The development of mobile communication systems for decades has been a technical update for the pursuit of higher spectral efficiency, from GMSK in the 2G era to CDMA in the 3G era to OFDM in the 4G era. At the same time, the design technology and production technology of large-scale integrated circuits have also changed from a few hundred nanometers to several tens of nanometers. The increasing bandwidth requirements of the system mean higher and higher processing power requirements for the terminal chip platform. The development of the system from 2G to 4G, the development of the wireless network itself also requires a long time and process, and the elimination of the existing 2G and 3G networks is not likely to be completed overnight, so the terminal chip platform is also proposed. Adaptive needs that change with the evolution of the network, that is, the working mode requirements for automatic switching of multiple modes. This article will discuss the requirements of several wireless communication systems for terminal baseband chips, and introduce a software wireless terminal baseband chip platform for 2G/3G/4G designed by the company.
1. Status of existing 4G terminal baseband chips
As shown in Figure 1 below, the basic technology of 4G wireless communication system is OFDM. OFDM system signals are array signals that are present in both the time and frequency domains. The conversion between time-frequency domain signals, the processing of a large number of array signals such as channel estimation and MIMO detection requires a large amount of parallel vector processing. Highly parallel vector processors are emerging from the OFDM system array signal processing requirements. Multi-core multi-threaded core plus vector processor is the trend of 4G baseband chip architecture.
Figure 1: Schematic diagram of OFDM system signal processing
From the published literature and information on wireless terminal baseband processors, the industry has made a lot of progress in programmable and vector processing applications, as shown in Table 1.
Table 1: Processing Capabilities in Baseband Processors
Others are ADRES from IMEC, ArdBerg from Michigan University, and others.
2. 2G/3G/4G terminal baseband system requirements analysis
Terminal system requirements are divided into two aspects, one is functional requirements, and the other is performance requirements. This chapter illustrates the power requirements of several systems in 2G/3G/4G through an abstract architecture diagram; and then gradually analyzes the performance requirements of different systems. Analysis of performance requirements, this paper starts with system bandwidth, sampling rate, and complexity analysis of link algorithm processing.
2.1 Abstract Architecture of Baseband Platform for Wireless Communication Terminals
Figure 2: Abstract Architecture of Baseband Platform for Wireless Communication Terminals
Functional requirements are shown in Figure 2 above, and all terminal baseband systems need to be completed:
â— interface with RF signals and control of frequency and gain of RF circuits;
â— Processing of uplink and downlink signals, modulation and demodulation circuits (or algorithms), receiving equalization and decoding circuits (or algorithms), and loop control of gain/frequency/power;
â— High-level protocol functions such as the establishment and release of communication links.
These functions on different chips, when building different system architectures, there will be different division of hardware and software.
2.2 Performance requirements of 2G/3G/4G systems
Figure 3: Schematic diagram of 2G/3G/4G system performance requirements
2G is a system based on voice communication, 3G is a system that combines voice communication and data communication, and 4G is a system that is based on high-speed data communication. The 2G air interface has a bandwidth of less than 200khz and can provide data traffic of several hundred Kbps. The 3G bandwidth is about 2Mhz, which provides several MBps of data traffic. The 4G bandwidth is higher than 20MHz and provides more than 100MBps of data traffic. 2G and 3G RF interface is simple, 1 receiving channel, 1 transmitting channel, the sampling rate of baseband signal is 2G at around 1MHz (taking 4 times oversampling as an example), 3G is about 10MHz; 4G RF interface has MIMO mode, 2 -- 4 receive channels, 1 - 2 transmit channels, and the baseband signal has a sampling rate greater than 30 MHz. The basic technology of the system 2G is GMSK modulation method and convolutional code; the link processing algorithm is simple, and the processed data volume is low; 3G is CDMA modulation mode, convolutional code plus Turbo code, and need to adopt matched filter and Turbo decoding, etc. Algorithm, but the amount of data processed is still relatively low; 4G uses OFDM technology, convolutional code plus Turbo code, link processing needs to use more complex algorithms such as MIMO detection and Turbo decoding, and the processing data volume is larger than 2G and 3G As the amplitude increases, the high-level protocol stack also needs to have a large data traffic processing technology. Based on the performance requirements of the system, it can be analyzed that the computational load of the physical layer is less than 50 MOPS per second, 3G is less than 500 MOPS per second, and 4G is over 5000 MOPS. High-level protocol stack processing, 2G is about 10MOPS per second, 3G is less than 100MOPS per second, and 4G is more than 1000MOPS. The requirements of the cached data area: 2G physical layer is lower than 128KBytes, high-level protocol stack is lower than 256Kbytes; 3G physical layer is less than 512KBytes, high-level protocol stack is less than 5Mbytes; 4G physical layer exceeds 2MBytes, and high-level protocol stack exceeds 20Mbytes.
2.3 Summary of Requirements Analysis of Baseband Platform for Wireless Communication Terminals
In summary, the terminal baseband platform from 2G to 4G has a very similar system functional architecture, but due to the revolutionary changes in system bandwidth and basic technology, there is a leap in performance requirements from quantitative to qualitative. But a similar functional architecture guides architects to pursue a terminal baseband platform that can take into account several eras. The solution described in this article is such a platform.
3. Customized multi-mode architecture design
Since the 4G network construction still takes a while, the 4G terminal chip needs to consider the multi-mode architecture compatible with 2G/3G in the architecture design, so that the terminal can fully enjoy the network resources when moving in the network.
In the 2G era, the traditional chip architecture design scheme uses part of the physical layer to implement circuit logic, and the high-level protocol uses a programmable core circuit to run the corresponding software. In the 3G/4G era, the traditional idea is to inherit the existing circuits of the former, add new circuit logic to implement the physical layer, and then increase the capability of the programmable core or multiple programmable cores to run high-level protocols. Thus, a conventional multimode architecture as shown in Fig. 4 is formed.
Figure 4: Traditional multimode baseband chip architecture
The advantage of this architecture is that the development cycle is relatively short, and the physical layers of 2G and 3G have been working stably, but the main drawback is that the area of ​​the logic circuit is also large, resulting in higher chip cost.
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