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Code division Duplexing
This document discusses code division duplexing techniques for wireless communications. It introduces code division multiple access (CDMA), frequency division duplexing (FDD), time division duplexing (TDD), and code division duplexing (CDD). CDD uses a combination of LAS codes, which have zero correlation values within an interference-free window, and TDD to eliminate interference. By using LAS codes and a TDD approach, CDD can create a noise-limited environment with higher spectral efficiency than traditional CDMA systems. The document also discusses how LAS codes are generated and how they reduce interference to allow CDD to be a simpler and more effective duplexing approach than FDD or TDD alone.
Code division Duplexing
- 1. CODE DIVISION DUPLEXINGCODEDIVISION DUPLEXING Under the esteemed guidance of Smt.V.LAVANYA,M.Tech Assistant Professor Presented by M.BHARATHI DEVI (09331A0469)
- 2. Topics to becoveredTopics to be covered INTRODUCTION CDMA FDD TDD CDD SMART CODES LAS _CDMA 1.Access codes in traditional CDMA and LAS-CDMA
- 3. Topics to beCoveredTopics to be Covered Generation of LAS-Codes Perspectives of LAS-CDMA for the 4G wireless systems CDD-A New Milestone CONCLUSION References
- 4. IntroductionIntroduction Reducing interference ina 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.
- 5. Two transmission techniques 1.Multipleaccess scheme 1.TDMA 2.FDMA 3.CDMA 2.Duplexing system 1.Frequency Division Duplexing (FDD) 2.Time Division Duplexing (TDD)
- 7. FDMAFDMA Different frequencies areallocated for multiple accessing Interference is more due to more number of frequencies. TDMA Different time slots are allocated for accessing Interference is even more than in FDMA.
- 8. CDMACDMA CDMA is thebest multiple access scheme when compared to all others for combating interference. 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. Traditional CDMA systems are all self- interference system.
- 9. FDDFDD Uses different frequenciesto handle incoming and outgoing signals. Currently, all cellular systems use frequency-division duplexing in an attempt to eliminate interference from adjacent cells.
- 11. TDDTDD Uses a singlefrequency but different time slots to handle incoming and outgoing signals. TDD can only be used for small confined area systems. TDD is not used for mobile systems.
- 13. CDDCDD Code-division duplexing isan innovative solution that can eliminate all kinds of interference. Eliminating all types of interference makes CDD the most spectrum efficient duplexing system. The smart code plus TDD is called CDD.
- 15. What type ofcodes are to be used inWhat type of codes are to be used in CDD?CDD? Walsh codes are used in CDMA,which are not to smart. Walsh codes are orthogonal in nature i.e., they arrive at different times. Thus, smart codes are efficient to be used.
- 16. What are smartcodes?What are smart codes? The codes which satisfy the following properties are called smart codes: Auto-correlation Rxx (τ) = 0 for τ = 0 δ for τ ≠ 0, within window τ0 Cross-correlation Rxy (τ) = δ for all τ, within window τ0 Where τ0 is a correlation window, δ can be zero or low correlation value.
- 19. How do smartcodes reduceHow do smart codes reduce interference?interference? Smart code sequences have two properties: 1.Auto-correlation equals zero, which causes no multipath interference. 2.Cross-correlation equals zero, which causes no multi-user interference.
- 21. LAS CDMALAS CDMA TheLAS- 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.
- 22. This family isconstituted by the combination of the so-called Large Area (LA) codes and Loosely Synchronous (LS) codes. The disadvantage of LAS codes is that the number of codes having an IFW is limited. When the LAS CDMA system operates in an asynchronous manner, it encounters a more serious MAI and Multipath Interference (MPI) than traditional CDMA.
- 23. -20 -10 010 20 τ τ G-1/2 G – Processing Gain 0 G-1/2 Auto-Correlation Cross-Correlation Relative delay Multiple Access Codes Correlations of Traditional CDMA
- 24. • Auto-correlation: • Cross-correlations:(Relative delay) τ 0 τ IFW IFW: Interference Free Window a b τ 0 0 0 0 0 0 τ Multiple Access Codes Correlations of LAS-CDMA
- 25. LAS CDMA continued…LASCDMA continued… LAS-CDMA introduces a novel multiple access scheme, based on two families of CDMA codes, LA codes and LS codes. LA codes are a family of pulse train with carefully designed pulse intervals. LA codes are mainly used to reduce the ACI (Adjacent Cell Interference).
- 26. LAS CDMALAS CDMA Thepulses of LA code are formed by passing through a LS code to its matched filter at the required pulse positions. LS codes are used for spreading. The family of LS codes has interesting correlation properties.
- 27. Generation of LAS-CodesGenerationof LAS-Codes LA Codes LA codes belong to a family of ternary codes having elements of ±1 or 0. Their maximum correlation magnitude is unity and they also exhibit an IFW.
- 28. DRAWBACK OF LACODESDRAWBACK OF LA CODES A specific drawback of this family of sequences is their relatively low duty ratio, quantifying the density of the non-zero pulses, since this limits the number of codes available and hence the number of users supported.
- 29. Loosely Synchronized (LS)CodesLoosely Synchronized (LS) Codes Loosely Synchronized (LS) codes exploit the properties of the so-called orthogonal complementary sets.
- 30. Seeding LS Codesin LA Codes toSeeding LS Codes in LA Codes to Generate LAS codesGenerate LAS codes The main problems associated with applying LA codes in practical CDMA systems are related to their low duty ratio and to the resultant small number of available codes. A specific family of LAS codes mitigates this problem by combining the LA codes and the LS codes.
- 31. Perspectives of LAS-CDMAfor thePerspectives of LAS-CDMA for the 4G wireless systems4G wireless systems Due to the existence of IFW, LAS-CDMA may have a much smaller interference level than traditional CDMA. Consequently, LAS-CDMA system may have a much higher capacity as well as a higher spectral efficiency than a traditional CDMA.
- 32. According to thebasic multi-user information theory, the optimum way to share (not to distribute) a channel capacity is the “Waveform Division” Multiple Access or CDMA technology. However, since traditional CDMA is a self- interference limited, that would drastically limit its capacity, in studying 4G systems, many people focus their attention on OFDM/OFDMA technology, space-time coding for MIMO channels etc…
- 33. CDD-A New MilestoneCDD-ANew Milestone 1.CDD Technology Is A New Milestone From FDD Systems To CDD Systems. Cellular technology before 1989 utilized frequency reuse in FDMA and TDMA networks. After 1989, CDMA was a breakthrough technology for maintaining the same frequency being used in all the cells.
- 34. The LAS smartcodes are the next breakthrough technology. Applying LAS technology to a TDD system becomes a CDD system. Without the sending and receiving occurring at the same time we can use the same smart code for both sending and receiving; thus, LAS + TDD = CDD. CDD is no more like TDD and is a better system for cellular.
- 35. 2.Reduce Interference-Limited Environment ToNoise-Limited Environment Before 1989, cellular systems were using multiple cells with multiple frequencies assigned in each cell. These systems generate an interference- limited environment. After 1989, multiple cells used the same frequency. Although the capacity increased using CDMA, the system is still in an interference-limited environment.
- 36. CONCLUSIONCONCLUSION CDMA is thebest multiple access scheme when compared to all others for combating interference. A set of smart codes can make a high- capacity CDMA system very effective without adding other technologies. A CDD system is simpler in design and lower cost.
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