An Ultra Wide Bandwidth System for In-Home Wireless Networking

Transcription

1 An Ultra Wide Bandwidth System for In-Home Wireless Networing iaoumis Geraoulis, Paola Salmi, Saeed S Ghassemzadeh AT&T Labs-esearch, Florham Par, NJ 7932, USA {geraoulis,saeedg}@attcom epartment of lectronics, Computer Science, and Systems University of Bologna, Viale isorgimento 2, Bologna, Italy psalmi@deisuniboit ABSTACT In this article we present an ultra-wide bandwidth (UWB) wireless networ for in-home cordless networing and wireless distribution of cable TV channels The article describes the networ services, the media access control, channel measurements, and the physical lin transmitter and receiver design The physical lin is based on an interference suppressing OFM (IS-OFM) in which the total number of freuency bins (sub-carriers) of the UWB channel is divided into groups very freuency bin in a group will carry all transmitted symbols for that group which are distinguished and separated from each other by orthogonal Hadamard seuences The proposed design may then suppress narrow-band interference which is often present in UWB channels The IS- OFM lin is also expected to satisfy the high bit rate needs and meet the reuired uality of service with a minimum transmit power so that it does not cause interference beyond the perimeter of the home 1 INTOUCTION A wireless channel is said to be ultra-wide if its bandwidth is wider than one uarter of its carrier (center) freuency Ultra-wide bandwidth (UWB) radio technology has been used in the past for radar and remote sensing applications ecently however it has been proposed for use in wireless communications [1], [2] An UWB communication channel has the advantage of providing single, integrated, homogeneous and seamless access for a wide variety of wireless services The UWB radio system, proposed in references [1] and [2], has based its design on a method similar to the one used in radar systems, which is the transmission of carrierless ultra short (4-8 nsec) pulses or impulses These impulses are transmitted at random or pseudo-random time intervals, in order to minimize other user interference in multiple access channels This method is nown as time-hopping impulse-radio (TH-I) In this article we use ultra-wide bandwidth for wireless in-home networing An UWB wireless in-home net- This wor was performed as an intern at AT&T Labs- esearch wor (WIN) will distribute cable TV channels and provide cordless lins for Internet access and local area networing in every room of the home The reuired bit rate for such a networ is estimated to be 5 Mb/s or more In order to meet such a bit rate reuirement the proposed UWB system has to have an alternative design Unlie the TH-I, the UWB lin presented here, has been designed in the freuency domain with carrier of about 5 GHz, still satisfying the definition of UWB system of USA FCC part 15 It is able to suppress narrow-band interference which is often present in UWB channels Moreover, the new system, unlie other methods used in mulituser communication such as multicarrier or multitone CMA systems, is a point-to-point transmission method (not a multiuser one) which also does not spread its transmission bandwidth In the next section we present some WIN services and the media access control, while in section 3 we present channel measurements and the design of the physical lin, and finally some conclusions are drawn in section 4 2 NTWOK SVICS AN ACCSS CONTOL The proposed networ is envisioned to provide wireless services within residential homes These services include: (a) Local area networing, (b) Wireless in-home distribution of broadcast cable channels and, (c) In-home cordless access and routing to external (outdoor) networs (a) The local area wireless services include broadcast video and in-home wireless data/voice networing The local broadcast video service allows wireless video transmission from one VC to one or more TV receivers Also, the in-home wireless networ may deliver interactive data traffic between home PCs, provide wireless networing for control devices (ie thermosts, switches etc) and intra-home voice communications (b) Cable TV channels may be distributed wirelessly to one or more TVs, thus avoiding wiring each TV-room in the home In this case we assume there is a need for simultaneously broadcasting within the home, of up to four TV channels (c) The wireless in-home networ (WIN) can also offer several in-home cordless lins for voice and internet ac-

2 cess, which avoids wiring to fixed locations while allowing mobility to the subscriber within the home The method of delivering the wireless traffic for the above services should not be specific to the type of traffic or system originating it For this reason, the received analog signal from a source networ or device will be sampled and pulse code modulated (PCM) in order to be transmitted over the WIN In particular, a cable TV channel having a bandwidth of 6 MHz will be sampled at a rate of 12 Mb/s Then, after using a 16-level PCM (2 4 = 16) this rate becomes 48 Mb/s (16 4) Assuming a forward error correction of rate 1/2, the resulting transmission rate for each TV channel will be 96 Mb/s and the reuired rate of having four simultaneous TV-channels transmissions is 384 Mb/s Now, taing into account the other services described above, we estimate that the total bit rate over the WIN need to be 5 Mb/s or more The above traffic volume will be carried by the physical lin which will occupy a bandwidth of 125 GHz with carrier freuency in the region of 5 GHz The physical lin design (presented in section 3), is based on a type of orthogonal freuency division multiplexing (OFM) called interference suppressing OFM (IS-OFM) The IS-OFM provides Ñ = 496 sub-carriers or freuency bins over the entire bandwidth These freuency bins are divided into L groups with M bins per group (Ñ = L M), where L=64, 128, 256 or 512 The IS-OFM then allows each transmitting user to occupy some or all L groups of bins In the case where a user does not transmit over all groups, a second user may transmit over those groups in which the first one does not Given the fact that transmission over different bins from non-colocated (non-synchronous) users destroys orthogonality, the use of such scheme with an ordinary OFM will not be feasible However, the IS-OFM allows simultaneous transmissions of different users over different groups, by providing the capability of recovering the interfering symbols in adjacent groups, see Fig 1 The WIN is comprised of one in-home base station (IBS) and a number of in-home terminals (IT) The IBS receives wired traffic from an external networ and broadcasts it to the ITs within the home or receives the wireless transmissions from the ITs and routes their traffic to an external networ The ITs may also transmit or receive internal in-home traffic directly to or from other ITs Transmissions may tae place simultaneously The media access control is based on a point coordinated function which is provided by the IBS More specifically, there is an assigned group of bins, called the controlgroup, for carrying control messages to or from the IBS ach IT, before its data transmission, sends a reuest via the control-group to the IBS The IBS eeps a record of the on-going transmissions and the available bandwidth at any time Based on the available and the reuested bandwidth the IBS responds to the IT with a message for granting or blocing the reuest If a reuest is granted the response message also indicates the groups of bins in which the IT may transmit its data 3 TH PHYSICAL LINK The physical lin design is based on the method of interference suppressing orthogonal freuency division multiplexing (IS-OFM) [3], which has been adjusted for use in UWB channels [4] Since, in our opinion, the nowledge of channel type is very important for system designing, before the description of the proposed transmitter-receiver scheme, we present some indoor measurements of the UWB channel 31 The UWB Channel The channel measurements shown in Figs 2 and 3 were performed in many residential homes in the northern and central New Jersey area, using the techniue of freuency swept channel sounding The channel bandwidth and center freuency were 125 GHz and 525 GHz, respectively Fig 2 illustrates a typical freuency response (top) and impulse response (bottom) of the UWB channel ue to nature of the measurements the channel freuency response did not exhibit significant variability in time and can be assumed time-invariant The impulse response in Fig 2 (bottom) indicates that the mean excess delay is about 25 nsec while the MS delay spread is 11 nsec Fig 3 (top) shows the percentile of signal energy in each path In almost all homes the first returns were either bloced or scattered By analyzing the time domain data we found that on the average the first return only carried 7% of the total energy Also, as shown, in order to collect 7% of the signal energy one would reuire to receive about 4 paths Fig 3 (bottom) shows a scatter plot of measured path gains as a function of distance for all non-line of sight locations The slope of the line is -32 corresponding to 32 db loss/decade The standard deviation around this line (median path loss) corresponds to shadow fading and is about 5 The complete description of the propagation measurements and the channel model are presented in [5] Also, baseband measurements of an indoor UWB channel are given in [6] In addition to the above propagation characteristics, the channel in its ultra-wide bandwidth will have significant interference from many narrow-band transmissions, such as cell phones, radio stations, etc Also, our system design has to obey Federal Communication Commission (FCC) rules on UWB emission limits so that the in-home transmissions does not cause interference outside the perimeter of the home Currently FCC sought comments on whether the existing Part 15 general emission limits are sufficient or appropriate for UWB operations 32 Transmitter escription The transmitter design is shown in Fig 4 As shown, an input data stream of rate bits/sec enters a serialto-parallel () converter which provides L parallel streams ach parallel stream of rate enters again a

3 second converter which provides M parallel streams each with rate /Ñ, where Ñ = L M At the output of the converter, a data signal x ( T sec long), of a parallel stream is spread by an orthogonal binary Hadamard seuence w = [w,, w,1,, w, ], for =,, M 1, (the entire seuence of length T has to overlay a single data symbol also of length T ) After the spreading operation the signal rate is bits/sec Assuming that x represents a complex-valued signal- = α ing point in a QAM constellation, ie, x the spread signal is given by, x +jβ, w, = α w, + j β w, where, =,, M 1 and l = 1,, L Let us now define, b = x w, (2) where, =,, M 1 and l = 1,, L For any pair (, l) we then define, {b } = α w, for i = b for i = L + l 1; and (, l) (, 1) a i = I{b } = β w, for i = Ñ (3) {b } for i = 2 ML (L + l 1) and (, l) (, 1) In the above euation we have assumed that Ñ = L M and N = 2Ñ This process taes place in the encoder which provides N parallel points a i to the input of the IFT, the output of which is given by, s n = 1 N 1 a i e j2π(in/n) for n =, 1,, N 1 N i= (4) Now, let us use the matrix M shown below for representing the distribution of the transmitted symbols over the freuency bins of the UWB channel M = x x x x x 1 x 1 x 1 x 1 x x x x f f fñ M fñ 1 w 1 w As shown, there are L sets or groups of bins with M freuency bins per group, so that Ñ = L M Then, every freuency bin in a set l contains all data points x for =,, M 1 This means that the transmitted power of each symbol is distributed over the M freuency bins of that set Therefore, if one or more bins are corrupted by narrow-band interference, the affected symbols can be recovered from the remaining bins If we assume L = 1, then the resulting system having Ñ = M is the basic IS-OFM In addition, if the spreading orthogonal Hadamard matrix W = [, w 1,, wñ 1 ] T is replaced with an identity matrix W = I, the resulting system is the ordinary OFM Also, if we tae M = 1 and Ñ = L the resulting system is again the ordinary OFM 33 eceiver escription The receiver design is shown in Fig 5 As shown, the received signal enters an OFM receiver, which provides Ñ parallel outputs Z Z where H = b H + I + J + n (5) is the transfer function of the channel at freuency bin in group l, I is the intersymbol and interchannel interference, J and n is the narrow-band interference is the AWGN Also, b shown in Fig 5, in each group l, the signal Z is given by e (2) As enters a converter the outputs of which are despread by the orthogonal seuences for recovering the data The output of the despreader- of group l = 1 is given by, Z = = = = = Z, (6) [b H + I + J + n ], The useful signal (represented by the first term above) provides the data x at the output of the despreader-, as shown below, b H w, = H x w,, = H x = = { H w,, = Mx for = for The above derivation is based on the assumption that the channel is freuency-flat within each freuency bin group l, that is, H = H is constant for =, 1, 2,, M 1 In order to satisfy this condition, we either choose a narrow group-width (narrower than the channel coherence bandwidth), or compensate each H (the compensation is based on the channel estimation Ĥ of each bin) Given the parameter value of Ñ for the UWB channel of 125 GHz eual to 496, the choices of the system parameters L and M are made so that the group-width on one hand is narrower than the coherence bandwidth of the channel and on the other is wide enough to have the capability of suppressing narrow band interference (since interference can only be suppressed if it has narrower bandwidth than the width of each group of bins) Therefore,

4 given that each bin is about 3 Hz, the UWB channel has delay spread less than 25 nsec, and assuming that the bandwidth of each interferer is less than 6 Hz the best estimated values are M = 8, 16 or 32 which corresponds to L = 512, 256 or 128, that is, to have each group-width eual to 25, 5 or 1 MHz Then, each of the L parallel receptions will maintain a satisfactory uality Also, considering the reuirement that the in-home transmission power should be low enough, so that it does not cause interference outside the perimeter of the home, we can enhance lin B performance by using an appropriate FC channel encoder 4 CONCLUSION In this article we have presented an ultra-wide bandwidth wireless in-home networ This includes networ services, media access control, channel measurements and the transmitter and receiver designs The proposed networ will broadcast within the home, cable TV channels and provide cordless networing and access to outdoor networs of a total estimated bit rate 5 Mb/s The proposed physical lin design is based on an interference suppressing OFM (IS-OFM) in which the total number of freuency bins in the UWB channel is divided into groups All bins in each group will carry all transmitted symbols for that group which are distinguished and separated from each other by orthogonal Hadamard seuences Such a system may then suppress narrow-band interference and avoid the effects of freuency selective fading by choosing the group-width to be wider than the interference bandwidth and narrower than the UWB channel coherence bandwidth The IS-OFM lin is also expected to satisfy the bit rate needs and meet the reuired uality of service with a minimum transmit power so that it does not cause interference beyond the perimeter of the home Finally, the proposed system provides multiple access capability by allowing independent users to transmit asynchronously in different groups of bins adio Channels, submitted to I Journal on Selected Areas in Commun Special Issue [5] S S Ghassemzadeh, Janna, C W ice, and W Turin, Measurements and Modeling of an Ultra Wide Bandwidth Indoor Channels, submitted to I Transactions on Commun [6] Cassioli, MZ Win, and AF Molisch, A Statistical Model for the UWB Indoor Channel, I Vehic Techn Conference, vol 2, pp , 21 f f f 1 M 1 f M f2m f 1 N M f N 1 Group Group -2 Group -L N = LM Figure 1: The physical lin IS-OFM sub-carriers in the UWB channel Power, db Freuency, GHz Normalized Power, db xcess elay, ns Figure 2: A typical UWB non line of sight (NLOS) freuency response (top) A typical UWB NLOS impulse response (bottom) FNCS [1] MZ Win and A Scholtz, Ultra Wide Bandwidth Time-Hopping Spread-Spectrum Impulse adio for Wireless Multiple Access Communications, I Transactions on Commun, vol 48, pp , 2 % Power in Multipaths Power of 1st multipath nls = 73% Number of Multipaths NLOS [2] F amirez-mireles, Performance of Ultrawideband SSMA Using Time Hopping and M-ary PPM, I Journal on Selected Areas in Commun, vol 19, pp , 21 [3] Geraoulis and P Salmi, An Interference Suppressing OFM System for Wireless Communications, submitted to I Int Conference on Commun, 22 [4] Geraoulis and P Salmi, An Interference Suppressing OFM System for Ultra Wide Bandwidth Path Gain, db ata, σ NLS = 493 db, γ NLS = T Separation, m Figure 3: The % energy captured from each path (top) Path loss as a function of distance (bottom)

5 x / Ñ x, x, x x w, M x M x w M, M x M w M, x M w M, M w M b b bm M s IFFT s(m) /A s N Add Prefix x x, x, / Ñ x w w, M b = M M x x w, = x M x M w x M w =,1,,M M, M, l=1,2,,l x M w M, M N C O a a N s(t) w M : Serial to Parallel Conversion : Parallel to Serial Conversion Figure 4: The IS-OFM transmitter with Ñ = L M r(t) A/ r(m) z Z FFT z emove N Z N Z Ñ Prefix C O Z Z Z Z M Z M Z Z M w M M M = M M Z M M M = Z Z M M Z M = w M Figure 5: The IS-OFM receiver with Ñ = L M