ZTE TD-LTE Common Frequency Networking Solution

As known, TD-LTE is one of the world's mobile communications field focuses. It adopts OFDM technology with orthogonal sub-carriers, which depresses the intra-cell interference. Thus, the mainly interference of TD-LTE system is the inter-cell interference. Therefore how to reduce inter-cell interference and achieve common-frequency networking are quite significant to enhance spectrum efficiency.

Intra-System Interference Analysis

The TD-LTE intra-system interference is divided into two parts, as link level interference and network level interference.

The link level interference is mainly caused by the loss of physical resources’ orthogonality. The interference is cancelled with special interference elimination technology, which drops physical layer demodulation performance. There are five kinds of link level interferences.

(1) ICI

(2) OFDM ISI

(3) Intra-cell sequence interference and interference between physical channels

(4) Phase noise

(5) Interference from the multi-antenna technology

Network-level interference occurs in the network scenarios. Scheduling, power control, ICIC and other strategies are used to control the interference control and coordination. There are five kinds of network level interferences.

(1) Common-frequency interference (from neighbor cell)

(2) Inter-frequency interference

(3) Inter-cell sequence interference and interference between physical channels

(4) Cross-slot interference

(5) Interference between indoor and outdoor coverage

Key technologies for service channel common-frequency networking

The key technologies for service channel common-frequency networking include power control, scheduling and Inter Cell Interference Coordination (ICIC). Power control can reduce the transmission power to avoid interference on RB, thereby reducing inter-cell interference. The RBs with lower interference are firstly scheduled to reduce inter-cell interference. The basic idea of ICIC is keeping the inter-cell interferences under control by radio resource management (RRM) methods.

• Power Control

(1) DL Power Control

The downlink power control is a passive process. The transmission power would be adjusted according to UE BLER performance until the power margin runs out

(2)UL Power Control

eNB sends the transmitting power control parameters via the uplink power control interface according to the characters of uplink channel and system complexity. UE can obtain the uplink transmitting power from the power control parameters.

• Scheduling

(1) Dynamic scheduling

The TD-LTE dynamic scheduling is based on service type, CQI, power, RB resources, antenna number and system load. The scheduling purposes are higher throughput and better QoS. The relative scheduling algorithm includes MAX CI, PF and RR.

(2) Time-Frequency resources scheduling

Dynamic scheduling algorithm is combined with ICIC in TD-LTE. The users from different direction are scheduled to the same timeslot. The cell center users are given more resources than the edge ones. Cell edge users are served in a frequency division multiplexing method to reduce common-frequency interference.

(3) Semi-persistence Scheduling (SPS)

SPS is designed for special service, which takes the pre-allocated resources at the fixed time and dynamic scheduled resources at other times during activating / releasing resources.

• ICIC

The basic idea of ICIC is keeping the inter-cell interferences under control by radio resource management (RRM) methods. ICIC is inherently a multi-cell RRM function that needs to take into account information (e.g. the resource usage status and traffic load situation) from multiple cells.

Control channel common-frequency networking

Whether control channel common-frequency networking will result in poor signaling coverage and high frame error rate are the main issues concerned during TD-LTE common-frequency networking. The main control channels of TD-LTE system are reference signal, synchronization channel, broadcast channel, control channel. The interference between downlink control channels are shown in the below table.

Note: P Stands for Interference；O Stands for No Interference

The required demodulation SINRs are different among different control channels. The order of downlink control channel anti-interference ability is PBCH > PHICH > PCFICH > PDCCH.

The relative simulation result shows that the upper and lower control channels achieve 90% or higher coverage at the common-frequency network, which meets the networking needs.