3G无线基本原理.ppt

* * 两个UE和基站之间的距离不同，信号就具有不同的衰耗 对于上行信号，如果两个UE以相同的发射功率进行发射，则基站处所接收到的信号的强度相差可达30-70dB 这样距离基站远的UE的信号很容易就“淹没”在其他UE的信号中 距离基站近的一个UE就可以完全阻塞整个小区 * 由于远近效应，WCDMA系统必须引入功率控制；引入功率控制后，还能带来很多好处。 * * * * * * The radio interface (Uu) is layered into three protocol layers: Physical layer (L1) Layer 1 supports all the functions required for transmission of bit streams on the physical medium. It is also in charge of the measurement function which consists in indicating to higher layers information such as, for example, FER (Frame Error Rate), SIR (Signal to Interference Ratio), interference power, transmit power, … It is basically composed of a “layer 1 management” entity, a “transport channel” entity, and a “physical channel” entity. Data link layer (L2) The layer 2 protocol is responsible for providing functions such as mapping, ciphering, retransmission, segmentation. Network layer (L3) Layer 3 is split into 2 parts: AS (Access Stratum) The AS is composed of the RRC (Radio Resource Control) entity and the “duplication avoidance” entity. NAS (Non Access Stratum) The NAS is composed of the CC (Call Control) and MM (Mobility Management) parts. * * * * As in GSM, UMTS uses the concept of logical channels. A logical channel is characterized by the type of information that is transferred. For example, some channels are used to transfer dedicated information, some for transfer of general control information, etc.. As in GSM, logical channels can be divided into two groups: control channels for control plane information and traffic channel for user plane information. The traffic channels are: DTCH (Dedicated Traffic CHannel): a point-to-point bi-directional channel, that transmits dedicated user information between a UE and the network. That information can be speech, circuit-switched data or packet-switched data. The payload bits on this channel come from a higher layer application (the AMR codec for example). Control bits can be added by the RLC (protocol information) in cas