Foreword
With the large number of applications of mobile data services and the emergence of new types of services, the requirements for the performance and quality of mobile communication networks are increasing. LTE is a system and network for long-term evolution. It is not actually a standard, but it has led to the full evolution of the 3G standard. At present, HSDPA and HSUPA have been widely introduced in 3G networks. The next step is to face the problem of HSPA+ and LTE evolution. The analysis of LTE evolution path and standardization process and its similarities and differences with HSPA+ will undoubtedly help to understand the current and deeper understanding. The evolution direction of the future network.
The evolution process of LTE technology and the main performance indicators of the LTE standard are introduced. Through the analysis and comparison of LTE technology and HSPA+ technology, the performance and advantages of LTE technology are expounded. Based on this, the LTE-A 4G evolution direction is forecasted.
1 LTE standard evolution process
The GSM network is the earliest digital mobile communication technology. It is based on FDD and TDMA technologies. Due to the limitations of TDMA, GSM network development is severely challenged by capacity and quality of service. From the perspective of service support, although GPRS/ EDGE introduces data services, but because it uses the original air interface of GSM, 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, the evolution direction of EDGE and EDGE+ is proposed for GPRS, but the emergence of 3G standard based on CDMA access method makes EDGE no longer enter people's sight.
CDMA adopts code division multiplexing. Although the CDMA standard in the 2G era is mature late, it has the technical advantages of strong anti-interference ability and high spectral efficiency. Therefore, WCDMA, TD-SCDMA and CDMA2000 in the 3G standard generally adopt CDMA technology. .
When migrating to a 3G network, the GSM system can adopt the WCMDA or TD-SCDMA route, while the CDMA uses the CDMA2000 approach. The early standards of WCDMA and TD-SCDMA were 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, LTE is introduced in R8 version, and CDMA2000 is extended to CDMA1x in CDMA series. Going to the direction of UWB, the evolution path is shown in Figure 1.
In each version, new technologies are used to improve network performance and quality of service, 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 work of LTE technology, and in March 2009 released the R8 version of FDD-LTE and TDD-LTE standards, which marked the LTE standard. The draft study is completed and LTE enters the substantive development phase. 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. It is in R8. Evolution and enhancement based on the version. The R10 version is perfected and is a key version of LTE-A.
LTE adopts physical layer key technologies such as Orthogonal Frequency Division Multiplexing (OFDM) and Multiple Input Multiple Output Antenna (MIMO) and network structure adjustment to obtain performance improvement. 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, to achieve greater improvement in performance indicators.
2 LTE basic performance requirements
At the beginning of the LTE system design, its goals and needs are very clear. As a revolutionary technology in the post-3G era, LTE has focused on reducing latency, increasing user data rates, and increasing system capacity and coverage. The specific performance requirements are as follows:
a) Support 1.4, 3, 5, 10, 15 and 20MHz bandwidth, flexibly use existing or new frequency bands; and support "paired" and non-"paired" 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) and 100Mbit/s (2&TImes; 2 antennas).
c) In a loaded network, the downlink spectrum efficiency is 2~4 times that of 3GPP R6 HSDPA, and the uplink spectrum efficiency is 2~3 times that of R6 HSUPA.
d) Under the condition of single user, single service flow and small IP packet, the user plane one-way delay is less than 5ms.
e) The transition time from the idle state to the active state is less than 100 ms, and the transition time from the sleep state to the active state is less than 50 ms.
f) Support low speed movement and high speed movement. The performance is good at low speed (0~15km/h), the best performance under high speed (15~120km/h), and the connection can be maintained at higher speed (350~500km/h).
In addition to performance requirements, LTE technology provides specific requirements for operability, interoperability, and service support, such as support for interoperability with existing 3GPP and non-3GPP systems; support for enhanced broadcast and multicast Service; 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, guarantee real-time services Quality of service; allows allocation of non-contiguous spectrum to the 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 LTE and HSPA+ performance comparison
As a direct evolution of HSPA technology, HSPA+ was introduced in the R7 version and experienced the development of the R8 and R9 versions together with LTE. The starting point of HSPA+ lies in the consideration of investment cost and smooth evolution, so it has certain limitations. This evolution can only be regarded as a technical "improvement". In contrast, LTE, as a mainstream evolution technology focusing on 4G, can be called a technological "revolution." The performance difference between LTE and HSPA+ is reflected in 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 of the performance of a wireless communication system. Factors affecting throughput include bandwidth, modulation scheme, signal quality, channel fading, noise interference, scheduling mechanisms, and so on.
Considering backward compatibility and upgrade costs, the carrier bandwidth of HSPA+ follows 5MHz since WCDMA. When using 2&TImes; 2 MIMO configuration and 16QAM modulation mode, the HSPA+ peak rate is 28 Mbit/s, and the peak rate is 42 Mbit/s when using 2&TImes; 2 MIMO configuration and 64QAM modulation. The LTE system can support 20MHz bandwidth, and LTE-A can support 100MHz bandwidth. The larger bandwidth allows the LTE system to have a larger transmission capacity than HSPA+.
The LTE system supports multiple multi-antenna array technologies such as SU-MIMO, MU-MIMO, and reference signal-based beamforming, supports 8 different MIMO and beamforming modes, and can simultaneously support multiple data streams. In LTE, each user can support 2 streams in the downlink, while LTE-A can support 8 streams in the downlink and downlink, and can also use 4×4, 8×8, etc., and the currently defined HSPA system only supports transmission. Diversity and 2×2 MIMO. The richness and diversity of MIMO technology applications make LTE's throughput better.
LTE uses a natural equalizer, and if the RMS delay spread is less than the CP length, no inter-system interference will occur. HSPA+ uses Rake receivers, which do not completely eliminate inter-system interference, so performance will degrade in multipath environments. In the LTE system, the MLD+SIC receiver is used in the downlink and the SIC receiver is used in the uplink. These advanced receiver technologies can further reduce interference.
In addition, HSPA+ does not use frequency selective scheduling and uses opportunistic scheduling only in the time domain. LTE benefits from the frequency selective scheduling mechanism, and can perform opportunistic scheduling in both the time domain and the frequency domain, and its capacity gain is about 10% to 15%. For the typical voice application of the PS domain, VoIP, the HS-SCCH is no longer used in HSPA+, and the downlink capacity is improved, but the uplink is still the limiting factor. LTE uses semi-persistent scheduling and TTI bonding techniques 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 visually from Figure 2 that when using the maximum bandwidth configuration, LTE's transmission performance far exceeds HSPA+, and its throughput is about 8 times that of the latter.
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