Similarly, if you have two users at SF8 or 4 users at SF16 on the same branch no users to the right on the same branch can be supported and SO on rightwards across the branch. A user at SF4 will have minimal spreading gain. A user at SF256 will have maximum spreading gain with a difference of just over 20 dB between the two extremes. As you would expect, this means that as a user's data rate increases, the spreading gain decreases. The occupied bandwidth (5 MHz in this case) remains the same. The spreading codes are used with scrambling codes (long codes) with the scrambling codes providing user to user/channel to channel selectivity on the uplink and base station to base station selectivity on the downlink. Additional short codes are used for uplink and downlink synchronisation. This is the original Release 99 WCDMA radio layer code scheme. It provides a significant amount of flexibility both in terms of being able to support a wide range of variable (and potentially fast changing) data rates per user and a significant amount of flexibility in being able to support multiple data streams per user. It does, however, require careful implementation both in terms of code planning and power planning. Although the variable spreading factor codes are orthogonal (hence their name orthogonal spreading factor codes), this orthogonality can be lost if code power is not carefully balanced or the nonlinearities inherent both in the radio system and the channel are not managed aggressively. Interrelationship of the Physical Layer with Other Layers of the OSI Model If orthogonality is compromised, unwanted error energy is projected across code chan- nels that will then suffer from an unacceptably high error vector magnitude that in turn compromises sensitivity and selectivity that in turn compromises coverage (range) and capacity. The mechanism for power management is an outer and inner control loop. The inner control loop also known as fast power control runs at 1500 Hz and if correctly implemented can be effective but in its 