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| WCDMA信号特点 |
| 2008年5月9日 09:19
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WCDMA Signal Characteristics
Andrew Wright，Director, Product Research
Oliver Nesper， DSP Design Engineer
The WCDMA down-link signal model for a single-carrier is shown in the following figures. Each down-link signal consists of a number of control and pilot channels that are always required. Additionally, each user operating in the cell can utilize one or more traffic channels (DPCHs) with a variety of spreading factors. Figure 1 shows the spreading operation for all downlink channels (DPCH and control channels), with the exception of the synchronization channel SCH. The incoming data streams are mapped to QPSK symbols and spread with the OVSF spreading code assigned to that channel. This operation provides the separation (orthogonality) between channels/users. The complex spread symbols are then multiplied by a scrambling code specific to the base station. This operation provides the signal separation between base stations.
Figure 1 Spreading for all downlink channels except SCH[4]
Figure 2 shows the combining of all physical channels with the primary (P-SCH) and secondary (S-SCH) synchronization channel. The synchronization channel provides radio frame and time slot synchronization. As WCDMA is an asynchronous system1, these sequences are needed to simplify the fast timing acquisition by the mobile subscriber unit. At the output of this block, the base band WCDMA signal samples are available. These are ordinarily pulse-shaped to form a bandlimited waveform. This waveform, depending upon the number of users and type of information being transferred, can cause very high peak to average (crest factor) waveforms to be generated. Combining individual information carriers on separate 5 MHz frequency allocations to form a multi carrier 20 MHz system further expands the peak to average waveform. Without intervention or additional signal processing crest factors that exceed 16 dB are not uncommon. Ordinarily this would lead to a very inefficient power amplifier design simply to ensure linearity is maintained.
Figure 2 Combining of all Down-link Channels including SCH[4]
1． WCDMA Signal Waveform Requirements
Table 1 defines the system requirements that need to be met by the base station transmitter [3] and that are of interest in the scope of this discussion. Error vector magnitude or EVM is a measure of the difference between the ideal transmitted waveform and the waveform actually transmitted. Peak code domain error or PCDE is a measure of how much cross-talk exists between different channels within the WCDMA signal. In other words, it is an indication of how well the orthogonality between different code channels is maintained. Adjacent channel leakage ratio or ACLR is the ratio of the transmitted power to the power measured in an adjacent channel. [3] The most important point to realize that each of these significant signal quality metrics is undermined as an microwave amplifier is driven close to saturation, which unfortunately is the most efficient operating point. Typically, a compromise between the efficient and distortion-free operating points is found. This results in amplifiers that are operated in a mode where the average operating point is set such that the signal crests are just less than the maximum saturated output power that the amplifier can deliver. Depending upon amplifier technology and circuit topology, this can result in very inefficient operation because the signal quality metrics defined in Table 1 can not be violated.
Table 1 3GPP Requirements
The 3G system specification[3] specifies different test models that are to be used for specific tests to be performed. The first test model, TM1, employs a user population of 64 with 4 carriers. This test case is used here to exemplify a typical traffic scenario that is well defined and so that results can be easily reproduced. Table 2 displays the settings that were employed for the test signal generation.
Table 2 Test Signal for 4 Carrier TM1 Signal with 64 Active Users
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