13-06-2012, 05:17 PM
Code-division-duplexing
[attachment=24406]
Introduction
To utilize spectrum efficiently, two transmission techniques need to be considered: one is a multiple access scheme and the other a duplexing system. There are three multiple access schemes namely TDMA, FDMA and CDMA. The industry has already established the best multiple access scheme, code-division multiple access (CDMA), for 3G systems. The next step is to select the best duplexing system. Duplexing systems are used for two-way communications. Presently, there are only two duplexing systems used: frequency-division duplexing (FDD), and time-division duplexing (TDD). The former uses different frequencies to handle incoming and outgoing signals. The latter uses a single frequency but different time slots to handle incoming and outgoing signals.
Interference and Capacity
One of the key criteria in evaluating a communication system is its spectral efficiency, or the system capacity, for a given system bandwidth, or sometimes, the total data rate supported by the system. For a given bandwidth, the system capacity for narrow band radio systems is dimension limited, while the system capacity of a traditional CDMA system is interference limited. Traditional CDMA systems are all self-interference system.
Three types of interference are usually considered. By ISI we mean InterSymbol Interference, which is created by the multi-path replica of the useful signal itself; MAI, or Mutual Access Interference, which is the interference created by the signals and their multi-path replica from the other users onto the useful signal; and ACI, or Adjacent Cell Interference, which is all the interfering signals from the adjacent cells onto the useful signal.
Why Reduce Interference In A Cellular System
Reducing interference in a cellular system is the most effective approach to increasing radio capacity and/or transmission data rate in the wireless environment. Therefore, reducing interference is a difficult and important challenge in wireless communications. The interference in an FDD system using a CDMA scheme is shown in Fig. 2. Looking at Fig. 3, we realize that a TDD system is very undesirable to use in a large-area cellular system.
LAS-CDMA
The LAS- CDMA utilizes the LAS codes which are a set of smart codes. The spreading sequences exhibit zero correlation values, when the relative delay-induced code offset is in the so-called Zero Correlation Zone (ZCZ) or Interference Free Window (IFW) of the spreading code. The attractive family of Large Area Synchronized (LAS) CDMA spreading sequences is constituted by the combination of the so-called Large Area (LA) codes and Loosely Synchronous (LS) codes. The resultant LAS codes exhibit an IFW, where the off-peak aperiodic autocorrelation values as well as the aperiodic cross-correlation values become zero, resulting in zero ISI and zero MAI, provided that the time-offset of the codes is within the IFW. In order to ensure that the relative time-offsets between the codes are within the IFW, the mobiles are expected to operate in a quasi-synchronous manner. More specifically, interference-free CDMA communications become possible, when the total time-offset expressed in terms of the number of chip intervals, which is the sum of the time-offset of the mobiles plus the maximum channel-induced delay spread is within the designed IFW. In case of high transmission-delay differences accurate timing-advance control has to be used, as it was also advocated in the GSM system. Provided that these conditions are satisfied, a major benefit of the LAS codes is that they are capable of achieving a near-single-user performance without multi-user detectors.
[attachment=24406]
Introduction
To utilize spectrum efficiently, two transmission techniques need to be considered: one is a multiple access scheme and the other a duplexing system. There are three multiple access schemes namely TDMA, FDMA and CDMA. The industry has already established the best multiple access scheme, code-division multiple access (CDMA), for 3G systems. The next step is to select the best duplexing system. Duplexing systems are used for two-way communications. Presently, there are only two duplexing systems used: frequency-division duplexing (FDD), and time-division duplexing (TDD). The former uses different frequencies to handle incoming and outgoing signals. The latter uses a single frequency but different time slots to handle incoming and outgoing signals.
Interference and Capacity
One of the key criteria in evaluating a communication system is its spectral efficiency, or the system capacity, for a given system bandwidth, or sometimes, the total data rate supported by the system. For a given bandwidth, the system capacity for narrow band radio systems is dimension limited, while the system capacity of a traditional CDMA system is interference limited. Traditional CDMA systems are all self-interference system.
Three types of interference are usually considered. By ISI we mean InterSymbol Interference, which is created by the multi-path replica of the useful signal itself; MAI, or Mutual Access Interference, which is the interference created by the signals and their multi-path replica from the other users onto the useful signal; and ACI, or Adjacent Cell Interference, which is all the interfering signals from the adjacent cells onto the useful signal.
Why Reduce Interference In A Cellular System
Reducing interference in a cellular system is the most effective approach to increasing radio capacity and/or transmission data rate in the wireless environment. Therefore, reducing interference is a difficult and important challenge in wireless communications. The interference in an FDD system using a CDMA scheme is shown in Fig. 2. Looking at Fig. 3, we realize that a TDD system is very undesirable to use in a large-area cellular system.
LAS-CDMA
The LAS- CDMA utilizes the LAS codes which are a set of smart codes. The spreading sequences exhibit zero correlation values, when the relative delay-induced code offset is in the so-called Zero Correlation Zone (ZCZ) or Interference Free Window (IFW) of the spreading code. The attractive family of Large Area Synchronized (LAS) CDMA spreading sequences is constituted by the combination of the so-called Large Area (LA) codes and Loosely Synchronous (LS) codes. The resultant LAS codes exhibit an IFW, where the off-peak aperiodic autocorrelation values as well as the aperiodic cross-correlation values become zero, resulting in zero ISI and zero MAI, provided that the time-offset of the codes is within the IFW. In order to ensure that the relative time-offsets between the codes are within the IFW, the mobiles are expected to operate in a quasi-synchronous manner. More specifically, interference-free CDMA communications become possible, when the total time-offset expressed in terms of the number of chip intervals, which is the sum of the time-offset of the mobiles plus the maximum channel-induced delay spread is within the designed IFW. In case of high transmission-delay differences accurate timing-advance control has to be used, as it was also advocated in the GSM system. Provided that these conditions are satisfied, a major benefit of the LAS codes is that they are capable of achieving a near-single-user performance without multi-user detectors.