Scrambling Code planning in WCDMA System

Abstract:The broadband code division multiple access (WCDMA) system uses a Code Division Multiple Access wireless access method without frequency planning. However, it is necessary to plan the disturbance codes of adjacent cells to distinguish different cells. Based on the disturbance code planning of the WCDMA wireless network, we can determine the multiplexing distance between two cells using the same disturbance code and distinguish each cell. During code scrambling planning, you can use the Planning Method Based on the disturbance code group to make the mobile station quickly search for the residential area. The basic principle of scrambling code planning is that the same primary Scrambling Code cannot be possessed for cells with overlapping coverage in the network.

Keywords:Scrambling Code; multiplexing distance; cell search

Abstract:For a WCDMA system, frequency planning is not required, but scrambling code planning for neighborhood cells is necessary. with scrambling code planning, the multiplex distance between adjacent cells using the same scrambling code can be determined and different cells can be distinguished. the Planning Method Based on scrambling code groups can be adopted for Mobile Station to quickly search cells. one principle for scrambling code planning is that overlapping cells in a network cannot use the same primary scrambling codes.

Key words:Scrambling Code; multiplex distance; cell searching

In the WCDMA network, the main difference between different users and base stations depends on different scrambling codes, so it is necessary to plan the scrambling codes. 1. Principle of scrambling code planning The disturbance code planning in the WCDMA system is similar to the frequency planning in the GSM system, mainly to allocate the main disturbance code (PSC) to the residential area ). In the WCDMA system, there are a total of 512 PSC links in the middle and lower lines. a psc is assigned to each cell as one of the identification parameters of the cell. When the number of residential areas exceeds 512, you can allocate a PSC to a residential area, as long as the distance between the residential areas using the same PSC is large enough, so that the received signal is below the threshold level within the coverage range of the same PSC. Therefore, the main idea of the scrambling code planning is to determine the minimum wireless propagation distance of two cells using the same scrambling code-reuse distance. The specific calculation process is as follows: 1. For the convenience of analysis, assuming that the transmit power of the basic stations in all the communities is equal and the noise is ignored, the I and J of the two communities use the same disturbance code, the Link Loss between the two cells is lij, and the coverage radius of the two cells is Ri and RJ, respectively. In order to avoid fuzzy interference between the two communities due to the same disturbance code, the distance between the two communities must be large enough, so that the wireless signal of the remote community with the same disturbance code is far smaller than the wireless signal of the local community. Therefore, the following inequality must be met: Alpha indicates the path loss index, and pgdb indicates the processing gain, in dB. The first item on the left of the above inequality indicates the minimum path loss of the remote community J, and the second item indicates the maximum path loss of the local community I. From the above inequality, we can obtain the lij that meets the inequality requirements: The above inequality describes the multiplexing distance that satisfies the disturbance code planning. The purpose of the scrambling code plan is to determine the reuse mode of the scrambling space. Replace max (Ri, RJ) with rmax, reuse the number of cells in the cell set to K, reuse the distance between cells, and rmin is the radius of the cell with the minimum coverage area, the minimum number of residential areas that meet the disturbance code planning is as follows: Taking the 12.2 kb/s AMR voice service as an example, if pgdb = 24 dB, path loss index α = 3, rmax/rmin = 3, the number of cell multiplexing k ≥ 160 can be calculated, 3 k = 480 for three cells, that is, the size of the multiplexing set is 480 scrambling codes, and 512-480 = 32 surplus scrambling codes can be used. Because the scrambling code is used to differentiate cells, it can be used for the initial access network of the Mobile Station, re-election and switching of cells, etc. Therefore, the distribution of scrambling codes is very important in system planning. In actual situations, factors such as the wireless transmission environment and irregular location distribution of base stations make it difficult to determine the effectiveness of the scrambling code planning. Therefore, scrambling code planning is usually completed by the network planning software. Generally, the above area multiplexing distance calculation method can be used to realize the automatic allocation of scrambling codes for software-implemented scrambling code planning. However, the principle of scrambling code planning is determined by the network planning engineers. 2. Principle of scrambling code planning 2.1 community search process The 18-bit long shift register can generate 218-1 scrambling codes. because too many scrambling codes may lead to long searching time on the Mobile Station, the system design is complicated, therefore, only 8 192 scrambling codes are selected in the 3GPP specification. These scrambling codes are divided into 512 sets, each of which includes a PSC and 15 auxiliary scrambling codes (SSC ). Each residential area uses one of the PSC. Further, these 512 PSC instances are divided into 64 groups, with 8 PSC instances in each group. The purpose of the scrambling code planning is to make the mobile station quickly and accurately complete the area search, identification and synchronization. To this end, we will briefly introduce the search process of the community. Generally, a terminal searches for a residential area without knowing any information about the residential area in advance, and requires three steps: time slot synchronization, frame synchronization, and PSC capturing. The time slot synchronization and frame synchronization involve the primary synchronous channel (P-SCH) and secondary synchronous channel (S-SCH ). The 10 ms wireless frame of the Primary and Secondary synchronization channels is divided into 15 time slots, each of which is 2 560 chips. Figure 2 shows the structure of the synchronous channel (Sch) wireless frame. The P-SCH consists of a PSC with a length of 256 chip. The PSC in each cell in the system is the same, and each time slot is transmitted once, which is expressed in ACP in Figure 2. The S-SCH retransmits a SSC with 15 sequences, with a code length of 256 chip and parallel transmission with the P-SCH. In Figure 2, SSC is used to represent, where I = 0, 1 ...... 63 is the serial number of the scrambling code group, K = 0, 1, 2 ...... 14 is the time slot number. SSC is a code selected from 16 different codes with a length of 256 chip. The serial number on the S-SCH indicates the code group of the downlink scrambling code of the residential area.   The first step of cell search is synchronization of time slots. PSC of all cells is the same, and the terminal knows the serial number of the cell in advance, therefore, you only need to use a well-performing matching filter to detect and capture the PSC, so as to determine the time slot boundary of each physical channel. The second step is frame synchronization, SSC is sent on the S-SCH, SSC is also 256 pieces of code, at the beginning of each time slot and sent together with PSC, each time slot uses a SSC. The difference is that SSC has a total of 16 different code piece sequences. These SSC are arranged into 64 different combinations, each of which is 15 SSC characters long for one wireless frame, note that the same SSC may appear several times in a combination, and each combination corresponds to a group of PSC. In this way, you can determine the group of PSC used by the community in step 2. After the block group is determined in the first two steps, find the PSC that matches the current Community from the eight PSC, and capture the PSC. 2.2 Scrambling Code Planning Method The principle of scrambling code planning is that the same primary Scrambling Code cannot be possessed for cells with overlapping coverage in the network. As described above, the scrambling code planning can be based on the scrambling code group or all different scrambling codes. The scheme based on all different scrambling codes is to allocate 512 PSC to each residential area as long as the multiplexing distance is met. The planning based on the disturbance code group is to allocate a different disturbance code group to each base station. The different sectors of each base station are allocated among the eight different disturbance codes in the disturbance code group. The difference between the two allocation methods is that the PSC sequence and SSC sequence of different sectors in the base station are the same in the Planning Method Based on the disturbance code group. Based on all the different disturbance codes, the disturbance codes of different sectors of the base station belong to different disturbance code groups. The PSC sequence is the same, while the SSC sequence is different. It can be seen that the Planning Method Based on the disturbance code group is more convenient and simple than the Planning Method Based on all different disturbance codes, and it is faster and more flexible in providing mobile station search community. Therefore, generally, the planning of Scrambling Code is based on the primary scrambling code group. After determining the planning principles, we should consider the multiplexing distance of the disturbing code group. This is mainly done by calculating the load-to-dry ratio (C/I) of signals. Table 1 provides an example of a scrambling code plan [1].   For distribution of disturbance code groups, we also need to fully consider the actual planned coverage of the residential area, and consider the distance of PSC reuse based on the actual situation of the region, especially the distribution of disturbance codes at the regional boundary. In addition, you should retain some PSC disturbing code groups as needed for network resizing. In the actual disturbance code planning, in order to enable the mobile station to search for the residential area as soon as possible and establish synchronization with the neighboring area, so as to allow fast switching, it is required that the disturbance code of the residential area and its adjacent area should belong to the smallest possible disturbance code group. Because every time one disturbance code group is demodulated, an additional 20 ms time is required [2]. It can be seen that it is very important to reasonably plan the scrambling code based on the network structure and wireless environment. For example, in dense urban areas, high-density sites form a complex list of neighboring areas and switching relationships. Therefore, a relatively small number of disturbance code groups should be used to reduce search time and improve network quality. In actual planning, not all disturbance code groups will be used up. The specific quantity of use should be determined based on the actual network plan in the future. In addition, if the network uses the second carrier, all the scrambling codes can be reused. 3 conclusion Because the WCDMA system introduces new business requirements and the wireless interface of the system is different from the second generation GSM system, the planning and design of the wireless network is different from that of the GSM system. This article discusses in detail the problem of scrambling code planning in the WCDMA system, from which we can see that the main principle of scrambling code planning is to combine the actual characteristics of the region with the code resources allowed, make the PSC reusable distance as large as possible. At the same time, when planning the disturbance code, the Planning Method Based on the disturbance code group can accelerate the cell search process of the mobile station, and the planning is flexible and simple. All these conclusions have good guiding significance for the wireless network planning engineers of the WCDMA system.