Wireless Local Area Network Adaptive Modulation and Coding Technology Research SUN Zeng-you, SUN Xiao-fan
(Northeast Dianli University,School of Information Engineering,Jilin 132012,China)
Abstract：Expound the existing modulation mode of the present wireless local area network. Directing at the problem that when single modulation method conveys data with high speed in the frequency selective fading channel, the system throughput will be declined and the efficiency of the spectrum will be lowered . Put forward the scheme that the OFDM sub-carrier will use different modulation modes after dynamic groups by amplitude frequency response. Simulation results show that the scheme will lower the complexity of the system achievement, and can get a higher spectrum efficiency.
Key words: WLAN; 802.11a; adaptive modulation; throughput;OFDM
中图分类号：TP393 文献标识码：A 文章编号：1009-3044(2012)28-6679-05
1 Introduction
The Wireless Local Area Network (WLAN) is a kind of Local Area Network that through the 2.4 GHz or 5.8 GHz exemption application ISM (Industry Science Medicine) frequency band transmission signal. IEEE 802.11 is the standard of wireless local area network that was released in 1998 by IEEE, that is also the first international recognition agreement in the field of WLAN. IEEE 802.11 is aimed at the physical layer (PHY layer) and the media access control sublayer (MAC layer) of the wireless network to the rules. Then it rolled out several new standards continuously such as 802.11 a, 802.11 b, 802.11 g and so on. The first launched IEEE802.11a occupies 5 GHz free frequency band, works at the orthogonal frequency division multiplexing (OFDM) mode among 5.15 ~ 5.25, 5.25 ~ 5.35, 5.725 ~ 5.825 GHz frequency band, and uses 52 subcarriers modulated by BPSK, QPSK or 16 - QAM or 64 - QAM. IEEE802.11a can provide the data communication ability of rate for 6, 9, 12, 18, 24, and 36, 48 and 54 MBPS, which must support for the 6, 12 and 24 MBPS data rate.
IEEE802.11 x supports for multiple transmission speeds in the physical layer, but different types of frame (data frame, management frame, control frame) regulation has different rate in the MAC layer, but do not provide a certain type of frame, especially data frames. In order to make better use of multi-rate ability of the physical layer, media access control (MAC) protocol should be able to choose the best transmission rate adaptively to adapt to the channel change and achieve the best system throughput. The MAC protocol of having the characteristics is called rate adaptive MAC protocol, multi-rate MAC protocol or automatic rate MAC protocol. Throughput is an important parameter of measuring the performance of communication system, hoping the data transmission with high rate as far as possible and error rate as low as possible in various channel environment. Usually choosing high rate transmission modulation method can obtain higher throughput when the link quality is good, but when the link quality is poorer, the high rate transmission modulation method will lower throughput instead because of the increase of the bit error rate. In fact, throughput is the function of transmission rate and bit error rate, and high transmission rate will cause higher error rate, reduce throughput. How to select and switch suitable transmission speed according to the channel condition in order to improve system performance, adaptive selection modulation mode on the basis of the channel quality becomes a current research hot spot [1]. At present the existing modulation modes in wireless local area network are BPSK, QPSK, 16 - QAM or 64 – QAM which are all based on OFDM. But any kind of single modulation mode mainly exists the problem of system throughput loss and low utilization ratio spectrum with the data transmission in the frequency selective fading channels. 2 The OFDM modulation technology of wireless local area network
The commonly used modulation mode in wireless local area network are MPSK (BPSK, QPSK), MQAM (16 - QAM, 64 - QAM), etc.
2.1 MPSK modulation[]
MPSK modulation mode is to show different sending information by using different phases of carriers. For binary phase shift keying (M = 2), its two carrier phases are [θ0=0], [θ1=π] 。For M system phase shift keying, [M=2k]( Here K is the information bit number that each sending symbol contains), the carrier phase’s range is [0<θ<2π],so MPSK modulation signal’s phase is [θM=2πm/M], m=0, 1, .... M-1。
MPSK’s signal waveform can be expressed as:
[um(t)=AgT(t)cos(2πfct+2πmM)] [m]=0 ,1 , …M-1 (1)
[gT(t)] is the filter pulse of the launch , deciding the sending signal’s frequency spectrum characteristics; [A] is the signal amplitude.
2.2 MQAM modulation
MQAM signal is using two orthogonal carriers [cos2πft] and [sin2πft], and use the two carriers to modulate two independent information bit sequences respectively. MQAM signal’s general expression is:
[um(t)=AmcgT(t)cos2πfct+AmsgT(t)sin2πfct] [m]=0 ,1 , …M-1 (2)
[gT(t)] is the filter pulse of the launch , deciding the sending signal’s frequency spectrum characteristics. [Amc]and[Ams]are level sets, which have been got by mapping the bit sequence for signal amplitude[2].
3 Wireless local area network adaptive modulation technology based on the OFDM sub-carrier group amplitude frequency characteristics
3.1 Bit error rate of OFDM system
In order to facilitate the instructions, simplified research environment, only aim at the ideal environment such as elaborating the channel in Gaussian white noise environment, choose BPSK, QPSK, 16 - QAM and 64 - QAM as multi-rate modulation mode and suppose that not use error correction coding. Bit error rate can be expressed as the following for BPSK modulation in Gaussian white noise channel environment.
[Pb=Q(2yb)] (3)
For QPSK modulation, Bit error rate can be approximated for:
[Pb≈Q(2yb)] (4)
For MQAM modulation, Bit error rate can be approximated for:
[Pb][≈][4log2M]（1-[1M]）Q([3yblog2MM-1]) (5)
[yb] is the signal bit signal-to-noise ratio at the receiving end in the equation. The relationship of [yb] and signal signal-to-noise ratio SNR can be expressed as: [yb=SNR/log2M] (6)
Q(.) is gaussian Q function, expressed as:
In Gaussian white noise channel environment , the performance curve between bit error rate BER and signal-to-noise ratio SNR can be got by the calculation of the above expressions in the various modulation modes. As shown in the figure 1,as you can see, at the same bit error rate performance, the higher rate of modulation mode needs higher signal-to-noise ratio. In the same SNR conditions, the higher speed modulation method has higher bit error ratio[3].
In the general digital communication systems, bit error rate performance is usually required to achieve for[10-5], so the signal-to-noise ratio (SNR) that the various mentioned modulation modes need is about:
3.2 OFDM system’s throughput
Assuming that there are only two sites of the network, the system throughput is restricted by the correct received probability (1 - BER) of the data bit. Assuming that the sender
sends a packet continuously,there is no hidden terminal problem, the rate of code element is 1 MBPS, and do not use the packet check, the throughput’s experience formula can be got:
[Throughtput=(1-BER)n?rate] (9)
In the equation, the throughput’s unit is bit/s, rate is the rate of code element, n is the length of data frame and the unit of it is bit. Figure 2 shows the curve of system theory throughput (received correct bag per second) and SNR for additive gaussian white noise environment. The packet length is 1500 byte, symbol rate is 1 M/s, and the bit rates are 1 Mbps, 2 Mbps, 4 Mbps, 6 Mbps respectively. The following conclusions can be got from the photos: each kind of modulation methods has its most effective SNR range respectively. For example, when SNR is 22 ~ 35 db, using 64 - QAM will get higher throughput, but when SNR is 15 ~ 22 db, using 16 QAM - will get higher throughput. In the channel environment that the SNR range changes greatly, in order to get higher throughput, it is needed to be able to switch in the variety of modulation modes, which is, in the variety of different rates[4].
3.3 OFDM subcarrier group division
Bit error is mostly caused by some seriously declined subcarriers, but most other carriers transmit little error bits in the OFDM systems based on IEEE802.11. The research is a kind of OFDM adaptive modulation scheme which is based on the IEEE802.11 a in the limited power environment. Specificly it is to show that the fading seriously subcarriers adopt the low spectral utilization modulation methods (such as BPSK) for transmission, even do not transmit information in order to ensure the transmission quality. But it should adopt multi-system modulation modes such as 16 - QAM, 64 - QAM to the subcarriers of high instantaneous signal noise ratio (SNR) in order to improve information transmission rate and spectrum utilization. In order to reduce the system’s complexit, it does not select modulation mode for each sub-carrier, but it will divide all the subcarriers (N) into several groups (n) according to the amplitude frequency response ,and each group’s subcarriers use the same modulation mode in this OFDM adaptive modulation scheme. The group’s partition can be decided according to the specific channel’s amplitude frequency response. In the simulation, we will use the modulation methods of BPSK ,4-QAM, 16-QAM and 64-QAM. 4 System simulation
Make the following hypothesis in the simulation process:
(1) Transmitting terminal and receiving terminal are ideal synchronizations and there is ideal channel estimation；
(2) The study is selective frequency fading channel, that is signal transmission bandwidth is greater than the channel correlation bandwidth, signal period is less than the channel coherence time；
(3) Channel fading is slow fading；
(4) CP duration is greater than the channel’s maximum delay spread[τmax][5];
In the IEEE802.11a’s OFDM adaptive modulation simulation model, subcarriers’ number N=512, IFFT(FFT) is 512 point. Carriers’ frequency fc=1.9GHz,channel’s bandwidth B=2.5MHz,sample’s rate Rs=2.5Msample/s.Protected interval GI=T/8=NTs/8=25.6μs＞＞[τmax].Doppler parameters are: V=50km/h=13.42m/s,the maximum doppler frequency shift fm=Vfc/C=13.42×1.9×109/3×108≈85Hz. Conventional normalization doppler frequency fm=fmTs=3.399×10[e-5],OFDM symbol normalized doppler frequency Fm=fmNTs=0.0174.The fm’s modulation mode is using BPSK,4-QAM,16-QAM,64-QAM. The error rate BER and spectrum utilization BPS’s simulation results can be got as shown in figure 3, figure 4. Spectrum utilization BPS refers to the average bit rate transmission in each subcarrier unit bandwidth[6].
It can be seen from the diagram 3’s BER curve that the greater the NS, the smaller the average bit error rate (All are less than the threshold value Pe=10[e-3]). When the signal noise ratio (SNR) is bigger, NS = 32 will get about 1 db gain more than NS = 16, and about 3 ~ 4 db gain more than NS = 8. When the signal noise ratio is less than 20 db, adaptive system mostly uses lower band utilization modulation mode such as BPSK, 4 - QAM, in order to achieve transmission reliability requirements. When the signal noise ratio is greater than 20 db, the modulation mode of high frequency utilization ratio such as 16-QAM , 64-QAM is dominant.The system has lower bit error rate, because at this time there is no higher order number modulation method that can be used. The biggest frequency spectrum utilization can be for [log2M=6bps/Hz]. Then we get the simulation results of spectrum utilization efficiency at three types of groups NS = 8,16,32. As shown in the figure 4, average bit error rate threshold is Pe=10[e-3]. It can be seen in the case of meeting the same bit error rate threshold, the greater the number of NS, the less number of subcarriers each group contains, the higher spectrum utilization rate could be got, because there are more number of subcarriers getting full used right now [7]. Of course, transforming each subcarrier’s modulation method adaptively according to the channel condition can achieve higher spectrum utilization. The corresponding curve in figure 4 is NS = 512, namely each group has one carrier. But it will be very complex for the realization of the actual system, so we adopt the way of group division in order to reduce the complexity of system as well as obtain the good performance. As shown in figure 4, the spectrum utilization rate when NS = 32 close to when NS = 512, about less than 1 db performance loss [8]. 5 Conclusion
The results can show that the scheme of OFDM subcarrier dynamic grouping to change modulation mode according to the amplitude frequency response reduce the complexity of system implementation, and at the same time it can obtain higher spectrum efficiency. The selection of group number has larger influence on system performance. The bigger the number of division, the more spectrum utilization rate, but the realization of the system will be more complex, so the above-mentioned factors must be considered in the actual operation.
References:
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