Research on 8-channel TD-LTE system

1 LTE system standard evolution

LTE (Long Term EvoluTIon) is a long-term evolution project of 3GPP, compatible with the current 3G communication system and the evolution of 3G. It has the characteristics of high transmission rate, high transmission quality and high mobility, improves and enhances the 3G air access technology, and adopts OFDM and MIMO technology as the only standard for its wireless network evolution. Under 20MHz spectrum bandwidth, it can provide the peak rate of 100Mbit / s downlink and 50Mbit / s uplink.

Since the launch of the LTE project in November 2004, 3GPP has gone all out to promote LTE research work with frequent meetings, and has completed the formulation of requirements in only six months. In June 2006, the 3GPP RAN (Radio Access Network) TSG has started the LTE working phase (WI), but after arduous discussions and integration, most of the basic technical frameworks have finally been determined. Before our eyes.

By introducing the evolution of the LTE standard, the origins of TD-LTE and LTE FDD are explained, and on this basis, the differences in antenna technology choices brought by the two standards due to their own characteristics are analyzed. The comparison of the simulation results under the same and reasonable simulation conditions illustrates the advantages of the TD-LTE system using multiple antennas.

The LTE system starts with defining requirements. The main demand indicators include:

â— Support 1.4 ~ 20MHz bandwidth.

â— Peak data rate: uplink 50Mbit / s, downlink 100Mbit / s. The spectral efficiency reaches 2 ~ 4 times of 3GPP R6.

â— Increase the bit rate at the cell edge.

â— User plane delay (unidirectional) <5ms, control plane delay <100ms.

â— Support interoperability with existing 3GPP and non-3GPP systems.

â— Support enhanced broadcast and multicast services.

â— Reduce the cost of network construction and realize the low-cost evolution from R6.

â— Realize reasonable terminal complexity, cost and power consumption.

â— Support enhanced IMS (IP Multimedia Subsystem) and core network.

â— The pursuit of backward compatibility, but should carefully consider the balance between performance improvement and backward compatibility.

â— Cancel the CS (circuit switched) domain, and the CS domain services are implemented in the PS (packet switched) domain, such as VoIP.

â— Optimize the system for low-speed mobile and support high-speed mobile at the same time.

â— Support paired (Paired) and unpaired (Unpaired) frequency bands with as similar technology as possible.

â— Support simple co-existence of frequency as much as possible.

For the positioning of WiMAX "low mobility broadband IP access", the LTE system has proposed corresponding requirements, such as similar bandwidth, data rate, and spectrum efficiency indicators, to optimize low mobility, and only supports the PS domain, emphasizing more broadcast Broadcast business, etc. At the same time, due to the emphasis on VoIP and online games, LTE has nearly stringent requirements for user plane delays. The requirements for backward compatibility seem ambiguous. Due to the large number of new technologies chosen, it has been difficult to maintain a smooth transition from the 3G system at the physical layer. Like the WiMAX system, both the LTE system chose OFDM as the basic technology rather than the CDMA technology.

As mentioned earlier, in the LTE system, more stringent requirements are imposed on the delay of the system:

â— Significantly reduce the control plane delay: 100ms: LTE_Idle â†’ LTE_AcTIve; 50ms: Dormant â†’ AcTIve 50ms.

â— User plane delay: defined as the single transmission time of the IP layer packet data from the UE or RAN edge node to the RAN edge node or UE IP layer packet data.

â— Demand: 5ms (in the case of no-load IP packets, a subsequent supplementary definition is required).

In order to meet the above requirements, in addition to changes in the air interface wireless frame length and changes in TTI to shorten the air interface delay, the network structure needs to be evolved to minimize redundant nodes, thereby reducing the transmission delay in the network. But no matter how the structure evolves, the radio access network and the core network still follow their respective development principles, 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 has selected OFDM, MIMO and smart antenna technologies as the basic physical layer technology at the beginning of the basic technology and retains the LTE technology of FDD and TDD. Below we make further analysis on some commonalities and differences between these two systems.

2 FDD and TDD spectrum efficiency is equivalent under the same conditions

The basic frame structure difference between LTE FDD and LTE TDD (ie TD-LTE) system is not analyzed in this article. As far as the basic frame structure is concerned, the TDD system retains 3 special time slots from the TD-SCDMA system design, and in order to adapt to the fusion of wireless frames, different uplink / downlink time slot ratios and special time slots are also designed. The number of different symbols matches. In terms of spectrum efficiency, our simulation results show that the two are basically equivalent.

Simulation conditions:

â— Network model: 19X3.

â— Band frequency and carrier bandwidth 2GHz, BW 20MHz.

â— Communication environment: Urban Macro.

â— Link model: SCM-E, 3km / h.

â— Base station transmit power: PBS_max: 46dBm.

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— Downlink: Scheme: rank1 / rank2 adaptive adjustment; No Power Control.

â— Uplink: Scheme: IRC (unified interference merge), uplink power control is turned on.

Based on the same conditions as above, through simulation, the results shown in Table 1 were obtained.

Table 1 Simulation results

From the comparison in Table 1, it can be seen that the spectrum efficiency of the TDD system and the FDD system are basically the same in both uplink and downlink, and the average spectrum efficiency of the downlink is DL: 1.5 ~ 1.6 (bit / s / Hz), the uplink results differ only by 0.1 bit / s / Hz. The spectrum 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 completely the same.

It can be seen from the comparison results of the simulation that the spectral efficiency of the TDD system and the FDD system are equivalent. So what are the differences between TDD system and FDD system?

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