At present, after 3G, how various communication technologies will evolve is a focus that the industry is very concerned about, especially for TD-SCDMA, whether it can achieve a smooth evolution to the next generation of communication technologies determines how long TD has. Vitality and how far our country ’s independent innovation strategy can go. In November 2007, the 3GPPRAN151 meeting adopted the proposal of the LTE TDD fusion frame structure jointly signed by 27 companies, unifying the two frame structures of LTE TDD. The integrated LTE TDD frame structure is based on the TD-SCDMA frame structure, which lays the foundation for the successful evolution of TD-SCDMA to LTE and even 4G standards.
TDD-LTE technical characteristics
The LTE system supports both FDD and TDD duplex modes. Under these two duplex modes, most of the design of the system, especially the high-level protocol, is the same. On the other hand, in the design of the bottom layer of the system, especially the design of the physical layer, due to the inherent differences in the physical characteristics of the two duplex modes of FDD and TDD, the LTE system has designed a series of special designs for the working mode of TDD. These designs have referenced and inherited the design ideas of TD-SCDMA to a certain extent, and we will briefly describe and discuss these designs below.
Wireless frame structure
Because TDD uses time to distinguish between uplink and downlink, resources are discontinuous in time, and the time interval needs to be protected to avoid transmission and reception interference between uplink and downlink. Therefore, LTE has designed its own frame structure for FDD and TDD, namely Type 1 and Type2, where Type1 is used for FDD and Type2 is used for TDD.
In FDD Type1, a 10ms radio frame is divided into 10 subframes with a length of 1ms, and each subframe is composed of two slots with a length of 0.5ms. In TDD Type2, a 10ms radio frame consists of two half-frames with a length of 5ms, and each half-frame is composed of 5 subframes with a length of 1ms, of which there are 4 ordinary subframes and 1 special subframe. The ordinary subframe is composed of two 0.5 ms slots, and the special subframe is composed of three special time slots (UpPTS, GP, and DwPTS).
The most significant difference between the TDD and FDD frame structure in LTE is that there is a special subframe of 1ms in the TDDType2 frame structure. The subframe consists of three special time slots: DwPTS, GP and UpPTS. The SCDMA system is similar, in which DwPTS is always used for downlink transmission, UpPTS is always used for uplink transmission, and GP is used as the guard interval for downlink-to-uplink conversion in TDD. The total length of the three special time slots is fixed at 1ms, and their respective lengths can be configured according to the actual needs of the network.
Upstream and downstream time allocation
Another physical characteristic of TDD that is significantly different from FDD is that FDD relies on frequency to distinguish between upstream and downstream, so its unidirectional resources are continuous in time; while TDD relies on time to distinguish between upstream and downstream, so its unidirectional resources are in time It is discontinuous, and time resources are allocated in two directions.
The following figure shows the 7 different uplink and downlink time ratios supported in LTE TDD, from allocating most resources to the downlink "9: 1" to the uplink occupying more resources "2: 3", when in actual use , The network can be flexibly selected and configured according to the characteristics of the traffic. In this way, the inherent difference between TDD and FDD in the composition of resources has become the reason why another part of LTE is specially designed for TDD. This part of the design mainly includes "relevant mechanisms of HARQ in the physical layer" and "random access channel using frequency division".
Allowing multiple random access channels (frequency division) at the same time is another design result of the structure of TDD uplink and downlink time division. In the design of LTE FDD, only one random access channel is allowed to exist at a time, that is, only the number of random access channels is changed in the time domain. In TDD, time resources have been allocated in the uplink and downlink. At the same time, due to the different uplink and downlink ratios, there may be a small number of uplink subframes (such as DL: UL = 9: 1), so in TDD A random access channel that supports frequency division is required, that is, multiple random access channels are provided by using different frequency divisions at the same time position to provide sufficient random access capacity for the system.
In the case of FDD, the uplink and downlink resources are continuous in one direction, and the number of subframes is equal. Therefore, in the following behavior example, when performing HARQ at the physical layer, a one-to-one correspondence can be established between downlink data and uplink ACK / NAK. Unlike this, in the case of TDD, the unidirectional resources are not continuous, so the corresponding temporal resources may not be obtained. In addition, the setting of the uplink and downlink ratio may make the number of uplink and downlink subframes unequal, so it is impossible to establish a one-to-one correspondence, so these need to be targeted design. In LTETDD, in order to solve the above problems, the concept of MulTIpleACK / NAK is introduced, that is, one ACK / NAK is used to complete the feedback of several consecutive downlink data, which solves the feedback problem caused by the asymmetry of the uplink and downlink time slots. On the other hand, at the same time, the transmission delay of the data is reduced, and the data does not need to wait until the next upstream time slot for feedback. Of course, the unnecessary over-multiplexing that may be caused by this solution also needs attention.
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