Defination and work of Eb/No in CDMA
Eb/No corresponds to energy per bit over interference plus noise density for a given target FER (typical FER target is 1%). In digital communications, it is customary to designate one-sided noise density with No. In CDMA, interference is dominated by the noise generated due to other users in the system. Here No, is refers to the total power density due to interference and noise.
Included in the CDMA Eb/No value is diversity improvement arising from performance in Rayleigh fading. This is distinct from the entry “Soft Handoff Gain” which represents an estimate of the performance improvement of soft handoff, relative to hard handoff, when experiencing log normal shadowing.
In general, the required downlink Eb/No, to provide an acceptable audio quality, improves at higher speeds and in soft handoff. In the uplink path, the required Eb/No improves at lower speeds (reverse of downlink). The worst case Eb/No value for the uplink is at about 30 Kmph.
The uplink Eb/No value accounts for rake (non-coherent combining) receiver, dual antenna, and interleaving/coding. The downlink Eb/No value accounts for rake (coherent, maximal ratio combining), and interleaving/coding. For mobile systems, the Eb/No target varies dynamically as the mobile moves around. However, FWTs are fixed and the only movement is that of people around the FWT in a building and buses or pedestrians close to an outdoor FWT antenna.
Optimized FWT deployment may significantly reduce the Eb/No target by avoiding the fading caused by the surrounding environment. In a mobile environment, the fading characteristic is Rayleigh. For a fixed system, the fading environment may be more Rician. The Eb/No value assumes a certain type of fading environment. The Eb/No requirement for a fixed system will therefore be different than for a mobile environment.
The Eb/No target value can range from 4 dB to 8 dB for CDMA WLL systems. The Eb/No target value should be set to 8 dB for isolated cells using indoor omni FWT antennas or for cells with little SHO benefits in the fringe areas. However, if external directional FWT antennas are used and a Line Of Site (LOS) path exists between the cell site and the FWT antenna, an Eb/No target value of 4 dB may be used.
As improvements are made to the hardware (chipsets) and to the software (how the energy is managed), the Eb/No requirement level may be lessened. Typical Eb/No values used for fixed systems are stated above. The current requirements for a mobile system are approximately 7 to 7.5 dB for the 8 kb and 13 kb vocoder respectively. From a link budget analysis, only one Eb/No value can be assumed for a given scenario. the Eb/No requirements needed to meet a desired frame error rate for each link that is being analyzed between the user and the site.