Hybrid networking of TD-SCDMA R4 and HSDPA (1)

In order to adapt to the increasing demand of data business, the original peak rate of TD-SCDMA has been unable to meet the demand, the development of HSDPA technology for higher data rate and higher capacity to provide a stable way of evolution. On the basis of the TD-SCDMA system networking scheme, how to introduce HSDPA has been paid more and more attention. With the rapid development of mobile communication and the Internet, many data services that require high traffic and latency performance, such as video and streaming media, have higher requirements on mobile communication systems. In this case, the peak rate of 2 Mbit/s originally proposed by the third generation mobile communication system cannot meet the market demand. Therefore, 3GPP introduced the new group access enhancement technology HSDPA in Versions later than R5 to increase the TD-SCDMA peak rate to 2.8 Mbit/s. Considering the combination of construction costs and economic benefits, HSDPA can adopt a hybrid networking mode with the R4 version system. Based on the discussion of the development of key technology of TD-SCDMAHSDPA, the hybrid networking scheme of R4 and HSDPA system is studied.

I. Structural Characteristics and Key Technologies of TD-SCDMA HSDPA

In R5, The TD-SCDMA system introduced HSDPA technology, added a new transmission channel and physical channel, and added a new media access control sub-layer (MAC-hs) to NodeB ).

New physical channels include high-speed downlink shared physical channels (HS-PDSCH), shared control channels (HS-SCCH), and shared indicator channels (HS-SICH. The HS-PDSCH is used to carry high-speed downlink data, the HS-SCCH/HS-SICH supports the corresponding signaling, controlled by NodeB, used to transmit the control information of the transmission channel and the feedback information of the terminal.

The new transmission channel includes high-speed downlink shared channel HS-DSCH, used to carry high-speed downlink data and map to high-speed downlink shared physical channel HS-PDSCH, it uses 1 and 16 different spread spectrum coefficients, nodeB can flexibly cope with different channel conditions and perform reasonable grouping data scheduling to improve the data transmission rate.

The main function of MAC-hs added to NodeB is to schedule sent data and control retransmission of data. Retransmission is directly controlled by NodeB, which avoids the delay from RNC to UE, increases the retransmission rate, and reduces the latency during data transmission. At the same time, NodeB can quickly schedule the size of data to be sent and objects to improve the cell throughput.

Because the shared channel uses the system to adaptive adjust the User modulation mode and encoding rate, HSDPA can better adapt to the characteristics of grouped data. Its main technical features include the following aspects.

1. Shared Channel

As mentioned above, HSDPA uses SHARED channels to provide services for grouped users. users share wireless resources by time or code.

2. adaptive coding modulation (AMC)

AMC is called Adaptive Modulation and encoding technology. It is a method that can adjust the modulation and encoding of transmitted data to compensate for the fading effect of Channel Changes on received signals, therefore, LinkAdaptation technology is used to improve the signal-to-noise ratio.

The implementation of AMC is that the system establishes a set of encoding and modulation formats based on its physical layer capabilities and Channel Changes. The transmission formats in each MCS include the transmission data encoding rate (transmission block length) and modulation method (PSK, 16QAM). When the channel conditions change, the system selects different transmission formats corresponding to the channel conditions to adapt to channel changes.

3. Hybrid re-transmission (HARQ)

HARQ is a combination of automatic retransmission request (ARQ) and forward correction (FEC) technologies. When decoding fails, the terminal saves the received data and requires the sender to re-transmit the data. The terminal merges the re-transmitted data and stored data and sends it to the decoder for decoding.

There are three types of HARQ. TypeIHARQ is relatively simple. After decoding at the receiving end, CRC is used to verify the group. If an error occurs in the group, the receiving end notifies the sender of the retransmission. The retransmission group adopts the same encoding as the previous one, and the error group is discarded. TypeII HARQ (incremental redundancy HARQ Scheme): the error group is not discarded, but merged with the incremental redundancy information re-sent by the sending end for decoding. This is called full ir harq. The retransmission information of the sender is not a simple duplication of the previous data, but incremental redundancy information. The redundancy information of multiple retransmission is different. The retransmission group cannot perform self-decoding, it must be merged with the data sent for the first time before decoding. Type iii harq: when receiving an incorrect data packet, it is not discarded. The receiver stores it and merges it with subsequent retransmission data for decoding. Different from the second type of HARQ, each retransmission contains a complete information bit. Therefore, the retransmission group has a self-decoding capability, which is suitable for the first time when data is seriously damaged. Currently, HSDPA uses the last two types.

4. fast scheduling of Base Stations

By adding MAC-hs entities, HSDPA can schedule and re-transmit data to NodeB, and adopt appropriate scheduling algorithms and reduce latency to adapt to channel changes more quickly, nodeB selects the user or adjusts the encoding mode based on the UE feedback and certain scheduling rules to optimize the system performance.