IEEE802.11 Working Group in recent years began to define a new physical layer standard IEEE802.11g. IEEE802.11 protocol standards and compared to the previous, IEEE802.11g draft has the following two characteristics: the 2.4GHz frequency band using orthogonal frequency division multiplexing (OFDM) modulation technology, enabling data transfer rates up to 20Mbit / s or more; to work with the Wi-Fi IEEE802.11b system interoperability, can coexist on the same AP in the network, thereby safeguarding the backward compatibility. Existing WLAN system can be such a smooth transition to high-speed WLAN, IEEE802.11b products, extended service life and reduce the user's investment. July 2003 IEEE802.11g IEEE802.11 Working Group approved a draft of the standard become a new focus of concern.
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key technology DSSS modulation technique OFDM modulation PBCC technology protocol frame structure of OFDM / OFDMCCK / OFDMCCK / PBCC performance analysis of key technology with the increasingly wide range of WLAN technology, users data transmission rate requirements of the increasingly high. But this is more complicated in the indoor electromagnetic environment, multipath effects, frequency selective fading and the presence of other sources of interference makes the wireless channel to achieve high-speed data transmission over cable channel difficulties, so WLAN need to adopt appropriate modulation techniques. IEEE802.11WLAN is to support higher data rates (1 ~ 54Mbit / s), using micro-cellular, cellular structure, a slight,
air force one high, self-management of computer local area network. The key technologies generally have three kinds, direct sequence spread spectrum modulation techniques (DSSS: Direct Sequence Spread Spectrum) and complementary code keying (CCK: Complementary Code Keying) techniques, including binary convolutional (PBCC: Packet Binary Convolutional Code) and orthogonal frequency division multiplexing OFDM: Orthogonal Frequency Division Mustiplexing. Each technology has its characteristics, the current spread spectrum modulation techniques are becoming mainstream, and OFDM transmission technology due to its superior performance become the new focus of concern. DSSS modulation techniques are based on DSSS modulation technique 3. IEEE802.11 standard was originally developed in the 1Mbit / s data rates using differential binary phase shift keying (DBPSK: DifferentialBinary Phase Shift Keying). If you want to provide 2 Mbit / s data rate, can be differential quadrature phase shift keying (DQPSK: Differential Quadrature Phase Shift Keying), this method of dealing with two bits per symbol, a two-bit. The third is based on the CCK's QPSK, is the basic data used IEEE802.11b standard modulation. It uses the complement sequence and the direct sequence spread spectrum technology, a single-carrier modulation technique, through the phase shift keying (PSK) transmission of data, transmission rate is divided into 1,2,5.5 and 11 Mbit / s. CCK by the Rake receiver and the receiver used in conjunction, can transmit data at high efficiency while effectively overcoming the multipath effect. IEEE802.11b CCK modulation through the use of technology to improve data transfer rate up to 11 Mbit / s. But when the transmission rate of more than 11 Mbit / s,
air force one low, CCK order to combat multipath interference, the balance and require more complex modulation, it is very difficult to achieve. Therefore, IEEE802.11 working group in order to promote the development of WLAN, but also introduced a new modulation technique. PBCC modulation PBCC modulation technology by Texas Instruments (TI) proposed by the company has the option as IEEE802.11g be adopted. PBCC is a single carrier modulation, but with CCK different,
nike air force one low, it uses a more complex signal constellation. PBCC uses 8PSK, and CCK with BPSK / QPSK; other PBCC convolutional code used, and CCK using block codes. Therefore, their demodulation process is very different. PBCC to complete a higher rate of data transmission, the transmission rate of 11,22,33 Mbit / s. OFDM technology is multi-carrier OFDM modulation technology (MCM: Multi-CarrierModulation) kind. The main idea is: the channel into many orthogonal subchannels, each subchannel on the narrowband modulation and transmission, thus reducing interference between subchannels. The signal bandwidth of each subchannel is less than the correlation bandwidth of the channel, so each sub-channel frequency selective fading on is flat, substantially eliminating the ISI. Since each sub-channel OFDM system, carrier orthogonal, so their spectrum is overlapping, so as to reduce interference between sub-carriers,
nike air force one, but also improve the spectrum efficiency. In each sub-channel in quadrature modulation and demodulation of this inverse fast Fourier transform can be used (IFFT) and fast Fourier transform (FFT) method to implement, with the large scale integrated circuit technology and DSP technology , IFFT and FFT are very easy to achieve. The introduction of FFT,
nike air force 1, OFDM implementation greatly reduces the complexity, improve system performance. Wireless data services are generally non-existence of symmetry, that is, the downlink data transmission is much greater than the uplink data traffic. Therefore, both high-speed data transmission services from the user's needs, or from wireless communications to consider their own, want to support non-symmetrical high-speed physical layer data transmission, and OFDM is easy by using a different number of subchannels to achieve the different uplink and downlink transfer rate. Because there is frequency selective wireless channel, all the subchannels will decline while the situation in a relatively deep, so the dynamic bit allocation can be dynamic subchannel allocation method and the full use of the high SNR subchannels, thereby enhancing system performance. The impact of narrowband interference only a small part of the sub-carriers, the OFDM system in a way to resist this interference. OFDM technology has very broad prospects for development, has become the core of the fourth generation mobile communication technology. IEEE802.11a / g standard to support high-speed data transmission using OFDM modulation techniques. Currently, OFDM with space-time coding, diversity, interference 〔including inter-symbol interference (ISI) and inter-channel interference (ICI)〕 suppression and smart antenna technology to maximize the reliability of the physical layer. If combined with adaptive modulation, adaptive coding and dynamic sub-carrier allocation, dynamic bit allocation algorithm technology, its performance can be further optimized. Protocol frame structure from the logical structure of the network point of view, IEEE802.11 only defines the physical layer and MAC sub-layer. MAC layer provides competition for shared use of wireless media and non-competition use, a wireless media access, network connections, data validation and security functions. Physical layer data link layer provides the physical connection, to achieve a transparent bit stream transmission, the transmission of data in bits. Defines the physical layer interface hardware communications equipment and machinery, electrical characteristics of functions and processes to establish, maintain and release physical connections. Physical layer consists of three parts: the physical layer management, physical layer convergence protocol (PLCP) and physical medium dependent sublayer (PMD). IEEE802.11g physical frame structure is divided into preamble (Preamble), and load letterhead Header Payload. Preamble mainly used to determine the mobile station and the access point when to send and receive data transmissions when the mobile station to the other so as to avoid conflict, simultaneous transmission and frame sync interval. Preamble to complete, the receiver had just started to receive data. After the Preamble Header? Be used to transmit important data such as load length, transfer rate and service information. Since the data rate and the number of bytes to send different, Payload of the packet size vary widely, can be very short can be quite long. In a signal transmission process, Preamble and Header more share of transmission time, Payload less time with the transmission, the lower the transmission efficiency. Taking these three kinds of modulation characteristics, IEEE802.11g OFDM and other key technologies used to protect their superior performance, respectively, on the Preamble, Header, Payload modulation, frame structure such as OFDM / OFDM mode. In addition, IEEE802.11g draft standard provides for optional and will be options, but also to protect can be compatible with the IEEE802.11b CCK / OFDM and CCK / PBCC modulation optional. Thus, OFDM modulation is mandatory to protect the transmission rate of 54Mbit / s; using CCK modulation for backward compatibility must-protection; CCK / PBCC and CCK / OFDM as an option. Frame structure are compared in IEEE802.11g. OFDM / OFDM Preamble, Header and Payload are modulated using OFDM transmission, the transmission rate of up to 54Mbit / s. OFDM is a good feature is that it has a short Preamble, CCK modulation signal header is 72μs, the OFDM modulation signal header is only 16μs. Header is a signal of the important part of the header occupies less time to improve the signal to send data. Header OFDM allows a shorter more time to transfer data with high transmission efficiency. Therefore, 11Mbit / s transfer rate, CCK modulation is a good choice, but must be used to continue to enhance the rate of OFDM modulation. Its maximum transfer rate of up to 54Mbit / s. IEEE802.11g protocol in OFDM? OFDM mode and Wi-Fi can co-exist, but it need to use RTS / CTS protocol to resolve the conflict. CCK / OFDM modulation which is a hybrid, is the IEEE802.11g's optional. The Header and Preamble transmission with CCK modulation, OFDM technology transfer load. CCK OFDM technology and the technology is the separation, and therefore must be between the Preamble and CCK and OFDM Payload conversion. IEEE802.11g with CCK / OFDM technology to protect and IEEE802.11b coexistence. IEEE802.11b OFDM demodulation format of the data can not, so data transfer conflict will inevitably occur, IEEE802.11g using CCK technology transfer Header and Preamble to make compatible with IEEE802.11b, IEEE802.11g the Header it can receive in order to avoid conflict. This protection IEEE802.11bWi-Fi devices with backward compatibility, but because of Preamble / Header using CCK modulation, increases the cost, transmission speed than the OFDM? OFDM way down. CCK / PBCC CCK / PBCC and CCK / OFDM, like, PBCC is also a hybrid waveform, the load header using CCK modulation using PBCC modulation, so that it can work for high-speed rate is compatible with IEEE802.11b. PBCC modulation maximum data transfer rate is 33Mbit / s, compared with OFDM or CCK / OFDM transmission rate is low. Performance analysis has not yet become a standard IEEE802.11g draft because of their different characteristics, become the focus of attention. IEEE802.11g and IEEE802.11b compatibility, and coexistence with the capacity and frequency of equipment problems OFDM technology itself will become a hot topic. IEEE802.11g IEEE802.11g compatible device compatibility refers to the IEEE802.11g and IEEE802.11b devices to an AP in the same node in the network interconnection. IEEE802.11g is to feature one of the greatest security and IEEE802.11bWi-Fi compatible. IEEE802.11g OFDM and CCK can receive data,
air force 1 high, but the traditional Wi-Fi system can only receive CCK information, which raises the question, namely, the coexistence of the two environments is not how to solve the IEEE802.11b OFDM demodulation format information header brought the conflict. In order to solve these problems, IEEE802.11g using the RTS / CTS technology. Initially, IEEE802.11 introduction of RTS / CTS mechanism is to solve the hidden station problem, that is not detected the sending station to send data to another stand, and thus the case of a collision in the station. IEEE802.11b and IEEE802.11g mixed work situation is very similar with the hidden station problem, IEEE802.11b equipment can not receive the information IEEE802.11g OFDM header format, so you can use RTS / CTS mechanisms. More Atlas Atlas entry Open Category: LAN I have to perfect