1 LTE system standard evolution
LTE (Long Term EvoluTIon) is a 3GPP long-term evolution project that is compatible with current 3G communication systems and evolves for 3G. It has high transmission rate, high transmission quality and high mobility. It improves and enhances 3G air access technology, and adopts OFDM and MIMO technology as the only standard for its wireless network evolution. It can provide a peak rate of 100 Mbit/s downlink and 50 Mbit/s uplink in a 20 MHz spectrum bandwidth.
Since the launch of the LTE project in November 2004, 3GPP has fully promoted LTE research work with frequent meetings, and completed the development of requirements in only half a year. In June 2006, 3GPP RAN (Radio Access Network) TSG has started the LTE working phase (WI), but after painstaking discussion and convergence, most of the basic technology frameworks have been finalized. A preliminary LTE system has gradually appeared in Before our eyes.
By introducing the evolution of the LTE standard, the origins of the two standards of TD-LTE and LTE FDD are explained, and on this basis, the differences in antenna technology selection between the two systems due to their own characteristics are analyzed. The comparison of simulation results under the same and reasonable simulation conditions illustrates the advantages of the TD-LTE system using multiple antennas.
The LTE system begins with defining requirements. The main demand indicators include:
â— Supports 1.4~20MHz bandwidth.
â— Peak data rate: 50Mbit/s uplink and 100Mbit/s downlink. The spectrum efficiency is 2~4 times that of 3GPP R6.
â— Increase the bit rate at the cell edge.
â— User plane delay (one-way) <5ms, control plane delay <100ms.
• Support interoperability with existing 3GPP and non-3GPP systems.
â— Support enhanced broadcast multicast services.
â— Reduce network construction costs and achieve low-cost evolution from R6.
â— Achieve reasonable terminal complexity, cost and power consumption.
Support for enhanced IMS (IP Multimedia Subsystem) and core network.
â— Pursuit of backward compatibility, but the balance between performance improvement and backward compatibility should be carefully considered.
â— Cancel the CS (Circuit Switched) domain, and the CS domain service is implemented in the PS (Packet Switching) domain, such as VoIP.
â— Optimize the system for low-speed mobile while supporting high-speed mobile.
• Support both paired (Paired) and unpaired (Unpaired) bands simultaneously with as similar a technique as possible.
â— Support simple cross-frequency coexistence as much as possible.
In response to the positioning of WiMAX "low mobility broadband IP access", the LTE system proposes corresponding requirements, such as similar bandwidth, data rate and spectrum efficiency indicators, to optimize low mobility, only support PS domain, emphasizing more broadcasts. Broadcast business, etc. At the same time, due to the emphasis on VoIP and online games, LTE has almost strict requirements for user plane delay. The requirement for backward compatibility seems ambiguous, and due to the selection of a large number of new technologies, it has been difficult to maintain a smooth transition from 3G systems at the physical layer. The LTE system, like the WiMAX system, has chosen OFDM as the basic technology, not CDMA technology.
As mentioned earlier, in the LTE system, more stringent requirements are imposed on the delay of the system:
◠Significantly reduce control plane delay: 100ms: LTE_Idle→LTE_AcTIve; 50ms: Dormant→AcTIve 50ms.
User plane delay: defined as the single transmission time of the UE or RAN edge node IP layer packet data to the RAN edge node or UE IP layer packet data.
â—Requirement: 5ms (in the case of no load IP packets, subsequent supplementary definitions are required).
In order to meet the above requirements, in addition to changes in the air frame length of the air interface and changes in TTI to shorten the delay of the air interface, the network structure needs to be evolved to minimize redundant nodes, thereby reducing the transmission delay in the network. However, no matter how the structure evolves, the radio access network and the core network still follow the principles of their respective development, and the air interface terminates in the radio access network. Therefore, the logical relationship between the radio access network and the core network still exists, and the interface between the radio access network and the core network is still clear.
Based on the above background, the LTE system selects technologies such as OFDM, MIMO and smart antennas as basic physical layer technologies and retains the LTE technologies of FDD and TDD in the basic technology. Below we will further analyze some commonalities and differences between the two systems.
2 FDD and TDD spectrum efficiency are equivalent under the same conditions
The basic frame structure differences between LTE FDD and LTE TDD (ie TD-LTE) systems are not analyzed in this paper. In terms of the basic frame structure, the TDD system retains three special time slots designed from the TD-SCDMA system, and in order to adapt to the convergence of the radio frames, different uplink/downlink slot ratios and special time slots are also designed. The ratio of the number of different symbols. In terms of spectral efficiency, our simulation results show that the two are basically equivalent.
Simulation conditions:
â— Network model: 19X3.
â— Band and carrier bandwidth 2GHz, BW 20MHz.
â— Communication environment: Urban Macro.
â— Link model: SCM-E, 3km/h.
Base station transmit power: PBS_max: 46 dBm.
â— TDD configuration: TDD UL: DL, 2:2; Special Frame: 10:2:2.
â— Terminal transmit power: PUE_Max: 23Bm.
â— Terminal height: 1.5m.
â— Downstream: Scheme: rank1/rank2 adaptive adjustment; No Power Control.
â— Upstream: Scheme: IRC (uninterrupted interference), the uplink power control is turned on.
Based on the above conditions, the results as shown in Table 1 were obtained by simulation.
Table 1 Simulation results
It can be seen from the comparison in Table 1 that both the uplink and the downlink, the TDD system and the FDD system are basically equivalent in spectrum efficiency, and the average spectral efficiency of the downlink is in the DL: 1.5 to 1.6 (bit/s/). Hz), the result of the uplink is only 0.1 bit/s/Hz. The spectral efficiency of the edge users of the two systems is almost the same, which means that the edge user experience of the two systems is exactly the same.
It can be seen from the comparison results of the simulation that the spectral efficiency of the TDD system is equivalent to that of the FDD system. So what are the differences between TDD systems and FDD systems?
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