Reducing interference in a cellular system is the most effective approach to increasing radio capacity and transmission data rate in the wireless environment. Therefore, reducing interference is a difficult and important challenge in wireless communications.
In every two-way communication system it is necessary to use separate channels to transmit information in each direction. This is called duplexing. Currently there exist only two duplexing technologies in wireless communications, Frequency division duplexing (FDD) and time division duplexing (TDD). FDD has been the primary technology used in the first three generations of mobile wireless because of its ability to isolate interference. TDD is seemingly a more spectral efficient technology but has found limited use because of interference and coverage problems.
Code-division duplexing (CDD) is an innovative solution that can eliminate all kinds of interference. CDMA is the best multiple access scheme when compared to all others for combating interference. However, the codes in CDMA can be more than one type of code. A set of smart codes can make a high-capacity CDMA system very effective without adding other technologies. The smart code plus TDD is called CDD.
This paper will elaborate on a set of smart codes that will make an efficient CDD system a reality. The CDMA system based on this is known as the LAS-CDMA, where LAS is a set of smart codes. LAS-CDMA is a new coding technology that will increase the capacity and spectral efficiency of mobile networks. The advanced technology uses a set of smart codes to restrict interference, a property that adversely affects the efficiency of CDMA networks.
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.
In the current cellular duplexing systems, FDD has been the appropriate choice, not TDD. Currently, all cellular systems use frequency-division duplexing in an attempt to eliminate interference from adjacent cells. The use of many technologies has limited the effects of interference but still certain types of interference remain. Time-division duplexing has not been used for mobile cellular systems because it is even more susceptible to different forms of interference. TDD can only be used for small confined area systems.
Code-division duplexing is an innovative solution that can eliminate all kinds of interference. Eliminating all types of interference makes CDD the most spectrum efficient duplexing system.
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.
Traditional synchronous CDMA systems employ almost exclusively Walsh-Hadamard orthogonal codes, jointly with PN sequence, and Gold codes, Kasami codes, etc. In these systems, due to the difficulty in timing synchronization and the large cross-correlation values around the origin, there exists a “near far” effect, such that in some typical system, fast power control has to be employed in order to keep an uniform received signal level at the base station. On the other hand, in forward channel all the signals’ power must be kept at an uniform level. Since the transmitting power of a user would interfere others and even may interfere itself, if one of the users in the system increases its power unilaterally, all other users power should be simultaneously increased; otherwise the controlled system power regime will be destroyed, and the capacity would be drastically decreased. This is because any radio channel, especially mobile channel, is a random time-varying time dispersion channel due to the multi-path effect, so that the received signal can not be reached at the receiver simultaneously.
In traditional CDMA, the auto-correlation functions as well as the cross-correlation functions are all not ideal, so that the signals at different arrival time can not be separated properly at the receiver. It is just such effect that makes the traditional CDMA a self-interference system. It would be practically impossible to enhance the traditional CDMA’s robustness in terms of interference resistance by increasing the user’s power when the network traffic load is high.
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.
Although FDD is the right choice for cellular systems, the interference is still very high. Today many enhanced technologies have been added together to reduce interference in FDD systems, but none of these technologies can be used solely and effectively. The Large Area Synchronous (LAS) Codes are a set of smart codes that can reduce interference very effectively. The effectiveness of smart codes applied to TDD makes it the right choice in cellular systems. The application of LAS Codes in a TDD system (called TD-LAS system) creates a CDD system.
LAS-CDMA (Large Area Code Division Multiple Access) employs a novel multiple access scheme, which is different from all the known traditional CDMA. The auto-correlation functions of all LAS-CDMA codes are ideal, and there exists an IFW (Interference Free Window), or a “zero correlation zone” (ZCZ) in their cross-correlation functions of its access codes around the origin. Due to the existence of IFW or ZCZ, a LAS-CDMA system can have a much higher system capacity and spectral efficiency than that of a traditional CDMA.
A Set of Smart Codes
The code used in today’s CDMA scheme is the Walsh code, which is not too smart. Walsh codes have the orthogonality property among codes while the time shift t = 0 (i.e., no time shift t or time delay spread). However, in the mobile radio environment the signal arrival can have a long time shift. The property of Walsh codes cannot properly be applied to this environment. Now there is a set of smart codes that have orthogonality among the codes for time shift t ≠ 0. The codes arrive at the terminals at different time shifts; because of the orthogonal nature, all undesired codes are blocked. Thus, smart codes are the proper codes to handle this situation. Therefore, in the future we can use different kinds of codes for CDMA schemes, which we will name a type of CDMA with a specified code (e.g., CDMA/Code A and CDMA/Code B). The properties of smart codes have to meet the following requirements:
Rxx (τ) = 0 for τ = 0
δ for τ ≠ 0, within window τ0
Rxy (τ) = δ for all τ, within window τ0
Where τ0 is a correlation window, δ can be zero or low correlation value. Outside the correlation window is beyond the time delay spread range of the received signal. Although the correlation value outside the range is high, there is no impact on our desired signal, as shown in Fig. 5.
With this property, we can illustrate the merit of using this smart code. Assume that eight smart codes are transmitted, as shown in Fig. 6. They arrive at the receiver of Code C1 at different times due to the multipaths caused by different reflectors. Because of the cross-correlation property, the desired code to be received is C1. However, many C1 codes can be received due to the effects of the multipaths. This does not occur though; due to the auto-correlation property, only Code C1 at time t1 is received, as shown in Fig. 6. We do not need the strength of more than one path signal to be added for increasing carrier-to-interference ratio (C/I) since we are only receiving carrier-to-noise ratio (C/N), not C/I. The rest of the signals from different paths do not cause any interference, and there is no need to use any means to collect them for the purpose of reducing interference. Hence, with this smart code property we can effectively eliminate interference, and we do not need other technologies.