Ovrrbu-bts. Pdf
RBU Capacity & Spectral Efficiency
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- 1.6.4 Power Control
| 1.6.3 RBU Capacity & Spectral Efficiency
One RBU can support 128 simultaneous 34 Kbps channels using the 2.72 MHz bandwidth giving it a spectral efficiency of 1.6 bits/Hz. Of this total capacity, 9 channels are used by the system and an additional 2 Kbps per channel is system overhead. Thus the effective traffic carrying capacity is 119 channels at 32 kbps. In order to further assure continued operations under less than ideal environmental conditions, the system is conservatively rated for 100 channels at 32 Kbps. The spectral efficiency of the system is 3 to 5 times that of competitive CDMA systems primarily because this system employs bi-directional Synchronous CDMA (S-CDMA). Competing systems, including those based on IS-95 are asynchronous or at best synchronous only in one direction. The bi-directional synchronicity permits this system to use near orthogonal codes and gain maximum possible data carrying capacity. As designed, the system is code limited unlike most other CDMA systems that are noise or interference limited. 1.6.4 Power Control Radio emissions lose energy as they travel in air over long distances. In order to ensure that the received signal energy from a distant subscriber is not completely overwhelmed by that of a near subscriber, the RBU controls the power level of the subscriber unit. In this system, only the reverse channel power (from SU to the RBU) is controlled by the RBU. This power control is primarily established at SU initialization. Subsequent power adjustments are likely to be infrequent and mostly in response to transient environmental conditions. The closed loop power control is implemented by comparing against a desired power level and making incremental adjustments until the desired level is achieved. The forward channel power control is not needed since each SU receives its entire signal at only one level. The RBU merely to ensures that the received signal strength by the farthest SU is sufficient for its application. The nominal average power in both directions is 14dbm. 1.6.5 Range The range of an RBU is nominally determined by the farthest SU that can be served by the RBU. This implies that the range is determined by the farthest SU that has a received signal strength that is large enough to keep the bit error rate below the acceptable threshold. This system is nominally designed to serve an SU at a distance of 10 km from the base station antenna with a bit error rate of less than 1x10 -6. Given good line of sight and environmental conditions, this range could be further extended up to 30 km or more. Typical Urban/Suburban Deployment Typical Range: 100m to 10 km. Typical Rural Deployment Typical Range: 10 to 30 km. Range Range |