Access Softswitch IAD performance test in several different WiMAX wireless environments

As a wireless broadband metropolitan area network standard, WiMAX technology has greater advantages than previous 3.5GHz MMDS products in terms of non-line-of-sight transmission and spectrum utilization efficiency. As one of the cores of the next-generation network, the soft switching technology based on IP network has also become very mature in terms of integrated service access and QoS level guarantee, and will soon replace the existing PSTN equipment. This article introduces the results of the access soft switching IAD performance test in several different WiMAX wireless environments, providing a flexible and convenient solution for operators' integrated service access in the "last mile".

In order to achieve the goal of improving the city image and the urban environment, more and more communication and power overhead poles and lines will be buried underground. In addition, in the initial planning of new urban areas and development zones, overhead poles and lines will not be allowed. This is a big problem for the construction of access networks of basic telecommunications operators. Especially for newly entered fixed network operators, there are no access network resources. Even if the municipal department allows the construction of underground pipelines, the investment is not only very expensive, but also irreversible. Therefore, all operators are looking for new access methods to replace wired access, and new access methods must meet the conditions of convenience, speed and low investment to be accepted by operators. In the WIMAX environment, providing users with integrated voice and data services by accessing the soft switch IAD is a very good solution.

Network topology and test environment

The network topology is shown in Figure 1

In this test, the outdoor CPE is about 2 kilometers away from the WiMAX base station, which can meet different transmission conditions such as line-of-sight and non-line-of-sight. The IAD is equipped with an RJ11 and an RJ45 interface, which can be connected to a regular phone and a data terminal device respectively, using a static public network IP address authentication method (IAD can use different authentication access methods such as DHCP, static IP, PPPoE, etc.). The CPE upstream and downstream bandwidths are set to 2Mbit/s each.

When IAD users communicate with PSTN users, IAD users access the IP bearer network through the WIMAX wireless link, which is controlled by the media gateway controller (Huawei SOFT3000) and sends the media stream to the integrated media gateway (Huawei UMG8900). There is a direct circuit between UMG8900 and the PSTN program-controlled switch (Huawei CC08) to complete the communication.

Voice communication between IAD users is controlled by the media gateway controller (Huawei SOFT3000) and completed through the IP bearer network.

Test results under different signal environments

To achieve the best access effect, WiMAX can automatically adjust the modulation method of the transmission signal according to different wireless environments. When the signal conditions are good (generally pure line-of-sight transmission or a small amount of obstruction), the QAM modulation method with higher spectrum utilization is used; when there is partial obstruction, the QPSK modulation method is used; when there is complete obstruction and only transmission by reflection and refraction is used, the BPSK modulation method is used. [page]

1. Line-of-sight transmission between CPE and base station, using QAM modulation

The test results show that when the CPE and the base station are in line-of-sight transmission and QAM modulation is used, the delay on the wireless side is 29ms and there is no packet loss. If data is not downloaded, the call effect is good and is no different from ordinary PSTN calls. The bandwidth occupied by a single voice channel is about 90kHz. On the speed test website, when downloading data (the instant transmission rate basically reaches 2Mbit/s) while talking, the voice effect is basically the same as that of a danchun call because the IAD uses a QOS priority guarantee mechanism.

In this environment, the receiving signal strength of the central station and CPE is generally above -78dBm, and the CINR value (carrier-to-interference ratio) is generally above 15.

2. There is some obstruction between CPE and base station, but it is not serious. QPSK modulation is used.

The test results show that when there is a shielding between CPE and base station but it is not very serious, QPSK modulation is used. Since some signals need to be refracted and reflected before reaching the base station and CPE, the delay on the wireless side is 49ms, which is much longer than that of QAM modulation, but there is no packet loss. At this time, the call quality is still very good, and it can still reach 2Mbit/s when downloading data. Although the voice quality of the call is slightly reduced, it is acceptable.

In this environment, the received signal strength of the central station is generally above -87dBm, and the received signal strength of the CPE cannot be less than -82dBm. The CINR value (carrier-to-interference ratio) is generally between 8 and 13.

3. There is serious obstruction between CPE and base station. When the signal is transmitted only by reflection, refraction or other methods, the received signal strength is weak and the interference is large. BPSK modulation is used.

The test results show that there is severe obstruction between CPE and base station. When BPSK modulation is used, the delay on the wireless side is 46ms, which is much longer than QAM modulation and not much different from QPSK modulation. At this time, the base station received signal strength is -90dBm, and the CPE received signal strength is -85dBm, which is the minimum value when there is no packet loss (once the received signal strength is less than the above two data and the CINR value is less than 6, packet loss will occur), and the download rate can no longer reach 2Mbit/s, but is about 1.2Mbit/s. At this time, the effect of making a voice call alone is still good, but the effect of making a call while downloading data is poor.

Since the received signal strength of the base station and CPE is already weak under the BPSK modulation mode, when the weather changes (the signal attenuation is large when it is cloudy or rainy), the received signal strength and CINR value will drop rapidly, which is likely to cause problems such as packet loss and excessive delay, thus affecting the call quality. Therefore, when accessing voice users, it is recommended that the signal modulation mode should be QAM and QPSK as much as possible, the received signal strength of CPE should be greater than -81dBm, the received signal strength of the central station should be greater than -85dBm, and the CINR value should be greater than 8.

Summarize

Since the test location and environment are relatively fixed, some other problems may occur in actual applications, especially under poor access conditions, the received signal strength and CINR value will fluctuate greatly. However, from the above test results, it can be seen that due to the use of OFDM technology, WiMAX has great advantages over other MMDS3.5GHz products in non-line-of-sight transmission and spectrum utilization. In addition, IAD based on soft switching also has good adaptability in terms of latency and bandwidth, and has strong QoS guarantee. Therefore, WiMAX+soft switching IAD is very suitable for development zones, small and medium-sized enterprise parks and other areas where wired access is difficult and costly, providing a good solution to solve the "last mile" access problem of operators.