Coherent uplink detection
Coherent detection allows data reception in bad channel conditions and can increasethe capacity or the data rate of the system. This technology is, for instance, used inGSM and requires so-called “pilot symbols” (“synch words” in GSM) to be transmittedalong with the data. The WCDMA system included such pilot symbols in both thedownlink and the uplink directions, already from its earliest versions and theCdma2000 standard later also adopted uplink pilots. This gave Cdma2000 aperformance improvement over IS-95, which did not allow coherent uplink detection.Not only the uplink pilot symbols but much of the modulation structure of the
Cdma2000 uplink was made similar to that of WCDMA, which in turn was largely nherited from the early WCDMA concept work in the FRAMES project.
OVSF codes
The purpose of 3G was to accommodate an anticipated need for mobile high-speed data communication, even though it was not easily predicted what that data would beor even in which direction it might predominantly flow – from the mobile device or tothe mobile device. Therefore, a major challenge in WCDMA was to create a highlyflexible bearer system that could carry all sorts of possible “multimedia” flows.One feature that provides flexibility of the so-called physical layer of the WCDMA airinterface is the use of Orthogonal Variable Spreading Factor (OVSF) codes [18].Such codes facilitate bit-rate variability while minimizing interference and are formedthrough a code-tree principle. Certain branches in the code-tree are prohibited foruse to minimize cross-talk between users. These codes, together with a flexible rate adaptation algorithm, allowed for a highly versatile variable rate configuration of thephysical layer that could easily meet the flexibility needs of 3G.
Data rate indication
Accommodating variable rate data flows on the physical layer also entails theproblem of how to notify the receiver about the data rate currently being used.Without this knowledge, the receiver cannot process the received signal in the first place. The main solution selected in WCDMA was to add specific information with afixed and known coding, the Transport Format Combination Indicator (TFCI), to the transmitted signal, notifying the receiver about the instantaneous data rate. More precisely, it informs the receiver about all the transport formats used in the currently used transport channels [19]. The alternative solution to TFCI would be to let the receiver blindly guess what transport formats are used, by some sort of trial-anderror procedure, often denoted “blind rate detection” [20].The WCDMA standard typically uses TFCI, but also allows for the optional omission of TFCI. In the downlink, for a pure low-rate speech call, blind transport format detection (blind rate detection of multiple parallel data streams) is used.The IS-95/Cdma2000 standard does not use TFCI and requires blind rate detection.
Speech codec
To transmit speech signals efficiently while maintaining high system capacity in a cellular system, some kind of speech compression or decompression algorithm, or speech codec, is always used. The Adaptive Multi-Rate (AMR) speech codec was originally developed and selected for use in GSM because of its high voice quality at a low bit rate [21]. AMR has also proven to meet higher demands and expected future needs and therefore the 12.2Kbps AMR codec was selected as the primary speech codec for WCDMA.
The speech codec used in IS-95/Cdma2000 was first the 13Kbps Qualcomm Code Excited Linear Prediction (QCELP) and later the 8.5Kbps Enhanced Variable Rate Codec (EVRC) and modifications thereof.
Coherent detection allows data reception in bad channel conditions and can increasethe capacity or the data rate of the system. This technology is, for instance, used inGSM and requires so-called “pilot symbols” (“synch words” in GSM) to be transmittedalong with the data. The WCDMA system included such pilot symbols in both thedownlink and the uplink directions, already from its earliest versions and theCdma2000 standard later also adopted uplink pilots. This gave Cdma2000 aperformance improvement over IS-95, which did not allow coherent uplink detection.Not only the uplink pilot symbols but much of the modulation structure of the
Cdma2000 uplink was made similar to that of WCDMA, which in turn was largely nherited from the early WCDMA concept work in the FRAMES project.
OVSF codes
The purpose of 3G was to accommodate an anticipated need for mobile high-speed data communication, even though it was not easily predicted what that data would beor even in which direction it might predominantly flow – from the mobile device or tothe mobile device. Therefore, a major challenge in WCDMA was to create a highlyflexible bearer system that could carry all sorts of possible “multimedia” flows.One feature that provides flexibility of the so-called physical layer of the WCDMA airinterface is the use of Orthogonal Variable Spreading Factor (OVSF) codes [18].Such codes facilitate bit-rate variability while minimizing interference and are formedthrough a code-tree principle. Certain branches in the code-tree are prohibited foruse to minimize cross-talk between users. These codes, together with a flexible rate adaptation algorithm, allowed for a highly versatile variable rate configuration of thephysical layer that could easily meet the flexibility needs of 3G.
Data rate indication
Accommodating variable rate data flows on the physical layer also entails theproblem of how to notify the receiver about the data rate currently being used.Without this knowledge, the receiver cannot process the received signal in the first place. The main solution selected in WCDMA was to add specific information with afixed and known coding, the Transport Format Combination Indicator (TFCI), to the transmitted signal, notifying the receiver about the instantaneous data rate. More precisely, it informs the receiver about all the transport formats used in the currently used transport channels [19]. The alternative solution to TFCI would be to let the receiver blindly guess what transport formats are used, by some sort of trial-anderror procedure, often denoted “blind rate detection” [20].The WCDMA standard typically uses TFCI, but also allows for the optional omission of TFCI. In the downlink, for a pure low-rate speech call, blind transport format detection (blind rate detection of multiple parallel data streams) is used.The IS-95/Cdma2000 standard does not use TFCI and requires blind rate detection.
Speech codec
To transmit speech signals efficiently while maintaining high system capacity in a cellular system, some kind of speech compression or decompression algorithm, or speech codec, is always used. The Adaptive Multi-Rate (AMR) speech codec was originally developed and selected for use in GSM because of its high voice quality at a low bit rate [21]. AMR has also proven to meet higher demands and expected future needs and therefore the 12.2Kbps AMR codec was selected as the primary speech codec for WCDMA.
The speech codec used in IS-95/Cdma2000 was first the 13Kbps Qualcomm Code Excited Linear Prediction (QCELP) and later the 8.5Kbps Enhanced Variable Rate Codec (EVRC) and modifications thereof.
References : http://www.ericsson.com/
