#2
文章发表于：2007-10-22 10:25
4G wireless: evolution or watershed in SOC architectures?
The next step in wireless technology could prove the tipping point for multicore embedded processing.
By Ron Wilson, Executive Editor -- EDN, 10/4/2007
AT A GLANCE
* Fourth-generation wireless service, or 4G, means different things to different people.
* Designers can achieve halfway measures with current chip architectures.
* Cost and energy constraints will eventually force some architectural innovation.
* The 4G that application developers dream of may require the use of radical architectures.
The almost-mythological fourth generation of wireless service—4G—could be the fountainhead of an entirely new way of thinking about SOC (system-on-chip) architectures. Or it could drive a simple evolution of today’s baseband wireless ICs. It could lead to entirely new kinds of mobile services for consumer clients. Or it could simply handle your e-mail attachments better. It could be a massive engineering challenge struggling into reality in 2015. Or it could happen in a couple of years.
To understand the impact that 4G is likely to have upon SOC design, you have to dig a little bit into just what people mean by the term, understand some of the computing challenges involved in supporting the service, and hear from some system architects on how they are approaching these challenges.
Many of the differences of opinion about the impact of 4G come from a single source: the lack of a clear definition. “You have to start with definitions,” warns Bill Krenik, Texas Instruments’ chief technology officer for wireless, “because all the controversy and confusion surrounding the term have left it meaning very little.”
Many people, Krenik says, think of 4G in terms of a new world of ubiquitous wireless connectivity—really anywhere, all the time—and in terms of the interactive, location-based, and media-rich services that such connectivity can support. Imagine walking down the street in an unfamiliar town, holding up your handset, and seeing it continuously display a real-time moving image of the street in front of you with map data, labels on buildings and sites of interest, paths to possible destinations, and locations of persons in your address file as you walk. Or imagine that same handset turning the streets of the town into a multiplayer video game, complete with avatars for other players, 3-D images of aliens and weapons, and realistic rendering of the damage resulting from the virtual battle.
Others—who have to implement the underlying systems—often see 4G in more concrete terms. “Within TI, we don’t try to have a dogmatic definition of what 4G is,” Krenik explains. “Instead, we refer to the actual technologies by name: HSPA+ [high- speed packet access plus], WiMax, LTE [long-term evolution]. Until 3G Americas comes up with a standard, everything else is just opinion,” he continues, referring to an organization whose mission is to promote the deployment of GSM (Global System for Mobile) communications and its evolution to 3G.
Still other engineers take a more quantitative view. Paralleling 3GPP’s (Third Generation Partnership Project’s) approach in defining LTE, these engineers frame 4G as “100-Mbps-peak throughput for mobile devices and 1-Gbps peak for nomadic devices such as notebook computers,” says Alan Brown, a senior radio-product manager at Nokia Siemens Networks. Each of these perspectives leads to a different set of expectations for the baseband SOC that will implement the 4G handset.
Evolving a baseband SOC
Start with the simplest set of expectations—those that LTE envisions—that the mobile device will somehow achieve a downlink peak data rate of at least 100 Mbps. “This [situation] leads to a baseband that is functionally no different from what we use today for UMTS [Universal Mobile Telecommunications System],” says Freescale Semiconductor Vice President and Senior Fellow Ken Hansen. Blocks include hardware accelerators for sample-rate functions, a CPU core to execute the MAC (media- access controller), a security engine, and a host interface.
At sample rate, data coming from the radio goes through analog-to-digital conversion, through some front-end digital processing, and into an FFT (fast-Fourier-transform) engine that separates the OFDM (orthogonal-frequency-division- multiplexed) signal into its many constituent frequency bands. The frequency-domain signal then goes through further digital conditioning and into a detector—not unlike the read channel in a disk drive—that decodes the 64 QAM (quadrature-amplitude -modulation) signal on each of the carriers, producing a symbol from each active carrier. The symbols go through turbo decoding for decompression.
The difference between 3G and 4G in this architecture is a difference of quantity, not kind. “In 3G, we extract about 1 bps per hertz of bandwidth,” points out Peter Carson, senior director of product management at Qualcomm CDMA Technologies. “To achieve 100-Mbps throughput, a 4G baseband would have to do significantly better than that: at least 3 or 4 bps per hertz, over a much wider band.”
In practice, this situation means many more carrier frequencies spread over a 20-MHz channel, compared, for instance, with the 5-MHz channel that UMTS 900 uses. It may also mean using multiple antennas in a MIMO (multiple-input/multiple-output) configuration. Today, MIMO configurations most often see use in channel equalization: You find a way to combine the signals from the two antennas to get the best possible reception. But 4G has something else in mind: using beam-forming algorithms to in effect make each pair of a base-station antenna and a receiver antenna into a separate channel, thus multiplying the effective bandwidth. “With multiple receivers, research has demonstrated, you can get about 1.75 times the data rate with two antennas,” Hansen says.
All of this capability requires silicon. The higher sample rate and wider channel mean a bigger, more power-hungry ADC and a faster, wider FFT engine. But the big hit comes from the need to provide for a 100-Mbps-peak throughput, which means faster symbol-rate processors, a lot more memory, and a faster processor for the MAC. “We are looking at 10 times the data rate coming into the MAC, with one-tenth the allowable latency on some transactions,” Hansen says. “But for power reasons, the MAC hardware has to run at a frequency much lower than the bit rate. This [problem] is interesting.”
Qualcomm’s Carson agrees. “Peak data rate turns directly into die size. One thing architects will have to ask themselves is whether the specified peak data rate and the die size it requires will be justified in the average data rate the network will actually deliver.”
Given sufficient insensitivity to chip cost, the baseband architecture for this rate can be evolutionary. Carson says that Qualcomm’s current Snapdragon architecture is still perfectly manageable extended to 30- to 40-Mbps-peak data rates. That speed doesn’t meet the LTE specification, but LTE will come later—some call it late-term evolution—possibly allowing time for 32-nm CMOS to once again bail out the architects.
Nonevolutionary design
One of the first challenges to evolutionary architecture will come from MIMO. “MIMO is used to improve the quality of the wireless link,” explains Thuyen Le, PhD, of the Feature Phone Business Unit, Communication Business Group of Infineon Technologies AG. “One idea is to use it for transmitter and receiver diversity to combat fading. The other idea is to exploit fading for spatial multiplexing, which then allows transmitting independent data streams over the multiple transmitting antennas at the same time, hence increasing the user data rate. That [idea], however, depends on how well- conditioned the channel matrix is. So, my take is that MIMO is necessary to achieve high data rates, in light of both ideas. ”
When air-interface designers shift from using a pair of receiving antennas to improve channel equalization to actually creating multiple channels through spatial-division multiplexing, the amount of duplicate hardware in the radio rises dramatically. Each antenna needs its own analog front end and digital front end, and the radio also needs either replication or increased throughput for much of the digital baseband (Figure 1). This requirement in itself does not mandate architectural innovation—just more of the same—but then there is the matter of power.
A limiting factor in any 4G architecture is that the radio must handle 10 times the peak data rate at a fraction of current energy consumption to make room for the dramatic increase in application-level processing energy. Will Strauss, president of research company Forward Concepts, estimates that a 4G handset will eventually require 100 times the computing power of a current 3G offering. “Everyone’s great hope is 32-nm processes,” Strauss observes, “but the reality is that energy consumption isn’t going down that much with new processes. What you gain in dynamic power you give back in leakage power. It may come down to finding novel architectures and power-management schemes or carrying a battery on your back for the handset. ”
Another factor drives consideration of novel architectures, as well. It is the previously mentioned disparity between simply stating a peak data rate, as the LTE specification does, and envisioning a new manner of using mobile devices, as do many of the visionaries who are evangelizing 4G to investors.
Imagining the future
“It’s true there is no clear definition of 4G,” says Liesbet Van der Perre, science director at IMEC (Interuniversity Microelectronics Center). “But I believe we should be talking of a heterogeneous network supporting much higher mobility and data rates than are currently possible. Today, if you are truly mobile, you will see less than 2 Mbps, but 4G should mean 10 to 20 Mbps of real throughput. At least 10 Mbits sustained—not peak—is essential for good video, for instance. One of the disappointments of 3G is that it could not deliver the sustained data rate for good video.”
Van der Perre and other researchers describe an environment far more dynamic than anything that today’s wireless networks can realize. “Today, a handset-silicon vendor faces something like 30 air interfaces, multiple noncontiguous channels, and many very different services running simultaneously,” she observes. But the fact that one phone from one vendor supports only a small subset of this cacophony simplifies much of this complexity.
In the future, to ensure both sufficient sustained bandwidth—think of that real-time video aligned to the moving handset’s location and orientation—and sufficient energy efficiency—always choosing only just enough bandwidth and coding strength for the current task mix—a mobile device may be in continuous negotiation with a number of vendors, using a number of air interfaces from a number of base-station sites all at once (Figure 2). Bursts of data, video streams, control information, and a return channel from the keyboard and camera may all be traveling over different services and may switch in real time. For instance, holding the camera still allows the motion compensation in H.264 to drastically reduce the bit rate necessary to link the camera to a game-server farm. This action thus allows the radio controller to select an air interface with a lower bit rate.
In this world view, using today’s hardware-processing pipelines with dedicated blocks is an intermediate option, Van der Perre says. She sees modular, heterogeneous clusters of similar processors that you can specialize and a configurable interconnect network that can allow real-time dynamic processor configuration and task mapping. Aggressive energy-management techniques, including rapid voltage-frequency scaling, moderately fine-grained power-gating of idle units, and agile shifting of algorithms between software and hardware, become possible in such an architecture. Indeed, this approach may be the only way to meet the energy-efficiency demands of the true 4G terminal, 32-nm CMOS notwithstanding.
All of these elements are taking shape at IMEC in various research projects, which perhaps explains Van der Perre’s world view. But it is far from an isolated point of view, at least in private. Companies publicly state their dedication to their pipeline-based hardware architectures, but one well-placed industry source claims that there are deeply embedded, heavily funded research teams at a number of major silicon suppliers exploring large multicore architectures for the 4G problem.
One major challenge with most large, multicore architectures is not an issue here: Much of the workload in high-bit-rate baseband processing is what the industry calls embarrassingly parallel. It’s not hard to spread around the tasks by simply dividing up the data. But the system-control, dynamic-load-balancing, and—perhaps most important—energy-management tasks are new, complex, and vital to the success of the design. In this respect, 4G may in fact not be evolutionary but rather the forge on which designers beat an entirely new style of real-time embedded processing into shape.
#3
文章发表于：2007-10-22 10:27
4G：SoC结构的继续演进还是分水岭？
http://article.ednchina.com/Communinet/2007-10/20071022102456.htm
总览：
4G通信服务对不同的人有不同的含义；
利用现在的芯片架构设计者可以实现前期测试；
成本和功耗限制最终会导致一些架构上的创新；
应用开发者期待的4G或许需要采用最基本的架构；
几近神话的第四代无线服务4G可以促使人们对SoC架构重新考量。或许它会驱动现今基带无线芯片的演变，给消费类客户端提供全新的移动服务 ；或许它只是对email附件的一个小小提升；它可能因为巨大的工程挑战而在2015年成为现实，也可能在未来几年就发生。
要想明白4G对SoC设计的影响，有必要先了解一些名词，明白支持这些服务面临的挑战，并听听系统架构师应对挑战的途径。很多关于4G的观点 都来自同一个问题：缺乏清晰的定义。我们必须从定义开始，定义的缺失只会导致争论和混乱。
很多人把4G看作是全新的无处不在的无线连接，它可以支持交互的、基于位置的并且内容丰富的服务。对另外一些需要实现系统的人来讲，4G 是更具体的技术：HSPA，WiMax，LTE。还有一些工程师采取更定量的看法，他们认为4G时移动设备可以达到100Mbps带宽，对诸如笔记本等便携 设备可以达到1Gbps。不同的观点对4G手持设备的基带SoC有不同的期待。
基带SoC的演进
从最简单的期待开始：移动设备的峰值下载速率可以达到至少100Mbps。它对基带的要求与UMTS相比在功能上并没有区别，处理模块包括：采样 速率的硬加速功能、执行MAC的CPU核，安全引擎以及主机接口。射频信号经过模数变化以及一些前端的数字处理后由FFT引擎实现OFDM处理。这 个频域的信号经过进一步数字调理后进入检测器进行64QAM信号解码，最后是Turbo解压缩。
3G和4G在这个架构上的区别不是类别而是数量。3G时1Hz带宽提取1bps，4G时为了达到100Mbps的吞吐率，1Hz需要提取至少3到4bps。实用中， 这就意味着更多的载波频率扩展在20MHz的带宽内，相应地UMTS900使用5MHz的带宽。这或许还意味着在MIMO配置中采用多天线。同时，4G还采 用波束成形算法。
所有这些性能都需要芯片。更高的采样速率和更宽的信道意味着更大的ADC和更快更宽的FFT引擎。但是最大的问题还在于100Mbps的峰值吞吐率 ，这意味着更快得符号速率处理器、更大的存储体以及更快的MAC处理器，但是从功耗角度考虑，MAC硬件又必须以低于比特率的速度运行。这 个问题很有趣，因为峰值数据率直接与晶片大小相关。如果不考虑芯片成本，那么这个速率上的基带结构将会继续演进。
非演进式设计
对演进式架构的第一个挑战来源于MIMO。MIMO用来克服衰减，提高用户数据率。但是它依赖于信道矩阵的完美设计。当空中接口设计从一对接 收天线变为空分复用的多天线，射频部分的重复硬件急剧增加。每个天线都需要自己的模拟前端和数字前端，射频部分也需要或者进行多个重 复或者增加吞吐率。这些需求并不要求结构上的更新，而是更多的重复，但是这会涉及到功耗问题。
4G结构的一个限制因素就是射频必须在当前功耗的条件下实现10倍的峰值数据率。据估计，4G手持设备的功耗是3G时的100倍。大家都在期待 32nm制程可以解决这个问题，但是实际上这是不可能的。所以，我们需要找到一个全新的架构和功耗管理方案。还有一个对全新架构需求的驱 动就是前面提到的差别，4G到底是简单的峰值速率提升还是对移动设备的一个全新应用。
规划未来
4G没有一个清晰的定义。但是4G网络应该比目前的3G支持更高的移动性和数据率。4G应该支持10M到20M的实际吞吐率，对于流畅的视频服务来 讲，最少需要10Mbits的持续速率，而3G令人失望的一点就是无法提供流畅视频所需要的持续数据流。
现在，手机芯片面临着多个空中接口、多个非连续信道以及同时运行的多个不同服务，而事实上，运营商的手机只是支持这些需求的一个子集 ，因此降低了复杂度。
未来，为同时保证足够的持续带宽和有效的功耗管理，移动设备可能会通过一系列的空中接口同时与很多基站通信。突发数据、视频流、控制 消息以及键盘和摄像头通道或许都通过不同服务实现，并进行实时的交换。
主动功耗管理技术，包括快速电压-频率调节等技术可以在可配置互连网络结构中实现。实际上，这个方法或许是4G终端在采用32nm CMOS技术 的情况下达到功耗要求的唯一办法。
对多核设计的一个大的挑战还在于高速率基带的并行处理。分配任务并不困难，但是系统控制、动态负载均衡以及功耗管理才是成功设计的关 键。如此看来，4G实际上可能不是演进式的，而是一个将全新嵌入式实时处理变为现实的过程。