What is system architecture evolution?

The research of LTE includes some generally considered important parts, such as the reduction of waiting time and higher users.

The improvement of data rate, system capacity and coverage, as well as the reduction of operating costs.

In order to achieve these goals, the evolution of wireless interface and wireless network architecture is equally important. Considering the need to provide more than

With the higher data rate of 3G and the spectrum that may be allocated in the future, LTE needs to support transmission bandwidth higher than 5MHz.

E-UTRA and E-UTRAN requirements

The goal of UTRA and UTRAN evolution is to establish a packet-based optimization system with high transmission rate and low delay.

Evolved wireless access architecture. Evolution technology of wireless interface and wireless access network architecture being developed by 3GPPLTE.

Includes the following contents:

(1) significantly improves the peak data rate. For example, in the bandwidth of 20MHz, the downlink transmission rate reaches100 mbit/s.

(5 bit/sec/Hz) and 50 megabit/sec uplink transmission rate (2.5 bit/sec/Hz).

(2) Increase the cell boundary bit rate while keeping the current base station position unchanged. Such as MBMS (multimedia

Broadcast and multicast services) can provide a data rate of 1 bit/second/hertz at the cell boundary.

(3) The spectrum efficiency is obviously improved. For example, 2-4 times the spectral efficiency of R6.

(4) The delay time of wireless access network (UE to E-NodeB user plane) is less than10 ms. ..

(5) obviously reduce the waiting time of the control surface, which is lower than100 ms.

(6) Bandwidth grades are: a)5, 10, 20MHz and possibly15mhz; B) 1.25, 1.6 and 2.5 MHz,

To adapt to the allocation of narrowband spectrum.

(7) Support cooperation with existing 3G systems and non-3GPP standard systems.

(8) Support further enhancement of MBMS.

The above-mentioned evolution goal involves the system's capability and performance, which is the most important part of LTE research and also

E-UTRA and E-UTRAN are the basis for maintaining the strongest competitiveness.

In LTE, some other requirements are also specified, such as requirements related to configuration, E-UTRAN architecture and migration requirements.

Requirements, wireless resource management requirements, complexity requirements, cost-related requirements and business-related requirements.

Compared with other wireless access methods, E-UTRAN has the advantages of high spectrum efficiency, wide coverage and support for high-speed mobile users.

The main characteristics of the system. In E-UTRAN, the highest data transmission can be achieved when the mobile rate is15 ~120 km/h.

Yes E-UTRAN supports inter-cell 120~350km/h or even up to 500 km/h. Over the entire rate range

In R6, voice and other real-time services in CS domain are supported in E-UTRAN through PS domain, and at least related voice and other real-time services are required.

The same performance of UTRAN.

LTE physical layer scheme and technology

In the process of soliciting the LTE layer 1 scheme, the 3GPPRAN 1 working group evaluated six schemes. They are:

(1)FDD, using single carrier FDMA(SC-FDMA) in uplink and OFDMA in downlink.

(2) FDD and OFDMA are adopted for both uplink and downlink.

(3)FDD, multicarrier WCDMA(MC-WCDMA) is adopted in both uplink and downlink.

(4)TDD, multicarrier time division synchronous CDMA(MC-TD-SCDMA) is adopted in both uplink and downlink.

(5) TDD and OFDMA are adopted for both uplink and downlink.

(6)TDD, using single carrier FDMA(SC-FDMA) for uplink and OFDMA for downlink.

According to the duplex mode, the above scheme can be divided into frequency division duplex (FDD) and time division duplex (TDD). according to

According to the wireless link multiple access mode, it can be mainly divided into code division multiple access (CDMA) and orthogonal frequency division multiple access (OFDMA).

The 5MHz spectrum is preliminarily evaluated at the system level, and the systems using CDMA and OFDM are at the rising frequency.

The spectral efficiency is similar. If CDMA evolution mode is adopted, the system will be upgraded smoothly from the previous UTRA version.

Level, physical layer can be widely reused. If OFDMA is adopted, the new layer 1 junction is completely out of the previous design constraints.

Structure, is conducive to the system in the design parameters to make flexible and free choice, it is easier to realize some definitions of E-UTRA.

Goals, such as waiting time, minimum bandwidth interval and fairness in different duplex modes; At the same time, for users,

For the receiver, the processing of OFDMA air interface is relatively simple, and it has wider bandwidth and higher-order MIMO.

MIMO configuration can reduce the complexity of the terminal.

Of course, based on the above factors, after intense discussion and difficult integration, TSGRAN was held in February 2005.

At the 30th plenary meeting, it was finally decided that the LTE feasibility study would focus on downlink OFDMA and uplink SC-FDMA. This also means that

OFDM technology has won in 3GPPLTE. On the one hand, this result is purely technical, that is, in the downlink.

Using OFDMA with high spectral efficiency as modulation mode and SC-FDMA in uplink can reduce the number of transmitting terminals.

Peak-to-average power ratio reduces the size and cost of the terminal; On the other hand, it is also to get rid of Qualcomm's monopoly since 3G.

Limitations of CDMA core patents.

Basic physical layer transmission scheme

LTE downlink transmission scheme adopts traditional OFDM with cyclic prefix (CP), and each subcarrier occupies 15kHz.

The duration of cyclic prefix is 4.7/ 16.7μs, corresponding to short CP and long CP respectively. In order to meet the requirement of data transmission delay

In order to achieve (user plane delay is less than 5ms under light load), LTE system must adopt a very short interleaving length (TTI).

And automatic repeat request (ARQ) period, therefore, the 10ms radio frame in 3G is divided into 20 subframes with the same size,

The length is 0.5 milliseconds.

The modulation of downlink data mainly adopts QPSK, 16QAM and 64QAM. Designed for broadcast services, a unique

Special hierarchical modulation mode is also considered. The idea of layered modulation is that, in

The application layer divides a logical service into two data streams, one is the basic layer with high priority and the other is the low priority.

The enhancement layer of. In the physical layer, these two data streams are mapped to different layers of the signal constellation. Due to the base layer data

The symbol distance of the mapping is greater than that of the enhancement layer, so the data stream of the base layer can be included far away from the base station.

And the enhanced layer data stream can only be received by users close to the base station. In other words, the same

According to the advantages and disadvantages of channel conditions, logical services can provide different levels of services in the network.

At present, Turbo coding of R6 is mainly used as LTE channel coding, for example, in the system.

Can be evaluated. However, many companies are also studying other coding methods and expect to introduce them into LTE, such as low-density parity check.

Check (LDPC) code. In the case of a large amount of data, LDPC codes can obtain higher coding gain and higher decoding complexity than Turbo codes.

Impurities are also slightly reduced.

MIMO technology is introduced into R7, which is an important feature of WCDMA enhancement. In LTE, MIMO is recognized.

It is the best technology to meet the average throughput and spectrum efficiency requirements of users. The basic configuration of the downlink MIMO antenna is that, when

The base station has two transmitting antennas, and the UE has two receiving antennas, that is, 2×2 antenna configuration. Higher downlink configuration, such as 4

×4 MIMO can also be considered. Open-loop transmit diversity and open-loop MIMO can be applied to transmission without feedback, as described below.

Control channel and enhanced broadcast multicast service.

Although macro diversity technology has played a very important role in 3G era, it is basically excluded in HSDPA/HSUPA.

Give up. Even the macro diversity of Fast Cell Selection (FCS) initially discussed is not defined in the actual specification.

LTE follows the idea of HSDPA/HSUPA, that is, gain is obtained only through link adaptation and fast retransmission, and macro is abandoned.

Diversity is a technology that needs the support of network architecture. At the RAN conference in March 2006, it was confirmed that there were no more packets in E-UTRAN.

Includes RNC nodes, therefore, in addition to broadcast services, macro diversity technology controlled by "central nodes" (such as RNC) is needed.

No longer considered in LTE. However, for multi-cell broadcast service, it is necessary to obtain high signal-to-noise ratio through soft combination of wireless links.

Than. In OFDM system, when the signal reaches the UE antenna within the CP window, soft combining can be realized by RF combining.

In implementation, this merging does not require any operation of the UE.

The uplink transmission scheme adopts SC-FDMA with cyclic prefix, uses DFT to obtain frequency domain signals, and then inserts zero symbols.

Spread spectrum is carried out, and the spread spectrum signal passes through IFFT. This process is called DFT-SOFDM for short. The purpose of this is to use the uplink.

Users can be orthogonal in frequency domain, and effective frequency domain equalization can be obtained at the receiving end.

The subcarrier mapping determines which part of the spectrum resource is used to transmit uplink data, while other parts are inserted.

Fuck zero. There are two ways to allocate spectrum resources: one is local transmission, that is, the output of DFT is mapped to continuous subchannels.

On the carrier wave; The other is distributed transmission, that is, the output of DFT is mapped to discrete subcarriers. Relative to the former, distribution

Additional frequency diversity can be obtained through the transmission of the formula. The uplink modulation mainly adopts π/2 shift BPSK, QPSK, 8PSK and.

16QAM. Like downlink, the uplink channel coding still follows the Turbo coding of R6. Other ways of forward error correction coding

Is under study.

The basic configuration of uplink single-user MIMO antenna is that the UE has two transmitting antennas and the base station has two receiving days.

Line. In uplink transmission, LTE adopts a special technology called virtual MIMO. through

It is usually 2×2 virtual MIMO, in which two UEs each have a transmit antenna and share the same time-frequency resources. these

UE adopts mutually orthogonal reference signal graphs to simplify the processing of the base station. From the perspective of UE, 2×2 virtual MIMO is similar to UE.

Single antenna transmission is different only in that the use of reference signal spectrum must be paired with other UE. But from the base station

From the point of view, it is indeed a 2×2 MIMO system, and the receiver can jointly detect the signals sent by two UEs.

Measure.

Basic physical layer technology

In the basic physical layer technology, E-NodeB scheduling, link adaptation and hybrid ARQ(HARQ) are inherited.

HSDPA strategy for fast data transmission based on data packets.

For downlink non-MBMS services, the E-NodeB scheduler dynamically allocates a specific UE at a specific time.

Time-frequency domain resources. Downlink control signaling informs the UE of what resources to allocate and its corresponding transmission format. The scheduler may

Select the best multiplexing strategy from multiple alternatives in real time, such as the allocation and multiplexing of subcarrier resources. This kind of election

The flexibility of selecting resource blocks and determining how to reuse UE can greatly affect the available scheduling performance. Scheduling and linking

Adaptation is closely related to HARQ because they operate together. Decide how to allocate and restore

The basis of use includes the following contents: QoS parameters, the amount of data to be scheduled in E-NodeB and the channel reported by UE.

Quality indicator (CQI), UE capability, system parameters such as bandwidth and interference level, etc.

By applying different modulation and coding methods on the * * * shared channel, link adaptation, that is, adaptive modulation and coding, can be adapted.

The same channel changes to obtain the maximum transmission efficiency. Combine coding and modulation mode changes into a list, E-NodeB.

According to the feedback from UE and other reference data, the modulation rate and coding mode are selected from the list and applied to the second layer.

And mapped to resource blocks allocated by scheduling. Uplink adaptation is used to ensure that

Small transmission performance, such as data rate, packet error rate and response time, maximizes system throughput. uplink

Adaptation can be combined with the application of adaptive transmission bandwidth, power control and adaptive modulation and coding.

The source, interference level and spectral efficiency are optimally adjusted.

In order to obtain correct data transmission, LTE still adopts forward error correction coding (FEC) and automatic repeat request.

(ARQ) combined error control, that is, hybrid ARQ(HARQ). HARQ applies the retransmission strategy of incremental redundancy (IR),

Chase merger (CC) is actually a special case of IR. So as to be easy to implement and avoid wasting time waiting for feedback messages.

At the same time, LTE still chooses N-process Parallel Stop Protocol (SAW) to connect multiple processes at the receiving end through reordering function.

Collect data and sort it out. HARQ can be divided into synchronous HARQ and asynchronous HARQ in retransmission. Synchronous HARQ method

The retransmitted data must be sent immediately at the time known by the UE, so there is no need to attach the HARQ processing sequence number, such as subframe.

Number. Asynchronous HARQ can retransmit data blocks at any time. From the point of whether to change the transmission characteristics, HARQ can also

It can be divided into adaptive type and non-adaptive type. At present, LTE tends to adopt adaptive asynchronous HARQ scheme.

Unlike CDMA, OFDMA cannot eliminate inter-cell interference through spread spectrum. In order to improve the spectral efficiency, it also

It is impossible to simply adopt the frequency reuse method with the reuse factor of 3 or 7 as GSM. Therefore, in LTE, we attach great importance to small.

Interval interference reduction technology. There are three methods to reduce inter-cell interference, namely, interference randomization, interference elimination and interference coordination/

Avoid. In addition, the solution of using beamforming antenna in base station can also be regarded as reducing the interference between downlink cells.

General method. Interference randomization can adopt cell-specific scrambling and cell-specific interleaving, and the latter is well known.

Interleaved multiple access (idma); In addition, frequency hopping can also be used. Interference cancellation discusses taking more measures, such as relying on UE.

Antenna receiving space suppression and cancellation method based on detection/subtraction. And interference coordination/avoidance usually adopts the following methods.

A method for limiting downlink resource allocation through coordination among cells, such as summing time-frequency resources of adjacent cells and transmitting them.

Due to the limitation of radio power allocation, the performance of signal-to-noise ratio, cell boundary data rate and coverage has been improved.

E-UTRAN architecture

E-UTRAN is completely different from UTRAN in architecture, except that RNC is a network device, and only the NodeB network element is reserved.

The purpose is to simplify the network structure and reduce the delay. The functions of RNC are assigned to the evolved base station (E-NodeB) and the access gateway.

(aGW)。 It is not clear whether aGW is located in E-UTRAN or SAE (System Architecture Evolution). But from LTE

From the initial point of view, only a single-layer structure composed of E-Node B should be adopted, and so should aGW.

It belongs to the boundary node of SAE, but the functions of some user planes and control planes related to E-UTRA are determined in LTE.

Justice.

The E-UTRAN structure includes several e-Nodes (eNBs), which provide an E-UTRA user plane terminating in UE.

(PHY/MAC) and control plane (RRC) protocols. E-Nodes are interconnected in a mesh manner, and the E-Nodes

The interface with aGW is called S 1 interface.

The protocol stack structure of E-UTRAN is still divided into user plane and control plane like URTAN, but it is much simplified. take for example

The RLC layer is removed, the entity function is merged into the MAC layer, and the PDCP function is moved to the aGW on the network side. Control plane RRC

The function is moved to the E-NodeB and terminated in the E-NodeB on the network side.

Compared with UTRAN, E-UTRAN greatly simplifies the channel structure. Although it has not yet been finalized, it comes from

According to the results of the current discussion, the transmission channels will be reduced from 9 to 5, and the logical channels will be reduced from 10.

Reduced to seven now. Uplink/downlink shared channel (DL/UL-SCH) is used to carry control signaling and service data of users.

It replaces the DCH, FACH, HS-DSCH and electronic DCH channels of R6. MCH only provides data for multi-cell broadcast/multicast services.

While the broadcast/multicast service data of a single cell is carried on the SCH channel. At this stage, LTE has not yet decided whether.

Define logical channels for mapping multicast services, such as inheriting separate MCCH and MTCH at R6.

The radio resource control (RRC) state in LTE is also simplified, and the RRC state of UTMS and PMM is merged.

It is a state set and contains only three states: RRC_IDLE, RRC_ACTIVE and RRC_DETACHED. In aGW

In the network element, the context of the UE must distinguish these three states. While in the E-NodeB, the UE only keeps RRC_ACTIVE state.

Since then, the original states of DCH cells, FACH cells, PCH cells and PCH URA cells have been merged.

Concluding remarks

In addition to studying the evolution of wireless access networks, 3GPP is also committed to the evolution of system architecture.

Defined as SAE. At present, some 3GPP members have initiated and participated in the formulation and technical research of LTE/SAE standards, such as

Alcatel and other equipment manufacturers are actively developing systems and equipment that meet 3GLTE/SAE technical standards.

It is to ensure the leading technology and system performance, at the same time, maximize the use and compatibility of the existing system platform, and maintain the system.

The system provides the best wireless communication solution.