The increased demand for unallocated idle spectrum resources will inevitably lead to the development of wireless communication systems operating at higher frequency terahertz (THz) bands. The instantaneous transmission of big data will use a higher carrier frequency to meet the high transmission rate requirements. A large number of studies have shown that the application of THz technology in communication field has more advantages than today's more mature microwave communication and optical fiber communication, for example, high transmission rate, good directionality, high security, small scattering, and Good penetration and so on.
The previous article "Detailed IEEE 802.11ad (60 GHz Wi-Fi) Technology" briefly introduced the PHY layer, MAC layer, and protocol adaptation layer of IEEE 802.11ad (60 GHz Wi-Fi). In this paper, the terahertz (THz) communication band in which the PHY layer of IEEE 802.11ad is located and the MAC layer working principle of IEEE 802.11ad will be described in detail.
1. The demand and possible applications of high-speed wireless communicationThe transmission rate increases as the carrier frequency increases. Generally, in the amplitude shift keying ASK modulation mode, the transmission rate is 10% to 20% of the carrier frequency. If the transmission rate reaches 10 Gbit/s to 100 Gbit/s, a carrier frequency of 100 to 500 GHz is required.
Currently, the bit rate of uncompressed HDTV data transmitted to TV devices via DVD or camcorder has exceeded 1.5 Gbit/s. Some manufacturers of consumer electronics have also introduced Gbit wireless interfaces into their latest products. Future wireless technologies require transmission rates above 10 Gbit/s. There is also a standard for high-speed wireless links that enables close transmission of huge amounts of data through mobile terminals and storage devices. It contains several technologies, namely, flash transmission support transfer jet technology and GigaIR (1 Gbit/s) using infrared transmission technology.
There is another type of application scenario in which the amount of data in terabits, such as secure digital SD memory and solid state hard disk memory, is quickly and centrally processed by solid memory media. This type of application, along with advances in memory internal access speed improvements, enables people to utilize high-speed wireless links to enable the instantaneous transfer of large amounts of data between personal mobile terminals and personal computers and between personal mobile terminals and cloud servers.
It can be seen from the above development trend that high-speed THz wireless communication technology can eliminate bottlenecks of network access speed, such as wireless broadband access in optical networks, wireless expansion of high-speed wired LANs, wireless bridging of low-speed wireless LANs and high-speed optical networks, and broadband indoors. Picocell networks, etc.
2. Characteristics and scope of THz communicationThe photon energy of the THz electromagnetic wave is about one-fortieth of the photon energy of visible light, so the use of the THz wave as the information carrier is much more efficient than the use of visible or near-infrared light. Compared with microwave technology, THz waves can detect smaller targets and achieve more accurate positioning, with higher resolution and greater confidentiality. Compared with infrared and laser technology, THz waves have sand penetration. The ability of smoke can achieve all-weather work, so THz technology is expected to play a huge role in military equipment and national security.
THz communication has many features, including:
(1) Atmospheric opacity
Since the water vapor in the atmosphere strongly absorbs the THz wave, THz exhibits opacity to the atmosphere, which makes THz communication unsuitable for terrestrial telecommunications. However, in areas other than a certain distance between the two sides of the communication, the THz communication signal has a high attenuation, making it difficult to detect the communication signal, so it is suitable for ground short-range secure communication.
(2) Large bandwidth and safer.
The THz band is between 0.3 THz and 10 THz, so it can be divided into more communication bands. Moreover, the THz band is approximately 1000 times the total bandwidth of long-wave, medium-wave, short-wave, and microwave (30 GHz). This determines that THz communication is a high-bandwidth communication, which can meet the current increasing demand for communication rates. If the communication frequency band used by both communicating parties is not known in advance, the probability of correctly capturing the frequency band used by both communicating parties in such a high bandwidth is very small, so THz communication is also safe.
(3) The antenna is small and the directivity is good.
Since the frequency of the THz wave is higher than that of the microwave and the wavelength is short, an antenna with a strong directivity and a small size can be made, which can greatly reduce the transmission power and reduce the mutual interference.
(4) THz scattering is small, and it has high permeability to aerosols and clouds.
This is mainly due to the small wavelength of the THz wave. The International Telecommunications Union has designated the 200 GHz band for next-generation inter-satellite communications. Further development must enter the range above 300 GHz. THz communication is suitable for inter-satellite interplanetary communication and stratospheric air-to-air communication. The so-called stratosphere refers to the atmosphere from the ground 10 km to 50 km. The air in the stratosphere is horizontally moved, the airflow is stable, the visibility is good, and it is suitable for surveillance and reconnaissance equipment flight.
From the characteristics of the THz wave introduced above, the applicable fields of THz communication can be: 1 interstellar communication; 2 air-to-air communication in the stratosphere; 3 short-range terrestrial wireless local area network; 4 short-range, safe atmospheric communication.
3. Ultra-high speed wireless LAN technology based on THzThere are two main standardization mechanisms for ultra-high-speed wireless network access that support data rates above 1 Gbit/s, which are specified in the IEEE 802.15.3C standard and the IEEE 802.11ad (60 GHz Wi-Fi) draft standard. The wireless access mechanism adopts a hybrid mechanism of TDMA+CSMA/CA, but each has its own advantages: the former has better compatibility with the access mechanism and can achieve higher data rates, but the principle is more complicated; the latter access mechanism The implementation is relatively simple, and the data rate of the Gbit/s level can also be achieved. The following describes the working principle of the IEEE 802.11ad MAC layer.
4, IEEE 802.11ad networking1) Composition of PBSS (Personal Basic Service Set)
The wireless network specified in the IEEE 802.11ad draft standard operates in the form of PBSS. PBSS is a wireless local area ad hoc network that allows network nodes (also referred to as STAs in the draft standard) to communicate with each other. The composition of PBSS is shown in Figure 1.
Figure 1 Composition of IEEE 802.11ad PBSS
One PBSS consists of nodes that can communicate with each other, one of which acts as the PCP (PBSS central node) of the PBSS, and the PCP provides basic timing mechanism and channel access control for the PBSS by broadcasting beacon frames. information. Two-way data transmission between any two nodes in the PBSS network.
2) PBSS networking process
When a STA acting as a PCP character starts broadcasting a millimeter wave beacon frame, it is considered that a PBSS has begun to form.
First, the STA acting as a PCP role should scan all channels to detect channel information and select a suitable channel to start PBSS. When receiving the MLME START. Request primiTIve from the SME (Site Management Entity), the PCP shall listen to the channel for at least aMinchannelScan for a long time to assess whether the channel is suitable for starting a PBSS. If the channel is suitable, the PCP should broadcast a millimeter wave beacon frame every other beacon interval (BI) to start a PBSS. Otherwise, the PCP shall reply the SME with an MLME START. confirm; and the ResuleCode field of the MLME START. confirm shall be set to INVALID_PARAMETERS to indicate that the PBSS failure has begun.
In addition, when the PCP receives the MLME STOP. Request from the SME, it indicates that the PCP should stop running the PBSS. At the same time, the PCP should stop broadcasting the millimeter wave beacon frame and use the de-authentication process to broadcast the de-authentication frame to the associated STAs in the PBSS. After the STAs receives the decryption frame, they will know that the PBSS will stop running; they will no longer communicate with each other. In addition, the PCP of one PBSS suddenly leaves and no other STAs can bear the PCP of the PBSS, which also causes the operation of the PBSS to stop.
5. Wireless access mechanism of IEEE 802.11ad1) Beacon interval of IEEE 802.11ad
In IEEE 802.11ad PBSS, its channel time is divided into a series of beacon intervals. BI is further divided into beacon frame period (BT), associated beamforming positioning period, and data transmission period. The structure of IEEE 802.11ad PBSS BI is shown in Figure 2.
Figure 2 BI of IEEE 802.11ad PBSS
In Figure 2, BT is used to transmit one or more beacon frames. The A-BFT is used to perform beamforming positioning by the PCP, and whether it exists in the BI is determined by the PCP. The AT (declaration time) is a request response time based on the management access period, and is used for exchanging request response frames between the PCP and the STAs, and is also determined by the PCP whether it exists in the BI. The DTT consists of several CBPs and several SPs, which are channel access periods and are used for data exchange between STAs.
2) Channel access mechanism of IEEE 802.11ad.
In IEEE 802.11ad PBSS, the basic channel access mechanisms of STAs are CSMA/CA and TDMA. STAs use different channel access mechanisms during different periods of BI; CSMA/CA is used during CBP and TDMA is used during SP period. Prior to these periods, the CP will dynamically assign SPs to STAs using polling. The specific channel access process is as follows.
(1) BT broadcast beacon frame, which does not require a CSMA/CA contention channel or is allocated a time slot by the PCP.
(2) The A-BFT is only a beamforming positioning time and does not necessarily appear in the BI. It is also not necessary to use the CSMA/CA contention channel or allocate time slots to it by the PCP.
(3) AT is the exchange period between the request frame and the response frame between the PCP and the STAs; during this period, after receiving the request frame of the STAs, the PCP should wait for one SIFS and then reply the response frame to the STAs. In addition, the AT period should be terminated at the beginning of the first SP in BI and at the beginning of the first CBP in BI.
(4) The basic channel access mechanism during the CBP period is DCF, namely CSMA/CA and backoff mechanism. First, when the STA detects that the channel idle time is up to one DIFS or EIFS, the backoff process begins. At the beginning of the backoff process, the STA first sets a random backoff counter time according to Back of TIme=Random()&TImes;a Slot TIme(3μs). In addition, the NAV (Network Assignment Vector) is also reduced as a counter at a uniform rate. When the NAV counter is reduced to 0, it indicates that the channel is idle and the backoff counter is reduced by one a Slot Time (3 μs); otherwise the backoff counter hangs. When the backoff counter is reduced to 0, it indicates that the channel is idle, and the STA can transmit data; otherwise, the channel is busy, and the STA cannot transmit data.
(5) The basic channel access mechanism during the SP period is TDMA. First, the PCP accesses all STAs in the PBSS by polling to collect relevant information in the network. Then, the PCP dynamically allocates SPs to the STAs according to the polling result; and then broadcasts the allocated SPs information, such as the duration, start and end time of each SP, to the STAs in the PBSS in the millimeter wave signaling frame. After receiving the signaling frame, the STAs will know the allocation of SPs. At this point, STAs can perform data transmission during the SPs period assigned to them.
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