4 Key Technologies in the 3G era (1)

HSDPA (high-speed downlink group access) is used to implement high-speed downstream data services of the WCDMA network, enabling the downstream data rate to reach 8 ~ 10 Mbps, known as the main solution of the 3G era. For HSDPA systems using multi-input, multi-output (MIMO) technology, the data rate can reach 20 Mbps. The emergence of HSDPA has aroused great attention in the industry. As the follow-up evolution technology of WCDMA system, many key technologies in HSDPA are similar to those in CDMA 20001xev/DV and some key technologies in TD-SCDMA, it is very important for us to fully understand the technological trend of the 3G era.

In the HSDPA technical solution, four key technologies are involved: adaptive coding modulation, H-ARQ, fast cellular selection (FCS), and multi-input multi-output antenna processing (MIMO ).

Adaptive coding modulation

Adaptive Modulation and encoding (AMC) are also in the category of link adaptation. The basic principle of AMC is to change the modulation and encoding formats and adapt it to the channel conditions within the limits of the system, while the channel conditions can be estimated by sending feedback. In AMC systems, users generally use higher-order modulation methods and higher encoding rates under ideal channel conditions, however, lower-level modulation encoding is used under unsatisfactory channel conditions.

The main benefits of using AMC are: users in a favorable position can have a higher data rate, thereby increasing the average throughput of the cell; in the process of link adaptation, the interference level can be reduced by adjusting the modulation encoding scheme instead of adjusting the transmit power.

Currently, AMC is faced with several challenges. First, AMC is sensitive to measurement errors and latencies. to select a suitable modulation method, you must first know the channel quality. An error in channel estimation may cause the system to select an incorrect data transmission rate, the transmission power is too high, the system capacity is wasted, or the error rate increases because the power is too low. Secondly, due to the time-varying characteristics of mobile channels, the delay in Channel Measurement reports reduces the reliability of channel quality estimation; in addition, interference changes also increase the measurement error, at this time can seek to combine with other technologies, such as the use of mixed decision feedback retransmission Technology (H-ARQ) it can reduce MCS requirement identification and sensitivity to measurement errors and traffic fluctuations.

H-ARQ

H-ARQ is also a link adaptive technology. In AMC, explicit C/I measurements are used to set the format of modulation encoding, while in H-ARQ, link layer information is used for retransmission decision.

There are many ways to achieve H-ARQ: Chase merge, compatible rate drill TurboCodes, and incremental redundancy. The merge strategy of Chase is to send a data group with the same encoding, and then the receiver can combine the multiple retransmission messages to obtain the diversity and then perform decoding. Incremental redundancy or H-ARQ-II is another way to implement H-ARQ. This policy transfers additional redundant information when decoding fails for the first time instead of resending the entire data code group. H-ARQ-type-Ⅲ is also one of the incremental redundancy solutions. However, in H-ARQ-type-Ⅲ, each retransmission can be self-decoded, this is different from HARQ-II. In multi-redundant H-ARQ-type-Ⅲ, different bits must be punched out each time redundant information is re-sent.

AMC can flexibly select the appropriate MCS based on UE measurement or information conditions provided by the network. However, UE is required to perform accurate Channel Measurement and be affected by the corresponding delay. The H-ARQ can automatically adapt to the change of channel conditions and is not sensitive to the measurement error and delay. The combination of AMC and H-ARQ can achieve the best effect-AMC provides a rough data rate selection while the H-ARQ can adjust the data rate according to the data channel conditions.