A Milestone of GSM BTS ― Application of Multi-Carrier Technology in GSM

Evolution of Broadband GSM BTS

Global GSM subscriber base is expected to reach a record of 4.2 billion at the end of 2010. As the most important part in the mobile communications system, the GSM EDGE Radio Access Network (GERAN) is advancing with the evolution of standards and technologies. The growth history of GSM BTS is exactly an epitome of the development of wireless technologies. The current GSM BTS has gone through four stages as shown in Figure 1.

GSM is a narrowband digital communications system. Both the 1st and 2nd generation GSM BTS adopted the Single-density TRX Unit (STRU). With the miniaturization of RF components and the development of DSP technology, the Dual-density TRX Unit (DTRU) was widely used in the 3rd and 4th generation GSM BTS. The dual-density module encapsulates two single-density RF modules in one unit to double the capacity of BTS. However, STRU and DTRU are in nature narrowband carrier technology, in which each carrier corresponds to a separate RF channel.

As operators are increasingly focusing on lower TCO and smooth evolution, the key for them to survive the competition is to fast and cost-effectively deploy or expand their networks. This requirement gives rise to the 5th generation GSM BTS that supports 3GPP R7/R8 EDGE, and 3GPP R9 EDGE Enhancement and Evolved-EDGE. The 5th generation GSM BTS can provide data rates of up to 2–4Mbps at the air interface for GSM and support WCDMA, HSPA, or even LTE. It also supports MIMO and OFDM. The introduction of software radio combined with the wideband Multi-Carrier Power Amplifier (MCPA) technology and the reduction in cost help to make a truly Software Defined Radio (SDR) BTS.

Introduction of MCPA into GSM

The core of the 5th generation GSM BTS is open architecture based on the wideband multi-carrier technology, in which MCPA is introduced into the GSM RF module to substitute the traditional narrowband analog power amplifier and achieve broadband RF.

As shown in Figure 2, the core of MCPA is to first couple signals in the digital Intermediate Frequency (IF) part, and then output them through the broadband power amplifier, which means that a physical module can be software configured as required into many logical radio sub-carriers without the need of coupling. This would undoubtedly help to develop a low-TCO BTS featuring higher integration, larger capacity and lower power consumption. Moreover, the Digital Pre-Distortion (DPD) technology widely used in WCDMA MCPA can also be widely used in GSM BTS, which will further improve the service support capacity of GSM BTS and greatly extend the life cycle of GSM network.

The multi-carrier technology was commercialized in WCDMA base station as early as in 2002, and then extensively used in CDMA2000 and TD-SCDMA base stations. However, it was not until 2008 that ZTE took the lead to introduce it into the GSM field. The reasons lie in technology, cost and standard restraints. Firstly, WCDMA and CDMA2000 are wideband multi-carrier systems, while each carrier in GSM system has only a bandwidth of 200kHz. The narrower the bandwidth, the more difficult the design of IF and Digital-to-Analog Converter (DAC). Secondly, due to the higher initial cost, MCPA is not suitable for the widely-used GSM system. Thirdly, no GSM standards prior to 3GPP R8 cover the multi-carrier technology.

Driven by factors like market potential, structure update, cost reduction, and standard evolution, major GSM equipment vendors begin to pay great attention to the development of GSM BTS. The BTS products are evolving to have higher integration, smaller size, lower power consumption and wider application, and to support higher-speed data services and multi-network convergence. The R&D of RF module has attracted increasing attention in the industry, with the view to reducing cost and improving performance to adapt to the new technologies and standards. All this leads to the introduction of MCPA into GSM. 

Multi-Carrier GSM BTS ― The Edge Tool to Cut TCO

Fast deployment, green coverage

The typical capacity of a traditional GSM BTS is 12TRX. The largest dual-density BTS of the industry launched by ZTE in 2007 has a capacity of only 18TRX. After adopting the multi-carrier technology, one MCPA can support a capacity of six GSM carriers, three times larger than the traditional DTRU; and the BTS is smaller in size, which can support 36TRX in a single cabinet and 72TRX in a stacked cabinet. The adoption of the multi-carrier technology makes it unnecessary to use the hybrid combiner or double filter combiner, and helps to reduce equipment failure and insertion loss. Therefore, the multi-carrier GSM BTS has the advantage of strong coverage over the traditional one.

In addition, the MCPA module allows the sharing of power resource, or a power pool, helping to implement dynamic coverage-capacity swapping and improve the coverage area of the BTS.

With the help of technologies like DPD, the efficiency of MCPA has been further enhanced. Because MCPA has a unique carrier pool, its power resources can be allocated dynamically based on the traffic and the power needed. This improves the BTS resource utilization. Besides, since MCPA needs no combiner, its power consumption is greatly reduced. Therefore, the carrier power needed to cover the same area is greatly reduced. For example, the power consumption of ZTE’s BS8800 SDR (multi-carrier GSM BTS) is only 70% of or even lower than that of the traditional GSM BTS, as shown in Figure 3.

Using the multi-carrier GSM BTS that features high integration, low power consumption, and high power output, the number of BTSs needed for network construction is significantly reduced. Hence, the goal of fast network deployment and green coverage can be achieved.   

Easy O&M, smooth expansion

The MCPA module enables higher BTS integration and fewer BTSs used, which helps to reduce the O&M cost and difficulty, and achieve smooth capacity expansion.

For example, a GSM operator adopted the configuration of S222 at the initial stage of network construction, but later it needs to expand the configuration to S666 due to the growing subscriber base. If the operator uses the conventional dual-density carriers, it is necessary to add one cabinet and six DTRUs, which would be very expensive. However, using the MCPA technology, the operators only needs to configure the software at the O&M center, without going to the BTS site to add hardware or modify connection. This would undoubtedly reduce the difficulty in capacity expansion and bring great savings in labor resource and hardware cost.

Cost-effective evolution

Both the variety of wireless standards and the growing complexity of network deployment have brought tremendous challenges to the long-term and sustainable development of operators. Traditional GSM BTS fails to achieve smooth expansion while multi-carrier GSM BTS can lay a solid foundation for SDR-based BTS. The RF Unit (RU) of SDR-base BTS is software reconfigurable and programmable, which helps to implement intelligent spectrum allocation and support multiple standards. The GSM BTS based on wideband multi-carrier technology can send and receive RF signals of many standards like GSM, WCDMA and LTE, through software configuration on the 20MHz continuous band. In practical networking, the RF module can be configured to support GSM/WCDMA dual modes, or WCDMA/LTE dual modes, or GSM/WCDMA/LTE multiple modes, thus realizing cost-effective system integration and evolution.

The new-generation SDR-based BTS solution using the wideband multi-carrier technology can not only meet the operators’ needs for building multi-standard, multi-band networks, but also help them effectively adjust their network development strategies to adapt to the market change and achieve sustained competitiveness and profitability.

Looking into the Future

The MCPA-based GSM BTS will evolve into a flat Intelligent, Information, and IP-based Base Station System (I3BSS), where the functions of BSC and CN will be integrated into a master BTS, and a slave BTS will be a low-cost, discrete access point. This kind of architecture will further reduce network complexity and construction cost to satisfy the need of a seamless converged high-speed data network in the future.

The introduction of advanced technologies like MCPA, all-IP architecture, high-speed DSP and CPRI into GSM makes a true 3G GSM that can boost profitability and lay a solid foundation for a seamless converged high-speed broadband network.