Foreword
With the large number of applications of mobile data services and the emergence of new types of services, the requirements on the performance and quality of mobile communication networks are becoming higher and higher. LTE is a system and network for long-term evolution. It is not actually a standard, but it has led to a comprehensive evolution of the 3G standard. At present, HSDPA and HSUPA have been generally introduced in 3G networks. The next step will be to choose the evolution direction of HSPA + and LTE. The analysis of the evolution route and standardization process of LTE and its similarities and differences with HSPA + will undoubtedly help to better understand the current and The evolution direction of the future network.
The evolution process of LTE technology and the main performance indicators of LTE standard are introduced. Through the analysis and comparison of LTE technology and HSPA + technology, the performance and advantages of LTE technology are expounded. On this basis, the 4G evolution direction of LTE-A is prospected.
1 LTE standard evolution process
GSM network is the earliest digital mobile communication technology. It is based on FDD and TDMA technology. Due to the limitations of TDMA, the development of GSM network is severely challenged by capacity and service quality. From the perspective of business support types, although GPRS / EDGE introduces data services, but because it uses the original GSM air interface, its bandwidth is limited, and it cannot meet the needs of data service diversity and real-time. In terms of the development of technical standards, EDGE and EDGE + evolution directions are proposed for GPRS, but the emergence of the 3G standard based on CDMA access mode makes EDGE no longer enter people's sight.
CDMA uses code division multiplexing. Although the CDMA standard in the 2G era matures late, it has technical advantages such as strong anti-interference ability and high spectrum efficiency. Therefore, WCDMA, TD-SCDMA and CDMA2000 in the 3G standard generally adopt CDMA technology. .
When evolving to a 3G network, the GSM system can use the WCMDA or TD-SCDMA route, while CDMA uses the CDMA2000 route. The early standard of WCDMA and TD-SCDMA was R99. Later, IMS was introduced in R4 version, HSDPA was introduced in R5 version, HSUPA was introduced in R6 version, HSPA + was introduced in R7 version, and R8 version was for LTE. Then to the direction of UWB development, the evolution path is shown in Figure 1.
In each version, new technologies are used to improve network performance and service quality, and throughput is used for comparison. The results are shown in Table 1.
LTE is a future-oriented mobile communication technology standard. As early as the end of 2004, 3GPP started the standardization of LTE technology, and released RDD FDD-LTE and TDD-LTE standards in March 2009, which marks the LTE standard The draft study is completed, and LTE has entered a substantial research and development stage. The concept of LTE-advanced (LTE-A) was further proposed in the R9 version. LTE-A passed the ITU evaluation in June 2010 and officially became one of the main technologies of IMT-A in October 2010. Evolution and enhancement based on the version. The R10 version improves it and is a key version of LTE-A.
LTE uses orthogonal frequency division multiplexing (OFDM), multiple input multiple output antenna (MIMO) and other physical layer key technologies and network structure adjustments to achieve performance improvements. LTE-A introduces some new candidate technologies, such as carrier aggregation technology, enhanced multi-antenna technology, wireless network coding technology, and wireless network MIMO enhancement technology, etc., so that performance indicators have been further improved.
2 LTE basic performance requirements
At the beginning of the design of the LTE system, its goals and needs were very clear. As a revolutionary technology in the post-3G era, LTE has the main goals of reducing latency, increasing user transmission data rates, and increasing system capacity and coverage. Specific performance requirements are as follows:
a) Support 1.4, 3, 5, 10, 15 and 20MHz bandwidth, use existing or newly added frequency bands flexibly; and support "paired" frequency bands and non- "paired" frequency bands with as similar technology as possible to facilitate flexible system deployment .
b) Under the condition of 20MHz bandwidth, the peak rate reaches 50Mbit / s (2 & TImes; 1 antenna) in the uplink, and 100Mbit / s (2 & TImes; 2 antenna) in the downlink.
c) In a network with a load, the downlink spectrum efficiency reaches 2 to 4 times that of 3GPP R6 HSDPA, and the uplink spectrum efficiency reaches 2 to 3 times that of R6 HSUPA.
d) Under the conditions of single user, single service flow and small IP packet, the unidirectional delay of the user plane is less than 5ms.
e) The transition time from idle state to active state is less than 100ms, and the transition time from sleep state to active state is less than 50ms.
f) Support low-speed movement and high-speed movement. The performance is better at low speed (0 ~ 15km / h), the best performance at high speed (15 ~ 120km / h), and users at higher speed (350 ~ 500km / h) can maintain connectivity.
In addition to the performance index requirements, LTE technology has put forward specific requirements in terms of operability, interconnectivity and service support, such as support for interoperability with existing 3GPP and non-3GPP systems; support for enhanced broadcast and multicast Business; reduce network construction cost; support enhanced IMS and core network; cancel circuit domain, all services are implemented in packet domain, such as VoIP, support simple adjacent frequency coexistence; provide QoS mechanism for different types of services to ensure real-time Quality of service; allows non-contiguous spectrum to be allocated to UE; optimizes network structure, enhances mobility, etc. Therefore, compared with other wireless technologies, LTE has higher transmission performance and is suitable for both high-speed and low-speed mobile application scenarios.
3 Performance comparison between LTE and HSPA +
As a direct evolution of HSPA technology, HSPA + was introduced in the R7 version, and has experienced the development of R8 and R9 versions together with LTE. The starting point of HSPA + is the consideration of investment cost and smooth evolution, so it has certain limitations. This evolution can only be regarded as a technological "improvement". In contrast, LTE, as a mainstream evolution technology focusing on 4G, can be regarded as a technological "revolution". The performance difference between LTE and HSPA + is reflected in aspects such as throughput, delay, and spectrum efficiency.
3.1 Throughput
Throughput refers to the amount of data successfully transmitted per unit time, and is an important indicator to measure the performance of wireless communication systems. Factors affecting throughput include bandwidth, modulation method, signal quality, channel fading, noise interference, scheduling mechanism, etc.
Considering backward compatibility and upgrade costs, HSPA + 's carrier bandwidth continues to use 5MHz since WCDMA. When using 2 & TImes; 2 MIMO configuration and 16QAM modulation, the peak rate of HSPA + is 28Mbit / s, and when using 2 & TImes; 2 MIMO configuration and 64QAM modulation, the peak rate is 42Mbit / s. The LTE system can support 20MHz bandwidth, and LTE-A can support 100MHz bandwidth. The larger bandwidth makes the LTE system have a larger transmission capacity than HSPA +.
The LTE system downlink supports multiple multi-antenna array technologies such as SU-MIMO, MU-MIMO, and beamforming based on reference signals, supports 8 different MIMO and beamforming modes, and can simultaneously support the transmission of multiple data streams. Each user in LTE can support 2 streams in downlink, while LTE-A can support 8 streams in downlink, and can also use 4 × 4, 8 × 8 and other types of transmission and reception methods, while the currently defined HSPA system only supports transmission Diversity and 2 × 2 MIMO. The richness and diversity of MIMO technology applications make LTE throughput better.
LTE uses a natural equalizer. If the RMS delay spread is less than the CP length, there will be no inter-system interference. HSPA + uses a Rake receiver, which cannot completely eliminate inter-system interference, so performance will degrade in a multipath environment. In the LTE system, the MLD + SIC receiver is used for the downlink, and the SIC receiver is used for the uplink. These advanced receiver technologies can further reduce interference.
In addition, HSPA + does not use frequency selective scheduling, only opportunistic scheduling in the time domain. LTE benefits from a frequency-selective scheduling mechanism, which can perform opportunistic scheduling in both time and frequency domains, with a capacity gain of approximately 10% to 15%. For VoIP, a typical voice application in the PS domain, HS-SCCH is no longer used in HSPA +, and the downstream capacity is improved, but the upstream is still the limiting factor. LTE uses semi-persistent scheduling and TTI bundling technology to reduce control channel overhead and greatly improve VoIP capacity.
The theoretical maximum transmission rate of LTE and HSPA + is shown in Figure 2. It can be seen intuitively from Figure 2 that when the maximum bandwidth configuration is adopted, the transmission performance of LTE far exceeds that of HSPA +, and its throughput is about 8 times that of the latter.
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