TD-HSDPA PS inter-system retry and field test

As one of the key algorithms for TD-SCDMA wireless resource management, access control (AC) is to admit or reject a request by establishing a radio access bearer (RB). When the RB is established or changed, the access control algorithm needs to be executed. When the system load is high, AC protects the communication quality of established connected users by rejecting new call requests. Common strategies include threshold strategy and reallocation strategy. The steps for implementing access control are as follows:

(1) Evaluate available resources and determine the resources that carry user services, which can also be achieved through dynamic channel allocation technology;

(2) Determine whether access is possible based on the selected carrier frequency, time slot and code channel. The purpose of TD-HSDPA admission control is to control users who are currently online but not scheduled. They do not occupy HS-PDSCH channel resources, but can also occupy related control channel resources. The scope of admission control includes all the accompanying channels and shared channels of the requested HSDPA services. Data users cannot access without restrictions. When the number of accepted HS services reaches the maximum number of simultaneous online users, the following strategies will be activated for the newly requested HS services:

(1) If the R4 carrier is allowed to access, it will be admitted to the R4 carrier;

(2) If the R4 carrier is not accessible, it is redirected to the GSM system.

When the system is congested, TD-HSDPA PS data services can be carried on HS carriers, R4 carriers or GSM systems without being rejected. This paper uses field tests to focus on the system's ability to carry TD-HSDPA PS services to the GSM network through RRC redirection when the service cannot access the TD network, as shown in Figure 1.

Figure 1 HSDPA PS service RRC inter-system redirection strategy

Field Test of TD-HSDPA Admission Strategy

1. Test Purpose

When HSDPA and R4 carrier resources are fully occupied, new users will be redirected to EDGE/GPRS, as shown in Figure 2.

Figure 2 Test method [page]

2. Test environment

The test case used in this article is based on the fixed-point test environment of an indoor distribution system in the existing TD-SCDMA network. The test terminals are statically and evenly distributed in the good coverage area of ​​the cell under test. The test cell is configured with 2 HSDPA carriers and 1 R4 carrier, with a 2:4 uplink and downlink time slot configuration. In each HSDPA carrier, HS-PDSCH occupies 2 time slots alone, and HS-SCCH/HS-SICH is configured in two groups.

3. Test terminal

The terminals used in the test are based on the three current mainstream chips, using TD-GSM dual-mode single standby and HSDPA data cards, supporting the "three new" mechanism, and using the latest software version. In the test, the maximum number of users accessing the HSDPA carrier was set to 3, and 6 terminals were used to apply for background services, with an application rate of UL64K/DL1024K.

4. Test steps

(1) Seven HS services (UL64K/DL2048K) are connected to the HS main carrier in sequence. After the seven HS services are connected, the uplink code channel resources of the main carrier are all occupied. It can be seen from the RAB assignment request message that the HS background service (UL64K/DL2048K) is connected.

(2) When the time slot ratio is 2:4, the auxiliary carrier sequentially accesses four VP services, all of which are carried on the R4 carrier and occupy all the uplink code channel resources of the R4 carrier. From the message RABASSIGN-MENTREQUEST, we can see that the requested service is CS64K videophone.

(3) Apply for another HS service (UL64K/DL2048K) based on step 2. Because the HS and R4 carrier code channel resources are all occupied, when applying for the HS service again, it can be seen from the signaling sent by the RNC to the UE that when the RRC connection is established in the TD network, the RRC is rejected and the service is connected to the GSM system. The rejected message in RRCCONNREJ contains GSM information.

Analyzing the test diagram of the RRC connection redirection message, it shows the access signaling process from the terminal receiving the RRCConnectionREJ message to the successful carrying of the EDGE/GPRS. A total of three tests were conducted, with delays of 6.12 seconds, 7.38 seconds, and 5.99 seconds respectively, and the average delay of redirected access was 6.5 seconds.

Test conclusions and deployment recommendations

After analyzing the field test results, the inter-system retry admission strategy and function of the TDH SDP APS service access process were verified, and the test conclusions were as follows:

*Data services (HSDPA, R4) can be accepted by HS carriers, R4 carriers or GSM systems without being rejected. Data services should be carried by TD as much as possible. If TD cannot be accessed, it should be carried by GSM instead of being rejected.

*When the HS carrier and R4 carrier code channels in the system are all occupied, if you apply for another HS (UL64K/DL2048K) service, you can see from the signaling that when establishing an RRC connection in the TD-SCDMA network, the RRC is rejected. From the RRCCONNREJ, you can see that the rejected message carries GSM information, and the PS service is redirected to the GSM system. The average access delay is about 6.5 seconds. Retry access between different systems in TD-HSDPA is one of the important strategies for TD wireless resource management. Through field test verification and in-depth analysis, the following deployment suggestions are proposed:

(1) In terms of deployment timing, the inter-system retry admission strategy in TD-HSDPA can only be deployed on commercial networks when the traffic volume grows to a certain level or when network resources are congested. If congestion control is enabled in the early stages of network construction when the traffic load is light, it will not only increase the complexity of equipment processing and the network signaling burden, but also some operations of the admission control itself will have a certain negative impact on network KPIs.

(2) In terms of implementation steps, it is recommended to first enable the admission control policy within a specific RNC range, observe the difference between the key performance indicators (KPI) before and after enabling, combine and optimize relevant parameters, and repeatedly perform live network verification until the KPI meets the requirements, and then promote it to the entire network.

(3) Pay close attention to and track other 3G commercial networks with high traffic volumes around the world, and learn from their technical means and successful experiences in solving load and congestion problems.