The forthcoming third generation networks (3G) will provide new opportunities to the wireless markets. However, the services offered by this technology barely ...
Virtual Cellular Networks for 60 GHz Wireless Infrastructure Maxime Flament and Arne Svensson Department of Signals and Systems, Chalmers University of Technology, SE-41296 Gothenburg, Sweden Email: {maxime.flament,arne.svensson}@s2.chalmers.se
Abstract— This paper gives a broad description of a broadband communication system using the 60 GHz frequency band. We describe the channel and its inherent problems such as interference situations and body shadowing. We introduce the use of Virtual Cellular Networks (VCN) and multiple receiving antennas (MRA) in order to solve these problems resulting in an overlapping cellular architecture. We explain how the diversity of the channel can be used to increase reliability and performance. Finally, we simulate the ideal link performance of a VCN using two receiving antennas for a particular set of OFDM parameters.
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I. I NTRODUCTION During the past decade, the demand for higher data rate wireless access has been growing at a dramatic pace. The forthcoming third generation networks (3G) will provide new opportunities to the wireless markets. However, the services offered by this technology barely compete with the fixed access possibilities in local or hot-spot situations, which opens the door to alternative ‘operatorless’ wireless technologies such as the IEEE802.11 and HIPERLAN2 standards. The 3G actors are now feeling the pressure caused by the fast penetration of these Wireless LAN (WLAN) forcing them to solve compatibility issues and integrate them in the 3G wireless standards under the name ”Beyond 3G”. In the near future, we will experience a revolution going from centralized mobile phone networks to decentralized mobile computer networks thanks to the success of the WLANs. Private and public hot-spots will cover most of the city areas and business centers leaving to the UMTS the small remaining market; in the same way, GSM did the same to the satellite mobile services such as Iridium. However, WLAN technologies are not answering all the expectations in the future. Five to ten years from now, a single wireless access will be expected to provide at least the same bandwidth as today’s office computers at a lower fee. This paper is meant to give a good overview at the characteristics of the 60 GHz indoor channel and identify the different sources of diversity that we could exploit. We introduce the concept of Virtual Cellular Networks (VCN) using OFDM to increase the Direction of Arrival (DoA) diversity at the receiver, which introduces channel macro-diversity. Using multiple receiving antennas (MRA), we exploit the relatively small human scale of the spatial diversity at the receiver. In addition, the diversity of the frequency-selective channel can be handled by forward error correction or using adaptive modulation across the subcarriers. We discuss the potential solutions and their implementation for future 60 GHz wireless infrastructure. For example, the combination of VCN and multiple antennas can yield a dramatic diversity gain in
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Fig. 1. Illustration of the Saleh-Valenzuela model with direction information [2]. Rays arrive in cluster from the same direction due to multiple reflection in materials. The taps in the discrete-time model are likely to contain power from one single cluster i.e. originating from one particular direction.
some cases and provide more stability in the network but this involves a centralized implementation, which reduces the overall system flexibility. In the last part of this paper, we propose an OFDM modulation scheme adapted to the channel and we simulate the link performance using different receiving techniques. In the case where the channel is known both at the transmitter and at the receiver, we can show that the performance of the system can reach promising figures. II. T HE 60 G HZ CHANNEL In the quest for ever-increasing bandwidth, we have chosen to investigate the unlicensed 60 GHz frequency band, offering up to 5 GHz of bandwidth. The large bandwidth offers a great capacity for wireless broadband systems. A. Propagation characteristics The propagation characteristics at this high frequency have been studied in many publications. In [1], we identified a series of inherent problems for the use of WLAN in offices and small open areas. Among others, the fast spatial change of the quality of the channel and the high attenuation due to body shadowing are the most destructive effects. The DoA and the Time of Arrival (ToA) of the rays play an important role in the rest of this paper. We illustrated it in figure 1.
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Typical shadowing situations for indoor Wireless LAN systems
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Fig. 3. CDF of the normalized received power using the VCN concept with 1 to 4 APs for shopping mall environment and in presence of shadowing
If we compare to a system like GSM 1800, the wave length at 60 GHz is about 30 times smaller and the bandwidth is 1000 times larger (if we assume just 200 kHz vs. 200 MHz1 ). The basic propagation problem could be explained if we imagine a cellular communication system (like GSM 1800) in which users are about 30 times bigger and system bandwidth is 1000 larger. Hills are our walls and buildings are our furniture. This simplistic approach gives us a good view on the 60 GHz propagation problems. By scaling the system down to the human scale, it becomes an indoor environment ( 30 times smaller than GSM 1800) with larger delay spread compared to the bandwidth (about 1000 times higher throughput than GSM 1800) and users are moving very fast compared to the 5 mm wavelength. Indeed a person moving at 1 m/s is comparable to a speed of 108 km/h in our imaginary giant world. An Access Point (AP) would have a typical range of 25 meters but the cell shape is mainly dependent on the surrounding furniture, walls and windows (likewise the cells in GSM 1800 are dependent on the buildings and hills). Since typical rooms are relatively small, coverage is not the main issue. Since, the bandwidth is large, the time variations of the channel are not the main problem either. On the other hand, the changes due to shadowing become dominant. In addition, due to the walls and doors, interference situations due to other cells occur in short bursts and create difficult handover situations [1]. The system triggers multiple handovers between different APs within a few centimeters (likewise the ”manhattan grids” create problems in cities for GSM systems). This issue is worse than at lower frequencies since users are moving fast compared to the wavelength. With the huge bandwidth available, we can accept some spectral efficiency losses in order to increase the link performance or the coverage of the cells. B. Destructive Shadowing effect The similarity with the lower frequency systems is not completely accurate. In the system we describe, people moving around the receiver destroy part of the signal by shadowing incident rays. The shadowing process becomes therefore a 1 Note that 200 MHz at 60 GHz is only a small faction of the 5 GHz licensed-exempt bandwidth
dynamic problem. If a user is standing still while other persons are walking inside the room, the mobile terminal will eventually lose the connection to an AP when a person is walking through its line-of-sight (LOS). It will then attempt to need to perform a hand-over to another AP having a stronger instantaneous signal. The situation is depicted in Figure 2. Previous studies [3] have showed that a person walking through the LOS of a communication link at 60 GHz can cause attenuation to the received power exceeding 18 dB. As showed in figure 3, achieving coverage in a crowded room is a critical problem. In order to avoid that this kind of situations result in a dropped network connection, the network must be designed in a way that AP’s coverage overlap each other. This effort to increase link reliability introduces even more interference problems for which we need to find a convenient solution. It could be solved using the usual frequency or code separation between APs. However, since in our case conventional methods would require high control traffic (up to one handover every 5s [1]), it becomes increasingly interesting to solve the problem by interference mitigation or by virtual cells. In the next section, we try to describe the advantage of the two methods and their combination. III. D ESIGN ALTERNATIVES IN OVERLAPPING CELLULAR NETWORK
A. Virtual cellular networks Smart network infrastructures, called VCN, were first introduced in [4] but we interpret it a little bit differently. We propose this technique in order to mitigate the 60 GHz channel shadowing problems. In our case, VCN uses distributed access points (AP) and Single Frequency Networks (SFN) within one virtual cell in order to form an adaptive wireless architecture. Our idea of VCN originates from systems such as Digital Audio Broadcast (DAB) but VCN uses adaptive cells. A VCN system intelligently selects the APs forming a virtual cell for a particular user: • Firstly, in order to preserve capacity, APs only transmit if their received power contribution at the terminal exceeds
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Fig. 4. Channel combination in VCN results in a new time delay profile. Whenever rays from different APs contribute to the same discrete tap, they might cancel each other resulting in power loss. Knowing the channel at the transmitter we could mitigate this problem.
a certain received threshold. Secondly, the number of APs in one virtual cell is limited in order to avoid increasingly time dispersive channels. • Thirdly, in the case of good quality of the different channels, the system tries to combine the contributions from each VCN AP in an optimum way to increase the diversity gain. The use of VCN is particularly relevant for 60 GHz wireless LANs. At this frequency, the propagation range is dramatically limited while the 5 GHz bandwidth is large. Also, Upcoming technologies such as Radio-over-fiber are candidates for supporting VCN backbone architectures. The major motivation for VCN is the increased DoA diversity at the receiver but it also introduces Time of Arrival diversity (frequency-selection). The VCN basically creates a cell for a user, which is going to follow him. The concept is interesting because the handover procedure doesn’t exist anymore on the terminal side. It is the network that is switching on and off APs in such a way that the signal forms a virtual cell around the user. It changes completely the way we think about a cellular network and new rules and adaptive techniques need to be implemented. The use of VCN requires a centralized network unit (or eventually a common control channel) deciding how virtual cells move with their user. This solution is therefore difficult to implement in public places where many APs belong to different wireless networks. However, it is the solution that will give the best results against the interference and shadowing problems. Also, Upcoming technologies such as Radio-over-fiber are candidates for supporting VCN backbone architectures. They can centralize the signal processing and reduce the required overheads. In this paper, VCN is used jointly with OFDM based modulation in order to mitigate the delay between different transmitting APs in a virtual cell. One major problem occur when rays from different channels contribute to the same discrete tap. Two taps from two different channels could cancel each other due to phase differences. This problem is illustrated by the example in figures 4 and 5. As a consequence, some subcarriers will suffer greatly from the interference. This effect •
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could be circumvented if we know the two channels before we send the signal from the transmitting APs but it would require to have a reliable feedback channel in order to provide this information at the transmitters. B. Multiple receiving antennas In contrast with the previous subsection, we consider the use of multiple receiving antennas at the receiver. MRA are interesting in order to select particular DoA of the desired signal. The DoA are detected by processing the slight differences between the estimated channels at the receiving antennas. At 60 GHz, the number of resolvable paths from one AP to the user is limited and the same is true for the eventual interfering APs if the cells are overlapping. In addition, the physical implementation of MRA on a small terminal will be relatively easy since the wavelength is very small, i.e. the size of the antennas and the required distance between them is very small. The use of smart antenna techniques is therefore interesting in order to solve the problem created by the overlapping cellular design. However, MRA necessitate an increased signal processing at the receiver at least proportional to the number of branches. When there is no interference, the ML combination of the signal is known as maximum ratio combining but it is also possible to cancel the interference. Figure 6 shows that multiple antennas could exploit the DoA to
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Using only the knowledge of the channels at the receiver, we can calculate the matrix U to perform a maximum ratio combination (MRC) of the signal coming from the different VCN antennas. The received signal vector z representing the contribution from VCN antenna i is expressed by
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separate signal falling in the same discrete tap. As soon as we know the wanted channel and the interferers, we can calculate the best way to combine the signal from the antennas and to mitigate the interference for each OFDM subcarriers. Using multiple antennas without a VCN is not extremely interesting since shadowing is still going to require temporary handovers from one APs to the others. However, due to the mitigation of the interference, it solves the problem created by overlapping cells. C. Joint use of VCN and MRA The VCN and MRA are two complimentary techniques. The system would use the multiple antennas as a safety net if the VCN has not activated one of the interfering AP in the virtual cell for a particular user. The terminal can use the 60 GHz coordination channel to ask the interfering channel to joint the virtual cell. If this AP doesn’t have any resource available, the terminal will continue to cancel its signals. Figure 7 gives the procedure for the formation of the virtual cells including the option of ignoring an AP. In addition, the system could increase dramatically the link quality as soon as the channels from different APs have a good quality without anyone shadowing it. Indeed, the multiple antennas can be used to separate the different channels coming from different APs. Even if the APs are sending exactly the same signals, it is interesting to detect them separately and combine them in an optimum way presented in the next section. Given the frequency response matrix H(k) formed by the channel impulse responses hi,j (k), i.e. nR × nT frequency responses representing the one-tap channel from VCN antenna i to the receiving antenna j on the k th OFDM subcarrier . The received symbol vector is y(k) = H(k)x(k) + n(k).
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. (3) On the other hand, if we consider also the knowledge of a channel at the transmitter, we can filter the signals already at the transmitter such that the signal will be sent on the singular dimensions of the channel matrix. In order to do that, the channel matrix H is expressed using a singular value decomposition (SVD)
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where S(k) is a rectangular matrix containing the singular values matrix diag(s1 (k), s2 (k), . . .), and, U(k) and V(k) are the matrices formed by the left and right singular vectors. UH (k) and V(k) are used at each side of the channel in order to form a multiple beamforming. The received symbol vector becomes z(k) = UH (k)H(k)V(k)x(k) + UH (k)n(k) = S(k)x(k) + UH (k)n(k)
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D. Exploiting frequency diversity The time dispersion of the channel can be interpret as a variation of the signal quality across all the subcarriers introducing a subchannel diversity similar to the time diversity of a flat Raleigh fading channel but with a different autocorrelation function. In section IV, forward error correction know as convolution codes are used to mitigate the deep fades in particular subchannels. However, when a good knowledge of the channel is present at the transmitter, it is possible to use adaptive modulation for each of the subcarriers. The techniques to use adaptive modulation in a VCN network might become very complicated but it could increase dramatically the capacity of a channel. IV. S IMULATIONS An indoor 60 GHz WLAN link using VCN and OFDM modulation depicted in Figure 8 is simulated in order to identify the link performance in an ideal situation, i.e. when no shadowing is present and the received signals from the 2 APs have the same average power. Signal bandwidth is fixed to 200 MHz divided into 256 subcarriers with a Cyclic Prefix (CP) length ∆=30 i.e. a CP time period of ∆/BW =150 ns. The subcarrier bandwidth is wide enough (781 kHz) and the symbol time is short enough (1.43 µs) such that the effect of speed can be ignored in one entire frame. 16–QAM baseband modulation is used. The diversity of the frequency-selective
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Overview of the OFDM communication system. Using VCN, the multipath channels are duplicated with uncorrelated noise n and channel h(t, θ).
channel is exploited by a rate R=1/2 convolutional coding with constraint length 7 using a block interleaver across the subcarriers. Perfect APs power control is assumed. In order to simulate the channels, we use a large set of Saleh-Valenzuela channel realizations represented by tap-delay models and we form the resulting time dispersive channels out of nT randomly picked channels. VCN AP delays at the receiver are uniformly distributed with 30 ns maximum. The nR channels from one VCN antenna have a similar power delay profile but experience different tap fading due to the path differences. They are therefore generated using independent Rayleigh random values on each of the taps. The results in figure 9 show the performance of the described system for two transmitting APs and two receiving antennas. Singular Value Decomposition (SVD) and Maximum Ratio Combining (MRC) are evaluated related to the case the channels are known or not at the transmitters respectively. For comparison, the SISO performance are shown, and, perfect and noisy estimates are considered. The estimated channels are generated at the beginning of the frames. channel estimation error variances are calculated from the noise power and the number of required training symbols for SVD or MRC [1]. Beside decreasing the flexibility of the system and requiring more computation, the pre-filtering at the transmitter using the SVD of the channel requires additional channel estimation and a reliable feedback channel. The nT × nR channels need to be known both at the transmitter and at the receiver making the system more complex. In our scheme, a SVD has to be performed for each subcarrier in order to shape the multiple beamforming. As long as the rank of the channel matrix is large then the SVD method gives good performance. However, this is not always the case since the received channels from one AP to the MRA have the same average power delay profile. The spatial fading correlation of the channel is high so that using all the singular values is, in reality, a waste of resources since the power is focussed in spatial channels having low gain. Instead, the power could be focussed on the most efficient beamforming vectors corresponding to the first few singular values of the SVD. V. C ONCLUSIONS In this paper, we described the use of OFDM, VCN and MRA in order to solve many problems identified in broadband wireless networks operating in the 60 GHz frequency band.
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Fig. 9. VCN using two antennas on each side. 16–QAM is evaluated with rate 1/2 convolutional coding (K=7). Perfect and noisy channel estimates are considered.
The different properties of the channel suggest that it is possible to build a system mitigating most of the 60 GHz problems. We identified each of the inherent diversities (time, space and frequency) and explained how the system can take advantage of it. The complexity introduced is relatively limited and simplified algorithms could be proposed. We also simulated the ideal link performance in case of no shadowing for a 16– QAM OFDM modulation scheme with two transmitting APs and two receiving antennas. The paper shows that 60 GHz wireless networks have a strong potential and opens many new issues related to the combination of OFDM, VCN and MRA. R EFERENCES [1] M. Flament, “Broadband wireless ofdm systems,” Ph.D. dissertation, Chalmers University of Technology, Dec. 2002. [2] Q. H. Spencer, B. D. Jeffs, M. A. Jensen, and A. L. Swindlehurst, “Modeling the statistical time and angle of arrival characteristics of an indoor multipath channel,” IEEE Journal on Selected Areas in Communications, vol. 18, no. 3, pp. 347–360, Mar. 2000. [3] K. Sato and T. Manabe, “Estimation of propagation-path visibility for indoor wireless LAN systems under shadowing condition by human bodies,” in Proc. of IEEE Vehicular Technology Conference. IEEE, May 1998. [4] H. J. Kim and J.-P. Linnartz, “Virtual cellular network: a new wireless communications architecture with multiple access ports,” in Proc. of IEEE Vehicular Technology Conference, vol. 2, June 1994, pp. 1055–1059.
