802.11g core technology and protocol test report

802.11g core technology and protocol test report

The IEEE802.11 working group has begun to define a new physical layer standard IEEE802.11g in recent years. Compared with the previous IEEE802.11 protocol standard, the IEEE802.11g draft has the following two characteristics: the use of orthogonal frequency division multiplexing (OFDM) modulation technology in the 2.4 GHz band increases the data transmission rate to 20 Mbit / s Above; can be interconnected with the Wi-Fi system of IEEE802.11b, can coexist in the same AP network, thus ensuring backward compatibility. In this way, the original WLAN system can smoothly transition to high-speed WLAN, extending the service life of IEEE802.11b products and reducing the investment of users. In July 2003, the IEEE802.11 working group approved the IEEE802.11g draft, which became the new focus of attention.

The key technologies implemented by IEEE802.11 WLAN With the increasing application of WLAN technology, users have increasingly higher requirements on data transmission rate. However, in the indoor complex electromagnetic environment, the multi-warp effect, frequency selective fading, and the presence of other interference sources make the realization of high-speed data transmission in wireless channels more difficult than wired channels. Therefore, WLAN needs to adopt appropriate modulation techniques.

IEEE802.11 WLAN is a computer local area network that can support a higher data transmission rate (1 ~ 54 Mbit / s), adopts microcell and picocell structure, and is independently managed. There are roughly three key technologies: DSSS (Direct Sequence Spread Spectrum) and Complementary Code Keying (CCK: Complementary Code Keying) technology, PBCC (Packet Binary ConvoluTIonal Code) and positive code OFDM: Orthogonal Frequency Division MusTIplexing. Each technology has its own characteristics. At present, spread spectrum modulation technology is becoming the mainstream, and OFDM technology has become a new focus of attention because of its superior transmission performance.

1. DSSS modulation technology There are three kinds of modulation technology based on DSSS. Initially, the IEEE802.11 standard was formulated to use differential two-phase phase shift keying (DBPSK: DifferenTIal Binary Phase Shift Keying) at a data rate of 1 Mbit / s. If you want to provide a data rate of 2 Mbit / s, you can use differential quadrature phase shift keying (DQPSK: DifferenTIal Quadrature Phase Shift Keying). This method processes two bit symbols at a time and becomes a double bit. The third type is QPSK based on CCK, which is the basic data modulation method adopted by the IEEE802.11b standard. It uses complementary code sequence and direct sequence spread spectrum technology, is a single carrier modulation technology, the data is transmitted by phase shift keying (PSK) mode, the transmission rate is divided into 1, 2, 5.5 and 11 Mbit / s. CCK can be used in conjunction with the Pake receiver at the receiving end to effectively overcome the multipath effect while transmitting data efficiently. IEEE802.11b uses CCK modulation technology to increase the data transmission rate, up to 11 Mbit / s. However, when the transmission rate exceeds 11 Mbit / s, CCK requires more complex equalization and modulation in order to combat multipath interference, which is very difficult to implement. Therefore, in order to promote the development of WLAN, the IEEE802.11 working group has introduced new modulation techniques.

2. PBCC modulation technology PBCC modulation technology was proposed by Texas Instruments (TI) and has been adopted as an option for IEEE802.11g. PBCC is also a single carrier modulation, but unlike CCK, it uses a more complex signal constellation. PBCC uses 8PSK, while CCK uses BPSK / QPSK; In addition, PBCC uses convolutional codes, and CCK uses block codes. Therefore, their demodulation process is very different. PBCC can complete data transmission at a higher rate, with a transmission rate of 11, 22, 33 Mbit / s.

3. OFDM technology OFDM technology is actually a kind of multi-carrier modulation (MCM: Multi-Carrier Modulation). The main idea is to divide the channel into many orthogonal sub-channels and perform narrow-band modulation and transmission on each sub-channel, which reduces the mutual interference between the sub-channels. The signal bandwidth on each sub-channel is smaller than the associated bandwidth of the channel, so the frequency selective fading on each sub-channel is flat, which greatly eliminates inter-symbol interference.

In the OFDM system, the carriers of each sub-channel are orthogonal to each other, so their spectrums overlap each other. This not only reduces the mutual interference between sub-carriers, but also improves the spectrum utilization rate. Such orthogonal modulation and demodulation in each sub-channel can be implemented using inverse fast Fourier transform (IFFT) and fast Fourier transform (FFT) methods, with the large-scale integrated circuit technology and DSP technology Development, IFFT and FFT are very easy to implement. The introduction of FFT greatly reduces the complexity of OFDM implementation and improves the performance of the system.

Wireless data services generally have asymmetry, that is, the amount of data transmitted in the downlink is much larger than the amount of data transmitted in the uplink. Therefore, regardless of the needs of the user's high-speed data transmission service or the wireless communication itself, it is expected that the physical layer supports asymmetric high-speed data transmission, and OFDM can easily achieve different uplink and downlink by using different numbers of sub-channels Transfer rate.

Due to the frequency selectivity of the wireless channel, all sub-channels will not be in a deep fading situation at the same time. Therefore, the sub-channels with high signal-to-noise ratio can be fully utilized through dynamic bit allocation and dynamic sub-channel allocation methods to improve the system performance. Since narrow-band interference can only affect a small part of subcarriers, the OFDM system can resist this interference to some extent.

OFDM technology has very broad development prospects and has become the core technology of the fourth generation mobile communication. IEEE802.11a / g standard adopts OFDM modulation technology in order to support high-speed data transmission. At present, OFDM combines 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, dynamic subcarrier allocation, dynamic bit allocation algorithm and other technologies, its performance can be further optimized.

4. The frame structure and technical details of the IEEE802.11g protocol are viewed from the logical structure of the network. IEEE802.11 only defines the physical layer and the MAC sublayer. The MAC layer provides competitive and non-competitive use of shared wireless media, and has functions such as wireless media access, network connection, data verification, and confidentiality.

The physical layer provides a physical connection for the data link layer to achieve transparent transmission of the bit stream, and the unit of data transmitted is bits. The physical layer defines the mechanical and electrical functions and process characteristics of communication devices and interface hardware to establish, maintain, and release physical connections. The physical layer consists of three parts: the physical layer management layer, the physical layer convergence protocol (PLCP), and the physical medium dependent sublayer (PMD).

The physical frame structure of IEEE802.11g is divided into a preamble, a header, and a payload. Preamble is mainly used to determine when data is sent and received between the mobile station and the access point. When the transmission is in progress, it informs other mobile stations to avoid collisions, and simultaneously transmits synchronization signals and frame intervals. The preamble is completed before the receiver starts to receive data. Header after Preamble? Used to transmit some important data such as load length, transmission rate, service and other information. Due to different data rates and the number of bytes to be transmitted, the packet length of Payload varies greatly, and can be very short or very long.

In the transmission process of a frame signal, the more transmission time occupied by Preamble and Header, the less transmission time used by Payload, the lower the transmission efficiency.
Based on the characteristics of the above three modulation technologies, IEEE802.11g adopts key technologies such as OFDM to ensure its superior performance, and respectively modulates Preamble, Header, and Payload. This frame structure is called OFDM / OFDM mode.

In addition, the IEEE802.11g draft standard specifies optional and mandatory options. In order to ensure compatibility with IEEE802.11b, optional modulation methods of CCK / OFDM and CCK / PBCC can also be used. Therefore, OFDM modulation is a mandatory option to ensure a transmission rate of 54 Mbit / s; CCK modulation is used as a mandatory option to ensure backward compatibility; CCK / PBCC and CCK / OFDM are optional.

Table 1 compares the frame structure of IEEE802.11g.


(1) OFDM / OFDM

Preamble, Header and Payload all use OFDM for modulation transmission, and the transmission rate can reach 54 Mbit / s. A good feature of OFDM is that it has a short Preamble. The frame header of the CCK modulated signal is 72 μs, while the frame header of the OFDM modulated signal is only 16 μs. The frame header is an important part of a signal. The reduction of the frame header occupation time improves the signal's ability to transmit data. OFDM allows shorter headers to give more time for data transmission, with higher transmission efficiency. Therefore, for the transmission rate of 11 Mbit / s, CCK modulation is a good choice, but to continue to increase the rate must use OFDM modulation technology. Its highest transmission rate can reach 54Mbit / s. OFDM in the IEEE802.11g protocol can also coexist with Wi-Fi, but it needs to use the RTS / CTS protocol to resolve the conflict.

(2) CCK / OFDM

It is a hybrid modulation method and is an option for IEEE802.11g. The Header and Preamble are transmitted using CCK modulation, and the OFDM technology transmits the load. Because OFDM technology and CCK technology are separated, there must be CCK and OFDM conversion between Preamble and Payload.

IEEE802.11g uses CCK / OFDM technology to ensure coexistence with IEEE802.11b. IEEE802.11b cannot demodulate data in OFDM format, so data transmission conflicts will inevitably occur. IEEE802.11g uses CCK technology to transmit Header and Preamble to make IEEE802.11b compatible so that it can receive IEEE802.11g headers to avoid conflicts. This ensures backward compatibility with IEEE802.11b Wi-Fi equipment, but because Preamble / Header uses CCK modulation, which increases overhead, the transmission rate is lower than that of OFDM? OFDM.

(3) CCK / PBCC

CCK / PBCC is the same as CCK / OFDM. PBCC is also a mixed waveform. The packet header uses CCK modulation and the load uses PBCC modulation, so that it can work at a high rate and is compatible with IEEE802.11b. The highest data transmission rate of PBCC modulation technology is 33 Mbit / s, which is lower than the transmission rate of OFDM or CCK / OFDM.

IEEE802.11g performance analysis has not formally become the standard IEEE802.11g draft due to its different characteristics, has become the focus of attention. The compatibility of IEEE802.11g and IEEE802.11b, the coexistence ability with the same frequency equipment and the problems of OFDM technology will become research hotspots.

1. IEEE802.11g compatibility IEEE802.11g compatibility refers to IEEE802.11g devices and IEEE802.11b devices can be interconnected in the same AP node network. One of the biggest features of IEEE802.11g is to ensure compatibility with IEEE802.11b Wi-Fi systems. IEEE802.11g can receive OFDM and CCK data, but the traditional Wi-Fi system can only receive CCK information, which creates a problem, that is, how to solve the OFDM format information cannot be demodulated due to IEEE802.11b in an environment where the two coexist The collision problem caused by the frame header. In order to solve the above problems, IEEE802.11g uses RTS / CTS technology.

Initially, IEEE802.11 introduced the RTS / CTS mechanism to solve the problem of concealed stations, that is, the sending station could not detect another station sending data, and thus a collision occurred at the receiving station.

The situation of the mixed work of IEEE802.11b and IEEE802.11g is very similar to the problem of hidden stations. IEEE802.11b devices cannot receive the IEEE802.11g information frame header in OFDM format, so the RTS / CTS mechanism can be used to solve it. In an environment where IEEE802.11g and IEEE802.11b work together (ie, there is both IEEE802.11g in the same AP service area

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