Impact of UWB interference on IEEE 802.11a WLAN System Santosh Reddy Mallipeddy and Rakhesh Singh Kshetrimayum Dept. of Electronics and Communication Engineering, Indian Institute of Technology, Guwahati, Assam, India, 781039. Email: santosh.m, krs@iitg.ernet.in Abstract The objective of this paper is to assess the effects of multiple UWB devices emission on the IEEE 802.11a WLAN receiver. The data rates and SINR of WLAN is evaluated with and without the UWB interference. Approximate path loss model is used to calculate the UWB signal power that interferes with the WLAN system. The three dimensional hemispherical distribution of UWB tramsmitters around the victim WLAN receiver is considered. Based on this model, the analytical analysis to assess the effect of the multiple UWB devices emission on the IEEE 802.11a system is presented. I. INTRODUCTION A. Ultra Wideband Technology Ultra Wideband (UWB) technology, due to its large bandwidth, is capable of supporting high data rate applications. The Federal Communications Commission (FCC) agreed in February 2002 [1] to allocate 7.5 GHz of spectrum for unlicensed use of UWB devices for communication applications in the 3.1 GHz to 10.6 GHz frequency band, the move represented a victory in a long hard-fought battle that dated back decades. With its origins in the 1960s, when it was called time-domain electromagnetics, UWB came to be known for the operation of sending and receiving extremely short burst of Radio Frequency (RF) energy. The UWB communication devices are a very good choice for indoor high data rate wireless applications. Potentially a large number of UWB devices may operate in close proximity of an indoor wireless application such as Wireless Local Area Network (WLAN). The UWB signal is a very low power signal and therefore the power of any single UWB device can be compared to that of a noise floor. But when a number of such devices operate simultaneously then the interference level could rise significantly above the level of the noise floor. B. IEEE 802.11a Wireless LAN The IEEE 802.11a standard for high speed physical layer operates in the 5 GHz Unlicensed National Information Infrastructure (UNII) band. The other standards of the IEEE 802.11 family operate at 2.4 GHz along with systems such as bluetooth, microwave systems and cordless telephones. The IEEE 802.11a standard inherently avoids interference with these systems. The data rate depends upon the modulation technique as well as the encoding scheme. Depending on the combination of modulation and encoding, data rates of 6 to 54 Mbps are possible. IEEE 802.11a WLAN uses Orthogonal Frequency Division Multiplexing (OFDM) at the physical layer for transmitting data. In OFDM data is transmitted by dividing the bit stream into parallel bit streams and then modulating each bit stream onto a sub-carrier. These orthogonal subcarriers are then modulated on a single carrier for transmission. The transmission of several symbols in parallel increases the effective symbol time leading to reduction in Inter Symbol Interference (ISI) and a potential increase in data rate. IEEE 802.11a WLAN uses eight channels of 20 MHz each in the lower 5 GHz band and each channel carries data over parallel channels in the form of sub-carriers. Each channel is divided in 52 sub-carriers of which 4 are pilot channels and 48 are data channels. Some of the researchers have investigated coexistence issues between UWB and other communication systems [2]-[4]. UWB regulations in Japan, Europe, 978 1 4244 6385 5/10/$26.00 2010 IEEE
Fig. 1. 3-D hemispherical distribution of UWB devices around the victim NB system (top view). Korea and China [2] has introduced Detect and Avoid (DAA) Schemes, in which UWB systems will first check whether there are NB systems existing in the environment and if it exists, NB avoidance techniques can be applied [5]. II. INTERFERENCE MODEL Here we develop the model which mathematically describes the interference of multiple UWB devices on the existing narrow band (NB) system in particular IEEE 802.11a WLAN systems. We will use 3-D distribution of UWB transmitters around a victim NB systems unlike [6], in which a 2- D distribution is assumed. This model should also form the basics for further analysis of the interference of cumulative UWB devices on other NB systems as well. Note that in placing many UWB systems in a room, we assume that there are no interference among themselves by applying a proper Time Hopping (TH) code in TH-UWB systems or Pseudo Random (PR) sequence in direct sequence UWB (DS-UWB) systems. It is assumed that UWB devices are transmitted at its full Effective Isotropic Radiation Power (EIRP) i.e. -41.3dBm/MHz in the frequency band of IEEE 802.11a WLAN and for brevity, the issue of multipath propagation is neglected in our analysis. A. Interference by single UWB transmitter We consider a single UWB interferer for each IEEE 802.11a WLAN down link channel first. We assume the quasi free space path-loss model, with the general assumption that UWB devices are short range devices. In this case we are dealing with only 20 MHz of the total UWB spectrum. To account for UWB interference, an extra source of interference is added linearly to the IEEE 802.11a noise floor. The interference power is calculated by assuming an UWB interfering source at different distance from the WLAN transmitter. Therefore, the UWB interference power (P(r)) for NB device placed at a distance r (considering free-space propagation loss before the break point distance d BP, and a fourth power loss law after d BP ) can be approximated as [6]-[7]: ( ) λ 2 ( ) 2 dbp P(r)=P UWB (1) 4πr d BP + r where P UWB is the UWB EIRP in dbm, λ is the wavelength of the signal, calculated using frequency obtained from the geometric mean of the highest and lowest frequencies of band and r is the distance between the victim and the transmitter. This a simplified model of UWB path loss model used for our analysis. We have also assumed that all interfering UWB device transmit equal power P UWB. The break point distance can be found approximately from [7] d BP = 12h T h R λ (2) where h T and h R are the heights above ground of the transmitter and the receiver respectively. Typically, we could assume h T = h R = 1.5m. The breakpoint distance is also a function of the propagation environment and can be set lower. The values of P UWB with which we carry our further investigation are obtained from the respective spectral masks released or proposed by FCC for indoor systems. Since, we are investigating the indoor environment, d BP is set to be 1m which is considered typical for indoor channels [8], [9]. B. Interference by multiple UWB transmitters Here our victim, which is physically located in an 3-D room as the receiver is located at the center of two concentric hemispheres of radius r min and r
Signal to Interference Noise Ratio in db UWB Device Density /cubic meter Fig. 2. Cumulative UWB interference at an IEEE 802.11a victim receiver with d 0 =1m. Fig. 3. SINR as a function of radius, for IEEE 802.11a WLAN for hemispherical distribution of UWB transmitters as depicted in Fig. 1. Here r min is the minimum radius of the hemisphere within which there are no UWB transmitters. Also all the UWB transmitters are in between the hemispheres considered. Then the cumulative distribution function (CDF) of the UWB devices between the two hemisphere and the corresponding probability density function can be obtained. From the CDF, we can write the probability density function (pdf) of the UWB transmitters as a function of the radius r as: { pd f (r)= 0, r < r min and r > r, r min < r < r (3) 3r 2 r 3 r 3 min The mean interference level is obtained by summing up the mean received power from all the interfering UWB transmitters, i.e, given by the following equation: r 3r 2 E {P} = P R = N P(r) r r=r 3 r 3 dr (4) min min where P(.) defines received signal power of one UWB transmitter at the victim receiver as a function N of distance and ρ = 2 is the density of 3 π(r3 rmin) 3 UWB transmitters per unit volume, N is the total number of UWB transmitters. The received power as a function of distance is considered from equation 1. Hence the above equation can be simplified as: ( ) λ 2 r P R (r)=2πρ P UWB 4π r=r min d 2 0 (d 0 + r) 2 dr (5) The total interference power is obtained from equation 1, equation 3 and equation 5 as follows: ( ) λ 2 ( d 2 ) I UWB = P R (r) r = 2πρP 0 UWB 4π r min + d 0 (6) where r tending to infinity is to accommodate all the UWB interferers starting from r min. III. IEEE 802.11A WLAN INTERFERENCE MODEL In order to compute and analyze the aggregate impact of multiple UWB devices on WLAN systems, we will use the model which we developed in previous section and we will investigate the effect of UWB interference on IEEE 802.11a WLAN in terms of its data rate and SINR. In our further step to assess the effect of UWB interference, we will first compute the SINR of the system. SINR is defined as the signal to Interference ratio and is
Data Rate (Mbps) Minimum SNR 6 5 9 8 12 10 18 13 24 16 36 19 48 22 54 25 TABLE I IEEE 802.11A MINIMUM SNR AND DATA RATE [5]. given as Useful Received Power SINR = (7) Inter f erence + Noise Power Since, in order to send data at a particular data rate we need to achieve the minimum SNR level. The IEEE 802.11a standard requires receivers to have a minimum sensitivity ranging from 82 dbm to 65 dbm, depending on the chosen and/or required data rate [10]. Table 1 provides the information of achievable data rates based on the minimum SNR requirements. Now, considering the hemispherical distribution, the variation in the UWB interference power on the NB victim receiver system with the distance is shown in figure 2 for various minimum separation distance between the victim receiver and the nearest UWB interferer. It implies that as r min increases i.e, the distance between the WLAN receiver and the nearest UWB transmitter, the interfering power from UWB transmitters decreases. IV. EFFECT OF UWB INTERFERENCE ON 802.11A WLAN AND SIMULATION RESULTS Considering the above described three dimensional cumulative UWB interference model, the effect of UWB interference on the SINR of the IEEE 802.11a WLAN system is depicted in figure 3. It can be observed that the SINR of the victim IEEE 802.11a WLAN system increases with the increase of the minimum distance between the WLAN receiver and the UWB transmitters. It is highest for Fig. 4. Data Rates as a function of radius of IEEE 802.11a WLAN Interference model considered. the case when there are no UWB transmitters in the vicinity of NB victim receiver in accordance with our intuition. As the distance r increases, SINR decreases for all the three cases plotted in figure 3. The effect of the UWB interference on the IEEE 802.11a WLAN is investigated in terms of the data rate. Consider the table 1 which gives the relation between then SNR and radius, from which we evaluate the data rate as a function of radius. Figure 4 shows the effect of UWB interference on the data rate of the IEEE 802.11a WLAN. All this calculations are made by assuming ρ = 0.2 users/m 3. Since, the imum data rate that can be achieved in IEEE 802.11a WLAN is 54 Mbps and this data rate can be maintained up to 43 meters if there is no interference because of UWB transmitters. But in the presence of UWB transmitters, the UWB interference limits the highest data rate of 54 Mbps only up to 16m for r min = 5m. It can also be noticed that as the minimum distance between the victim receiver and the UWB transmitter increases, the imum achievable data rates over the distance also increases. V. CONCLUSION The range for the minimum data rate of the IEEE 802.11a WLAN get reduced by five times of the initial distance in case of hemispherical distribution of UWB transmitters around the WLAN receiver. Since, the IEEE 802.11a WLAN operate inside the UWB spectrum, so the co-existence of both
technology will be a big challenge. If we consider 1 db degradation in SINR corresponds to about a 20 percent decrease in the received SINR, which is generally acceptable for any system and calculate the corresponding number of UWB user permitted we obtained ρ = 3.35 10 11 users/m 3, which is considered to be quite low for any practical usage. Therefore, we conclude that the density found is too small to consider realistic UWB densities for coexistence of UWB and IEEE 802.11a devices to be operated within the same region. Consequently, UWB deployment will be nearly impossible in an IEEE 802.11a network area if we can only tolerate 1 db degradation in the SINR, which indeed demands for the change in present FCC mask. REFERENCES [1] Federal Communication Commission, FCC 02-48 First Order and Report, www.fcc.gov, April 22, 2002. [2] B. T. Ahmed and M. C. Ramon, Coexistence between UWB and other communication systems - tutorial review, Int. J. Ultra Wideband Communications and Systems, vol. 1, no. 1, pp. 67-80, 2009. [3] M. J. Hamalainen, J. Iinatti, M. Oppermannn, M. J. Latva- Aho, J. Saloranta and A. Isola, Co-existence measurements between UMTS and UWB systems, IEE Proc.- Communications, vol. 153, no. 1, pp. 153-158, 2006. [4] K. Shi, Y. Zhou, B. Kelleci, T. W. Fischer, E. Serpedin and A. I. Karsilayan, Impacts of narrowband interference on OFDM-UWB receivers: analysis and mitigation, IEEE Transactions on Signal Processing, vol. 55, no. 3, pp. 1118-1128, 2007. [5] S. M. Mishra, S. Brink, R. Mahadevappa, R. W. Brodersen, Detect and avoid: an ultra-wideband/wimax coexistence mechanism, IEEE Communications Magazine, vol. 45, no. 6, pp. 68-75, 2007. [6] F. Nekoogar, Ultra-wideband communications - Fundamentals and Applications, Prentice Hall, 2005. [7] T. S. Rappaport, Wireless Communications - Principles and Practice, Prentice Hall, 1996. [8] B. Manny and E. Tsui, Adaptive Radio Constructs, Technical Report Intel Corporation, 2003. [9] B. Quijana, G. Valera, A. Alvarez, R. P. Torres, J. L. Garcia and K. S. Shanmugan, UWB aggregate interference an a cellular victim receiver from a statistical perspective, in IWUWBS, 2003. [10] Recommendation ITU-R M.1450, IEEE Standard for Information Technology: High-Speed Physical Layer in the 5 GHz Band, IEEE, June 2004.