code and the second using a scrambling PN code. These codes are used to provide different levels of separation. The OVSF spreading codes are used to identify the user services in the uplink and user channels in the downlink whereas the PN code is used to identify the individual node B or UE.
On the uplink there is a choice of millions of different PN codes. These are processed to include a masked individual code to identify the UE. As a result there are more than sufficient codes to accommodate the number of different UEs likely to access a network. For the downlink a short code is used. There are a total of 512 different codes that can be used, one of which will be assigned to each node B.
By Ian Poole
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