Multifrequency Networking Solution for TD-SCDMA

 In radio communications, system capacity and interference directly affect user experience.  Interference limits system capacity, so the primary task of radio network planning and optimization is to solve this contrary relationship. Users are also increasingly aware of networking technology within the system. How to provide a reasonable networking scheme, and increase system capacity and performance to the largest possible extent by reducing interference has become a recent focus of study.

1 Multifrequency Networking Mode
    Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) is a 3G standard being pursued in China. It is a multiaccess technology and a hybrid of Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA) and Space Division Multiple Access (SDMA). TD-SCDMA adopts smart antennas, joint detection technology, and Time Division Duplex (TDD) mode. Symmetric frequency band is not required, but high frequency spectrum utilization and asymmetric data service are supported. TD-SCDMA carrier bandwidth is 1.6 MHz, and compared with 5 MHz Wideband Code Division Multiple Access (WCDMA), three carriers can be provided within the same bandwidth. Therefore, frequency planning is less complicated than in other 3G networks, and multiple carriers can be used for networking. Before describing the differences between networking modes, it is useful to look at the concept of a traditional cell. In TD-SCDMA, each carrier section is an independent cell by default. The Uu interface between user equipment and radio access network is configured and operated only in one carrier. To establish a lub interface cell, one cell is configured with an absolute carrier. For multiple carriers, each frequency is regarded as a logic cell.  In the case of three sectors and three carrier frequencies, there are nine logic cells that are independently operated. Each cell transmits pilot frequency and broadcast information. Nine carrier frequencies must be configured with nine complete public channels.  Broadcast Channel (BCH), Forward Access Channel (FACH) and Paging Channel (PCH) are omnidirectional channels. Therefore, for multicarrier configuration in traditional cell mode, shared frequency networking and difference-frequency networking [1] are typically used.

1.1 Shared Frequency Networking Mode
    In shared frequency networking, each cell has the same number of carriers and these carriers are the same. Each carrier is an independent logic cell with its own common control channel, downlink pilot channel and independent broadcast channel.  On the 10 MHz bandwidth, TD-SCDMA supports up to six carriers. Configuration of shared frequency networking is shown in Fig. 1.

    Using the shared frequency networking mode, band utilization can be improved. On the 15 MHz bandwidth, nine carriers are supported, and the station model can be S9/9/9. Multiple logic cells can exist in the same physical environment. On the service channel, service quality can be ensured through intelligent smart and joint detection technology. But the broadcast channel uses omnidirectional transmission. Therefore, interference between carrier frequencies is serious, and can significantly impact system performance and capacity.

1.2 Difference Frequency Networking Mode
    Compared to shared frequency networking mode, difference frequency networking mode is used to distribute different carriers in adjacent cells. In the case of 15 MHz bandwidth, TD-SCDMA contains nine carriers. As shown in Fig. 2, the maximum station model is S3/3/3.

    In difference networking mode, users of the same carrier can be separated to increase the reuse distance and to reduce interference between frequencies. In this way, the system performance and capacity are improved.

1.3 Multifrequency Technology
    Due to defects in these two networking modes, Multifrequency technology is proposed. If multiple carrier frequencies exist, one can be selected from the assigned Multifrequency carriers to serve as the primary carrier of the whole cell. The remaining carriers are secondary carriers. Cell division in multifrequency technology is different from that in traditional cells: for multifrequency technology, multiple carrier frequencies of the same section belong to the same logic cell. The primary carrier and secondary carrier use the same scramble code and training sequence code. As a result, multiple carriers in the same cell have the same cell ID. The common control channel is configured on the primary carrier, and on the secondary carrier there is no common control channel. The service channel is configured on both the primary carrier and secondary carrier. Multiple time slot service should be configured on the same carrier to reduce complexity in terminal implementation. Uplink and downlink of the same user are configured on the same carrier. Switch points of the primary carrier and secondary carrier are the same. This restriction is caused by characteristics of the transceiver. If the uplink and downlink switch points of the primary carrier and secondary frequency are different, the base station needs to transmit and receive signals at the same time in some time slots. As a result, transmission signals of the base station can be properly received but other signals cannot. Multifrequency technology can therefore improve system performance and raise frequency spectrum utilization. There are two implementation modes for multifrequency technology: Multicarrier joint networking (of shared frequency and difference frequency) and multicarrier shared frequency networking. For each networking mode, there are three frequency plans covering 15 MHz, 10 MHz and 5 MHz [2].

1.3.1 Multicarrier Joint Networking of Shared Frequency and Difference Frequency
    In this networking mode, difference frequency mode is used by major carrier of adjacent cells, and shared frequency mode is used by secondary carriers. For 15 MHz bandwidth, three carriers are selected as the primary carrier, and six are selected as secondary carriers. The maximum station model is S7/7/7. As shown in Fig. 3, the primary carrier is red and the six secondary carriers are black.

1.3.2 Multicarrier Shared Frequency Networking
    In this networking mode, frequency spectrum can be utilized to the greatest possible extent. Both the primary carriers of adjacent cells and secondary carriers use difference frequency mode. Crossover of adjacent cell main frequency points is contained in the secondary carrier. For 15 MHz bandwidth, the maximum station model is S9/9/9, as shown in Fig. 4.

1.3.3 Carrier Planning 
    For a network with multiple carriers, carrier planning is very important. Reasonable planning can improve system performance. Hierarchical planning based on multifrequency technology and concentric circle technology of multicarrier shared frequency networking is recommended. This improves system performance and reduces shared frequency interference on the service channel. The cell is divided into two layers, from the center to the margin. Each layer uses a different carrier allocation scheme. The primary carriers of adjacent cells use difference frequency networking modes, and the primary carriers with pilot channel cover the whole cell. Other carriers are called secondary carriers. Compared to the primary carrier, the traffic channel of secondary carriers works for the users of the internal layer in a high priority [3],[4].

    As shown in fig. 5, the secondary carrier (internal layer of the cell) is contracted within the yellow area, and the white space is the external area of the cell. When multifrequency technology with secondary carrier contraction is used for hierarchical carrier planning, shared frequency interference of users in the crossover area of adjacent cells is reduced.

2 Simulation Verification

2.1 Simulation 
    A hierarchical planning model is set for the 5 MHz frequency bandwidth with three carriers. The typical topology model defined in the Universal Mobile Telecommunication System 30.03 (UMTS 30.03) is used and each cell has three carriers. The primary carriers of adjacent cells are different and each primary carrier of one cell is contained in the secondary carriers of another adjacent cell [5].

    As shown in Fig. 6, the red rings divide the cell into two layers. The secondary carriers of each cell are contracted within the internal layer to provide resources for internal layer users. External layer users access the primary carrier point first. Adjacent cells use difference frequency switching, and shared frequency users are isolated.

    In this simulation, a mobile model is used (with a speed of 12 km/h. For other parameters, refer to the protocol). The handover algorithm is based on the electrical level, and the handover redundancy value is 1 dB. Open-loop, closed-loop and outer-loop power control algorithm are enabled. Load control is also enabled.

2.2 Major Simulation Parameters
    Nine hundred users are placed in the simulation, and each call lasts 200 s. Other parameters are shown in Table 1.

3 Performance Result

3.1 Performance Comparison
    When compared with a scheme that does not use hierarchical frequency planning, the performance of the new scheme is as shown in Table 2. When the new scheme is used, transmission power, received interference and call drop rate are reduced. System performance is also significantly improved.

    The contraction area of the secondary carrier area is different. When the internal radius is changed, system performance under the new scheme also changes. As the internal radius is contracted, the reuse distance of the shared carrier increases, and interference of the shared frequency decreases. Transmission power and call drop rate are improved. The results of the simulation are listed in Table 3.

3.2 Improvement of New Algorithm
    In simulation, hierarchical carrier planning improves system performance. But when users are at the cell edge or in the external layer of the cell, they access the primary carrier first until it cannot provide resources. At this point, the primary carrier becomes overloaded while the secondary carrier is underloaded or not loaded at all. The advantages of a new algorithm cannot be demonstrated and calls will be dropped. An improved scheme[6] that can solve the special distribution scenario needs to be found. In the preceding analysis, the algorithm is improved. One carrier is selected from the secondary carrier to serve as the middle one. When primary carrier resources are sufficient, the middle carrier serves as the normal secondary carrier and is contained within the internal cell. When the primary carrier resources are limited, users who excessively access resources may affect system performance. The middle carrier can supplement the primary carrier to provide resources to external layer users. As a result, shared frequency interference can be isolated and system performance in a special user distribution scenario can be ensured.

    If the simulation involves two opposite cells each with a radius of 50 meters, the maximum transmission power of R4 time slot is 30 dBm. Each cell has three carriers, and 40 users are placed in the external layer of the cell. The simulation results are listed in
Table 4.

    The middle carrier of adjacent cells should adhere to the principle that shared frequency interference should be isolated. According to the simulation results, reasonable selection of the middle carrier can improve system performance and save power.

4 Conclusion
    From the above simulations, some conclusions can be drawn. The secondary carrier area should be contracted in order to perform networking mode simulation of hierarchical carrier planning. After the shared frequency interference is isolated, system performance is significantly improved. This is useful for the study of multifrequency networking mode and actual networking scheme design. Different user distribution and environment will affect the networking scheme. In the future, a suitable scheme will need to be worked out according to theoretical analysis and actual conditions.

References:
[1] Muan Duan, “TD-SCDMA Frequency Planning and Networking Models,” Telecommunications Technology, No.11, pp.98-101,Nov.2006.
[2] Hui Xie, Chunjiang Zhu, Rupeng Xu, “TD-SCDMA Wireless Network Planning,” Telecom Engineering Technics and Standardization, No.10, pp.53-56, Oct.2006.
[3] Zhuo Wang, Lijun Song, Shunyong Yu, “TD-SCDMA System Frequency Planning Analysis,” Telecommunications Technology, No.5, pp.119-123, May 2006.
[4] Mugen Peng,Wenbo Wang et al., TD-SCDMA Mobile Communication System. Beijing: China Machine Press,2006.
[5] Selection procedures for the choice of radio transmission technologies of the UMTS, UMTS 30.03 v3.20. 1998.
[6] Rong Ren, “Analysis of TD-SCDMA Networking,” Telecommunications Technology, No.11, pp.29-32, Nov.2005.

Birgraphies
Min Jin works in the Wireless Signal Processing and Network laboratory at the Beijing University of Posts and Telecommunications (BUPT). He is engaged in researching TD-SCDMA system-level simulation, wireless resource management algorithm, networking planning, and LTE-A physical layer technologies. He has published one article and has applied for one patent.

Wenbo Wang is a professor and doctoral supervisor at BUPT. He is mainly engaged in researching mobile communications radio transmission theory and technology, wireless communication network theory, digital signal processing, and software defined radio theory. He has participated in many state and national science and technology projects, and has received many awards from provincial departments. He receives a special allowance granted by the State Council and is supported by the "New Century Talented Persons Project" of the Ministry of Education. He has published more than 200 articles and 10 books.

Mugen Pen is an associate professor at BUPT. He is engaged in researching signal processing and the key theories, and networking policies of the wireless communication system. His research is now focused on wireless network cooperation information theory, wireless resource management algorithm and protocol design in next-generation wireless broadband, multi-hop wireless relay networking, and network coding technology. He has recently published about 10 books, has translated two books and more than 80 articles, and has applied for more than 20 patents.

[Abstract] This paper introduces the characteristics of TD-SCDMA, and analyzes some networking schemes and methods of multifrequency. For the 5 MHz frequency bandwidth, a frequency planning scheme containing three frequencies is examined, and a simulation model is built to validate the performance of this scheme. Finally, this paper analyzes the advantages and disadvantages of the scheme, and proposes some directions for the future study of networking planning.

[Keywords] TD-SCDMA; multifrequency networking; simulation; frequency planning; multi-frequency