频带、频率分配、无线电、ISM频段、4G

最近写论文碰到不同的通信技术(比如cellular, femotocell, WiFi),都涉及到了频带。于是想粗略了解下频带和频率分配。

1. 频带和频率分配

频带(frequency band),

A frequency band is an interval in the frequency domain, delimited by a lower frequency and an upper frequency. Many systems are characterized by the range of frequencies to which they respond. Musical instruments produce different ranges of notes within the hearing range (The human range is commonly given as 20 to 20,000 Hz). The electromagnetic spectrum can be divided into many different ranges such as visible lightinfrared or ultraviolet radiation, radio waves, X-rays and so on, and each of these ranges can in turn be divided into smaller ranges. A radio communications signal must occupy a range of frequencies carrying most of its energy, called its bandwidth. A frequency band may represent one communication channel or be subdivided into many. Allocation of radio frequency ranges to different uses is a major function of radio spectrum allocation.

频率分配(frequency allocation or spectrum allocation),

Frequency allocation (or spectrum allocation) is the division of the electromagnetic spectrum into radio frequency bands. This spectrum management is regulated by governments in most countries.

我的理解是这样的,国际电信联盟从全球的角度对频率进行分配,各个国家(联合国193成员国都是ITU的成员)在这个大框架下进行频率细分配与管理。比如中国无线电管理由国家无线电监测中心/国家无线电频谱管理中心(一个班子,两块牌子,是工业和信息化部的直属事业单位)负责,其官方网站是:srrc.org.cn。关于国际电信联盟介绍,摘抄维基百科如下:

The International Telecommunication Union (ITU) is a specialized agency of the United Nations (UN) that is responsib for issues that concern information and communication technologies. The ITU coordinates the shared global use of the radio spectrum, promotes international cooperation in assigning satellite orbits, works to improve telecommunication infrastructure in the developing world, and assists in the development and coordination of worldwide technical standards.

直观感受下美国频率分配表,如下:

united_states_frequency_allocations_chart_overview
Fig. 1: Overview of United States Frequency Allocations Chart (image from here)

1.1 一些常见的电磁波

一些常见的电磁波(资料来源于维基百科,一些网站给的数据不尽相同):

  • 无线电波(radio wave):3 kHz ~ 300 GHz
  • 微波(microwave):300 MHz ~ 300 GHz, 波长介于红外线和无线电波
  • 红外线(infrared):300 GHz ~ 430 THz, 波长介乎微波与可见光
  • 可见光(visible light):430 Thz ~ 790 THz
  • 紫外线(ultraviolet):750 THz ~ 30 PHz
  • X射线(X-ray):30 PHz ~ 30EHz
  • 伽马射线(Gamma ray):30 EHz ~ ?

PS: Hz不同单位从小到大的关系(x1000):Hz -- KHz -- MHz -- GHz -- THz -- PHz -- EHz

上述的电磁波对应的一些应用如下[3]

examples-of-some-uses-of-common-electromagnetic-waves
Table 1: Examples of some uses of common electromagnetic waves

2. 无线电频谱

2.1 无线电频谱表

无线电频谱(radio spectrum)是电磁波谱(Electromagnetic spectrum,包括电磁辐射所有可能的频率)的一部分,其频率从3 Hz到3000 Ghz。

The radio spectrum is the part of the electromagnetic spectrum from 3 Hz to 3000 GHz. Electromagnetic waves in this frequency range, called radio waves, are extremely widely used in modern technology, particularly in telecommunication. To prevent interference between different users, the generation and transmission of radio waves is strictly regulated by national laws, coordinated by an international body, the International Telecommunication Union (ITU).

中国无线电频率划分图

china_radio_frequency_allocations_chart
图2: 中国无线电频率划分图(图片来源于这里

美国无线电频谱表

united_states_frequency_allocations_chart
Fig. 3: United States Frequency Allocations on the Radio Spectrum Chart (image from here)

2.2 Licensed vs. Unlicensed

无线电波在不同的波段具有不同的传播特性[4]

频率越低,传播损耗越小,覆盖距离越远,绕射能力也越强。但是低频段的频率资源紧张,系统容量有限,因此低频段的无线电波主要应用于广播、电视、寻呼等系统。

高频段频率资源丰富,系统容量大。但是频率越高,传播损耗越大,覆盖距离越近,绕射能力越弱。另外,频率越高,技术难度也越大,系统的成本相应提高。

移动通信系统选择所用频段时要综合考虑覆盖效果和容量。UHF频段(微波分米波,300 MHz ~ 3 GHz)与其他频段相比,在覆盖效果和容量之间折衷的比较好,因此被广泛应用于手机等终端的移动通信领域。当然,随着人们对移动通信的需求越来越多,需要的容量越来越大,移动通信系统必然要向高频段发展。

无线电波的速度只随传播介质的电和磁的性质而变化。无线电波在真空中传播的速度,等于光在真空中传播的速度,因为光就是一种电磁波。无线电波在其他介质中传播的速度为Vε=C/sqrt(ε)。其中ε为传播介质的介电常数。空气的介电常数与真空很接近,略大于1,因此无线电波在空气中的传播速度略小于光速,通常我们近似认为就等于光速。

平时新闻看到的工信部向运营商发放*G牌照,实际上就是频谱授权,运营商有了授权频谱(licensed spectrum),就可以在该频段上自主规划,即有了专有权(exclusive rights),这样就避免了干扰。摘抄一段英文描述[6]

Licensed bands means that individual companies pay a licensing fee for the exclusive right to transmit on assigned channels within that band in a given geographic area. Licensing is a way of ensuring that wireless operators do not interfere with each other’s transmissions. Without licensing, interference would garble both transmitters’ signals, preventing decent reception. With licensing, the only place where interference occurs is usually at the outer edge of the license-holder’s assigned coverage area.

2.3 ISM频段

关于ISM频段介绍,摘抄维基百科词条ISM频段

ISM频段(Industrial Scientific Medical Band),各国挪出某一段频段主要开放给工业,科学和医学机构使用。应用这些频段无需许可证或费用,只需要遵守一定的发射功率(一般低于1W,对功率进行管制,使得发射台只有很短的距离,避免相互干扰),并且不要对其它频段造成干扰即可。ISM频段在各国的规定并不统一。如在美国有三个频段902-928 MHz、2400-2484.5 MHz及5725-5850 MHz,而在欧洲900MHz的频段则有部份用于GSM通信。

2.4GHz为各国共同的ISM频段。因此无线局域网(IEEE 802.11b/IEEE 802.11g),蓝牙,ZigBee等无线网络,均可工作在2.4GHz频段上。

ITU-R指定的ISM频段如下(详情见ISM band, Frequency allocation):

itu-r_ism_band
图4:ITU-R指定的ISM频段

ISM频段使用,最常见的是微波炉,其工作在2.45 GHz。

2.3.1 Non-ISM uses

In recent years ISM bands have also been shared with (non-ISM) license-free error-tolerant communications applications such as wireless sensor networks in the 915 MHz and 2.450 GHz bands, as well as wireless LANs and cordless phones in the 915 MHz, 2.450 GHz, and 5.800 GHz bands. Because unlicensed devices are required to be tolerant of ISM emissions in these bands, unlicensed low power users are generally able to operate in these bands without causing problems for ISM users. 

一些例子如下:

  • Bluetooth: 2.45 GHz
  • IEEE 802.11 (e.g., Wi-Fi): 2.45 GHz, 5.8 GHz
  • IEEE 802.15.4 (e.g., Zigbee, 6LoWPAN): 使用868(Europe)/915(North America)/2450(worldwide) MHz中的一个

3. 国内移动通信频谱

国内移动通信频谱分布示意图如下:

%e4%b8%ad%e5%9b%bd%e7%a7%bb%e5%8a%a8%e9%80%9a%e4%bf%a1%e9%a2%91%e8%b0%b1%e5%88%86%e5%b8%83%e7%a4%ba%e6%84%8f%e5%9b%be
图5:国内移动通信频谱分布示意图(图片来源于这里

一些专有名词的解释:

  • WRC-07表示World Radiocommunication Conference 2007(2007年召开的世界无线电通信大会)。
  • DCS表示Digital Cellular System,实际上是GSM-1800。
  • 电信EVDO是指CDMA2000 1x EV-DO(Evolution-Data only),已被国际电联ITU接纳为国际3G标准。
  • 4G网络(LTE, Long-Term Evolution)分为TDD(time-division duplexing)和FDD(frequency division duplexing)。

配合下表“三大运营商2G/3G/4G频率分配和网络制式”一块看,一目了然。

%e4%b8%89%e5%a4%a7%e8%bf%90%e8%90%a5%e5%95%86%e9%a2%91%e7%8e%87%e5%88%86%e9%85%8d%e5%92%8c%e7%bd%91%e7%bb%9c%e5%88%b6%e5%bc%8f
表2:三大运营商2G/3G/4G频率分配和网络制式(图片来源于这里

3.1 4G

2008年3月,国际电信联盟-无线电通信部门(ITU-R)规定了一组用于4G标准的要求,命名为 IMT-Advanced规范,要求静态传输速率峰值达到1 Gbps,用户在高速移动状态下可以达到100 Mbps。IMT-Advanced的4G标准有两个[8]

  • LTE-Advanced:由3GPP所主导制定,是第一批被国际电信联盟承认的4G标准,也是事实上的唯一主流4G标准。峰值速率:下行1Gbps,上行500Mbps。LTE有两种技术:FDD、TDD。
  • Mobile WiMAX Release 2 (也叫WirelessMAN-Advanced): 由IEEE主导制定,IEEE 802.16m是WiMAX(Worldwide Interoperability for Microwave Access)的升级演进,从fixed stations接收可高达1 Gbps,接收下行与上行最高速率可达到100 Mbps。是国际电信联盟承认的4G标准,不过随着Intel于2010年退出,WiMAX技术也已经被运营商放弃,并开始将设备升级为TD-LTE。

International Mobile Telecommunications-Advanced (IMT-Advanced Standard) are requirements issued by the ITU-R of the International Telecommunication Union (ITU) in 2008 for what is marketed as 4G (Or sometimes as 4,5G) mobile phone and Internet access service.

3.2 LTE

LTE指定的频带列表,可以参见维基百科词条LTE frequency bands

china_fdd_td_lte
表3:中国LTE频谱分布(图片来源于这里

下图更直观展示了中国TD-LTE频段分配:

lte_tdd_china
图6:中国TD-LTE频段分配图(图片来源于这里

注:该博文是看了网上不少资料(主要是维基百科)整理的,不一定准确(缘于资料间相互矛盾),如有错误,恳请指出。

参考资料:
[1] Wikipedia: Frequency allocation, Frequency band, International Telecommunication Union, GSM frequency bands
[2] ITU: Frequency allocations
[3] Everything Maths and Science: Electromagnetic spectrum
[4] 百度百科:无线电波
[5] 中国无线频谱资源分配详细图解
[6] TechTarget: What’s the difference between licensed and unlicensed wireless?
[7] UtilityProducts: Licensed vs. Unlicensed Spectrum for Utility Communications
[8] 维基百科词条:4G

发表评论

电子邮件地址不会被公开。 必填项已用*标注