An Approach to Cooperative Satellite Communications in 4G Mobile Systems

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1 JOURNAL OF COMMUNICATION, VOL. 4, NO., NOVEMBER 9 85 An Appoach to Coopeative atellite Communications in 4G Mobile ystems Yui Labado, Masoumeh Kaimi, Deng an, and Jey Mille Floida Intenational Univesity, Depatment of Electical and Compute Engineeing, Miami, FL, UA s: {ylab, mkai, pand, millej}@fiu.edu Abstact In this pape we focus ou attention in the main two methods of Coopeative Communications: Decode and Fowad, and Amplify and Fowad, and how they can be used in a new concept of Coopeative atellite Communications. We pesent an analysis of both in tems of ymbol Eo Rate and owe Allocation and analyze which would be moe efficient when elaying infomation fom the satellite to a mobile node in the teestial netwok. We popose a potocol that combines elective and Incemental Relaying to optimize the coopeative scheme. Index Tems Decode and Fowad, Amplify and Fowad 8, 6QAM, ymbol Eo Rate. I. INTRODUCTION Futue 4G mobile systems will allow a subscibe to eceive sevices anywhee, anytime at low costs. uch 4G systems will be capable of coveing any geogaphical aea by eithe using the teestial netwoks o the satellite netwoks. To this aim, it is necessay to combine both netwoks into a hybid achitectue that allows the flexibility to tansmit high data ates fom the souce to the end use. To obtain such high data ates it is also necessay to use highe ode digital modulations, i.e., M- o M-QAM, along with a bandwidth efficient scheme like Othogonal Fequency Division Multiplexing (OFDM) [5]. It is also impeative to adapt the ecent tend of Coopeative Communications (CC) to this Hybid atellite/teestial netwok so the link is as eliable as possible and the tansmission of infomation is guaanteed. CC woks on the basis of a elay node that etansmits the signal to the destination node. CC combines two tansmission phases; in hase I, the souce tansmits a signal to both the elay node and the destination node and in hase II, the elay node etansmits the eceived signal to the destination node. Two methods ae being used by CC, they ae known as Decode and Fowad (DF) and Amplify and Fowad (AF). AF is just an amplification of the signal by the elay node and then, the amplified signal is tansmitted. DF is a moe complex appoach in which Manuscipt eceived June, 9; evised August 5, 9; accepted August 5, 9. the elay node eceives a signal, decodes and e-encodes it, and then is tansmitted to the destination node. CC can be categoized in Fixed Relay and Adaptive Relay schemes. Fixed Relaying has the advantage of easy implementation but it is not efficient in the bandwidth usage since half of the channel esouces ae allocated to the elay fo tansmission. This educes the oveall ate. Adaptive Relaying includes selective and incemental elaying, and it is bandwidth efficient. We will conside the case of satellite tansmissions whee the satellite acts as the souce node. A elay node is placed in aeas whee the mobile uses may lose link with the satellite and theefoe a way of elaying the signal is needed. Examples of this can be a mobile use taveling and appoaching places whee the satellite link may be intemittent, o completed disupted (tunnels, vegetation aeas, building, etc.) as depicted in Figue. Fig.. Coopeative atellite Communications showing hase I and hase II In hase I, the eceived signal (y) at elay and destination nodes is: s, d = hs,d x(t) ns,d and y = h x(t) + ns, () y + whee is the tansmitted powe at the souce, x(t) is the tansmitted infomation symbol, n s,d and n ae the additive noise in the souce-destination s,d and souceelay channels, and h s,d and h ae the channel coefficients fo the s-d and s- channels. The channels ae consideed as zeo-mean, complex Gaussian andom vaiables with vaiances δ² s,d and δ². The noise tems n s,d and n ae modeled as zeo-mean complex Gaussian andom vaiables with vaiance N. In hase II, the elay sends a signal to the destination based on what it eceived fom the souce: 9 ACADEMY UBLIHER doi:.434/jcm

2 86 JOURNAL OF COMMUNICATION, VOL. 4, NO., NOVEMBER 9 y = κ + (), d h,d (y ) n,d whee,d epesents the link between the elay and the destination, and κ vaies depending on the type of scheme (AF o DF). The destination eceives two copies fom the signal x(t) though the s,d link and the,d link. It is necessay to combine both incoming signals at the destination. The best technique that allows the best ignal to Noise Ratio (NR) is the Maximal Ratio Combine (MRC). At the MRC output we obtain a NR that is equal to the NR fom both the s,d and,d links. The outage pobability [3], [4] is the pobability that the mutual infomation is less than the ate R, in AF the outage pobability is [6]: [ MI < R] AF δ δ R ( ) /,d - δ δ N s,d,d + δ (Achieving divesity two) whee I AF is the mutual infomation between souce and destination, R is the ate. The same analysis can be extended to DF systems, giving an outage pobability as follows: [ MI R] DF R (3) - < (4) δ / N (Achieving divesity one) The emainde of this pape is oganized as follows. Fist, we descibe the Adaptive Coopeation chemes in ection II. ymbol Eo ate Analysis of DF and AF ae pesented in section III. ection IV is dedicated to the analysis of powe distibution in DF chemes. Then, we explain the DF and AF pefomance in ection V. In ection VI, we pesent a chaacteization of the atellite Channel Model. We then descibe elective and Incemental Relaying in atellite/ Teestial Coopeation in ection VII. imulation esults ae shown in ection VIII. Finally, we povide some concluding emaks in ection IX. II. ADATIVE COOERATION CHEME With Fixed Relaying thee is a 5% loss in the spectal efficiency due to the tansmission in two phases. The pefomance of DF is limited to the weakest souce-elay and elay-destination link educing the divesity gain to one. ome othe appoaches [] ae aimed at esolving this limitation. They ae known as: elective Relaying and Incemental Relaying. In the following, we biefly analyze each one of them. A. elective Relaying In DF elective Relaying (DF) the elay node decodes and fowad the signal only if its NR is above a cetain value known as the theshold value [3],[4]. If the souce-elay link suffes fom fading o attenuations making the NR value less than the theshold, the elay will not decode and fowad the infomation to the destination node. When the eceived signal at the elay node is stong enough (NR > Theshold), the NR of the combined MRC signal at the destination is the sum of the eceived NR fom the souce and elay, as stated above. In ode to an outage event to happen, both the souce-destination s,d and souce-elay channels should be in outage o the combined souce-destination, and elay-destination channel should be in the outage [7], [6], giving a divesity of two. The outage expession is given by: [ MI < R] DF δ + δ δ ( δ δ s,d,d,d ) We can see that it has the same divesity gain as the AF case above; we can conclude that with high NR both selective elaying DF and AF have the same divesity gain. B. Incemental Relaying In this case thee is a feedback channel fom the destination to the elay, as shown in Figue. The destination will send an acknowledgement message to the elay [8] if it coectly eceived the signal sent by the souce. If this happens the elay does not need to tansmit in hase II []. This scheme has the best spectal efficiency among the above descibed appoaches because the elay not always need to tansmit and the hase II tansmission will depend on the channel chaacteistics in hase I between the souce and destination. Fig.. hase II occus only if the destination node asks the elay node to fowad infomation If the tansmission in hase I fom souce to destination was successful, then hase II will neve occu and the souce will use the next time fame to tansmit new data. On the othe hand, if the hase I tansmission was unsuccessful then hase II will take place and the elay will send infomation to the destination. This could be the case when the mobile use loses the link with the satellite. The outage expession [9], [6] is given by: whee [ MI R] (5) δ δ R - +,d AF < (6) δ δ δ /N s,d,d R R - R = + exp ( ) /N (7) 9 ACADEMY UBLIHER

3 JOURNAL OF COMMUNICATION, VOL. 4, NO., NOVEMBER 9 87 The pefomance degades when the ate R inceases, but it degades faste fo incemental elaying because of the inheent loss in the spectal efficiency []. Fo high enough R, diect tansmission is moe efficient than elaying. Incemental elaying pefoms bette because incemental elaying woks at a much highe spectal efficiency than the est of the elaying mechanisms and gives a divesity gain of two. III. ymbol Eo ate Analysis of DF and AF A. DF analysis We based the analysis of ymbol Eo Rate using DF [6] with 8 and 6QAM modulations. In futue 4G systems, it is necessay to use high ode modulations to guaantee that high data ates ae deliveed to the end use. These high data ates ae needed by many applications but especially by those that use multimedia such as video, data, etc. Having the infomation of the channel coefficients h s,d and h,d between souce and destination and elay and destination, and assuming that the tansmitted symbol x has aveage enegy, the NR of the MRC output is given by [6]: NR hs, d + R h, d MRC = (8) N fomulations fo both 8 and 6QAM ae given by the equations: Ι Ι QAM (M )π)π bϑ ( ϑ ) = exp dθ π (9) in θ 9 ( ϑ ) = 3Q( bqam ϑ ) Q ( bqam ϑ ) () 4 wheeϑ is the NR, b = sin²(п/8), b QAM = /5, and Q is the Gaussian function. If 8 is used in a DF Coopeation system, with instantaneous NR I, then the conditional of the system with channel coefficients h s,d, h, h,d can be expessed as () and (): = Ι (NRMRC ) () If 6QAM is used in the system, then the conditional [6] of such a system is given by the following expession: QAM = ΙQAM (NRMRC ) () In the case of Q and 4QAM modulation, the conditional given by () and () is the same. This is because Q and 4QAM have the same constellation so the detection of the phases has the same complexity. In hase II if the elay node decodes the symbol coectly, it is fowaded to the destination with powe R = R. If the symbol is not decoded coectly then it will not be fowaded and R =. If 8 is used the chances of incoectly and coectly decoding at the elay ae: h Ι and N - Ι h N On the othe hand, if 6QAM is used the chances of incoectly and coectly decoding a symbol at the elay ae: h Ι QAM and N - Ι QAM h N The link between the elay node and the destination node can be modeled as a Rayleigh fading channel because the path between them can be obstucted and a diect line of sight may not exist. The ymbol Eo Rate fo a Decode and Fowad Coopeation cheme unde a Rayleigh fading channel using 8 modulation can be expessed as (3), simila to the one in []: b + N δ s, d F in θ b + N δ + F in θ = F b + N δ s, d b + in θ N δ - F in θ b + N δ in θ (3) Fo a system using 6QAM ove a Rayleigh fading channel with Decode and Fowad Coopeation the ymbol Eo Rate is given by (4): F b QAM δ s, d + F N in θ bqam δ + + F N in θ QAM = QAM QAM QAM b QAM δ s, d b QAM δ, d F N in θ N in θ QAM whee F and FQAM depend on x(θ). B. DF appoximation b QAM δ + N in θ (4) The ymbol Eo Rate of Decode and Fowad Coopeation [6] system using 8 and 6QAM modulations can be uppe bounded as shown in (5): (M )N M M b δ s, R, ( N + b δ ) + ( N + b δ ) + ( N + b δ ) s, d + (M - )b δ d + (M )N R, d (5) whee b=b fo 8 signals and b=b QAM fo 6QAM signals, M = 8 in 8 and M = 6 in 6QAM. If δ² s,d, δ², and δ²,d, it means that all of the link channels (h s,d h and h,d ) ae available then /N and R /N go to infinity, the ymbol Eo Rate of the system using 8 and 6QAM modulation can be appoximately as shown in (6): N A B + (6) b δ s, d δ R δ, d 9 ACADEMY UBLIHER

4 88 JOURNAL OF COMMUNICATION, VOL. 4, NO., NOVEMBER 9 whee b, A, and B depends on the type of modulation [] and will be A =.494, B =.377 fo 8; and A =.64, B =.53 fo 6QAM. C. AF appoximation An appoximate expession fo using Amplify and Fowad can be obtained. If all the channels (h s,d, h and h,d ) ae available (meaning that δ² s,d, δ², and δ²,d ), then when /N and R /N tend to infinity the of AF using 8 and 6QAM modulation is given by [5] as shown in (7): A N δ + Rδ,d (7) b Rδ s,dδ δ,d whee A and b depends on the type of modulation and ae given by A=.374 and b=b fo 8; and A=.53 and b=b QAM fo 6QAM. Figue 3 and Figue 4 show Decode and Fowad and Amplify and Fowad ymbol Eo Rate gaphs vesus /N [db]. The thee esults showed ae: the exact fomulation, the uppe bound fomulation and the asymptotically tight appoximation, consideing δ²s,d = δ² = δ²,d =, and N = /No [db] Uppe bound Appoximation Exact Fig. 3. DF Coopeative Communications system with Q /No [db] imulation 35 Appoximation Exact Fig. 4. AF Coopeative Communications system with Q IV. ANALYI OF OWER DITRIBUTION IN DF CHEME In this section we aim to obtain the optimum powe distibution both at the souce and the elay node []. Note that as stated befoe, the powe at the souce is and the powe at the elay is R. In a Decode and Fowad Coopeation cheme using 8 and 6QAM modulation [6], if all the channels ae available (h s,d, h and h,d ), and δ² s,d, δ², and δ²,d fo high NR and = + R the powe distibution [] is shown in (8) and (9): R δ + δ s, + 8(A /B)δ, = (8) 3δ + δ + 8(A /B)δ d, d δ = (9) 3δ + δ + 8(A /B)δ, d whee A and B depends on the type of modulation 8 o 6QAM as stated in the pevious section. It is impotant to note that the expessions (8) and (9) do not depend on the souce-destination channel; they only depend on the links between souce-elay and elaydestination. We can also note that the optimum powe atio of the souce powe ove the total powe is less than one and lage than ½ [], on the othe hand the optimum atio of R at the elay ove the total powe is geate than and less than ½ [6]. ½ < / < and < R / < It shows that we should always put moe powe at the souce and less powe at the elay. This consequence is impotant in ou case because the satellite is the souce and it has to have the geate powe. If δ² << δ²,d, link quality between souce-elay is less than that of elaydestination; tends to and R tends to, meaning that we must use all the powe at the souce given that the link quality between elay-destination is bette. This should be the case when the satellite link pesents stong fading due to ain, o any othe atmospheic impaiment. On the contay, if δ² >> δ²,d, it means that the souce-elay channel is in much bette condition than that the elaydestination link. In this case and R go to ½, and we should allocate equal powe at both the souce and elay. In the satellite link case, since the satellite powe cannot be inceased, we must find a way to incement the powe at the elay evey time the elay-destination link fades consideably. It is impotant to note that the elaydestination link is modeled as a Rayleigh fading channel which is a type of channel when thee is no diect line of sight between elay and destination, thus having stong fading. In ode to obtain divesity two, the souce-elay and elay-destination links should be appopiately balanced. If the souce-elay link is unavailable, it is had fo the elay to pefom its task of Decode and Fowad the eceived symbol. Theefoe, the fowading task of the 9 ACADEMY UBLIHER

5 JOURNAL OF COMMUNICATION, VOL. 4, NO., NOVEMBER 9 89 elay is less impotant, so it makes sense to put moe powe at the souce. On the contay, if the souce-elay channel quality is vey good, the elay can decode the signal fom the souce. In this case, we can conside the elay as a copy of the souce and put the same amount of powe on both. It is impotant to note that the amount of powe also depends on the constellation size; fo high ode constellations such as 8 o 6QAM the amount of powe must be geate than in the case of Q o 4QAM. We now conside thee cases of powe allocation using Decode and Fowad [].. Relay-destination channel is not available (δ²,d = ). ouce-elay channel is not available (δ² = ) 3. ouce-destination channel is not available (δ²s,d = ) Case. If the elay-destination channel is not available, fom (3) the ymbol Eo Rate of Decode and Fowad Coopeation ystem using 8 modulation can be expessed by (): b δ s, d A N = F + () N inθ bδ s, d whee F and A ae defined above fo the 8 case. Analyzing (4) we obtain a simila equation fo the case of 6QAM when the elay-destination link is not available as shown in (): QAM b QAM δ s, d AN = F QAM + () N inθ b QAM δ s, d whee F QAM and A ae specified above fo the 6QAM case. Fom () and () we conclude that the optimum powe distibution is = and R =. As expected if thee is no elay-destination link then the only option is to use diect tansmission between souce and destination allocating all the powe at the souce. Case. If the souce-elay channel is not available, fom (3) and (4), the ymbol Eo Rate of Decode and Fowad Coopeation ystem using eithe modulation ae given by (): A N () b δ s, d whee A will vay if the system uses 8 o 6QAM and b = b fo 8 and b = b QAM / fo 6QAM. In this case, the optimum powe distibution is = and R =. Case 3. If the souce-destination channel is not available (causing hase II tansmission, see ection VI) fom (3) and (4) the ymbol Eo Rate of Decode and Fowad Coopeation ystem with 8 o 6QAM is given by (3): b = + δ b + + R δ, d b + δ Fi F i F i N in θ N in θ N in θ (3) whee i= and b = b fo 8AK, and i= and b = b QAM / fo 6QAM. If the souce-elay and elaydestination ae available the in (3) can be appoximate as shown in (4): AN + (4) b δ R δ, d whee b = b fo 8, and b = b QAM / fo 6QAM. A also depends on the type of modulation as expessed above []. In this last case the powe distibution fo both 8 and 6QAM is: δ,d δ = and R = δ + δ δ + δ,d When the souce-destination channel is not available, the system is modeled as a two-hop system. This conclusion is impotant in the case the satellite loses the link with the mobile use and needs to use the elay node to tansfe the sevice. The mobile node may have enteed a zone out of the satellite each and then will depend on the elay node to eceive the signal, as shown in Figue 5. The powe at the satellite will depend on the channel quality between the elay and destination, the channel quality between the satellite itself and the elay and the oveall powe. Fig. 5. Case 3 when thee is no souce-destination link The optimum powe distibution fo an Amplify and Fowad system using eithe 8 o 6QAM modulation can be expessed as (5) and (6), simila in [5]: R,d δ + δ s, + 8δ, = (5) 3δ + δ + 8δ d, d δ = (6) 3δ + δ + 8δ, d Fom (5) and (6) we can deduce that the optimum powe distibution in an Amplify and Fowad system does not depend on the type of modulation used. This diffes fom the Decode and Fowad scheme whee the optimum powe distibution depends on the type of modulation. This is because in the AF case, the elay 9 ACADEMY UBLIHER

6 8 JOURNAL OF COMMUNICATION, VOL. 4, NO., NOVEMBER 9 eceives, amplifies and fowads the signal egadless of the modulation type. In DF, the elay uses the modulation type in ode to decode and e-encode the data that is why in DF the powe distibution depends on the modulation. Also fom (5) and (6) we can see that optimum atio of to the oveall powe is less than and lage than ½, and the atio of powe R to the oveall powe is lage than and less than ½. A. Decode and Fowad V. DF AND AF ERFORMANCE We saw that fo high NR the ymbol Eo ate pefomance of a DF system is given by (6), substituting the optimum powe distibution given by (8) and (9) in (6) thus, we have (7) and (8), simila in []: D DF = b δ s, d B δ δ ( Φ) DDF (7), d / ( δ + δs, + 8(A /B) δ, d ) ( 3δ ) / + δ + 8(A /B) δ, d (8) whee b = b fo 8, and b = b QAM / fo 6QAM; and Φ = /N. Analyzing (7) we can see that adaptive DF Coopeation gives us a divesity of, depending only on the chaacteistics of the channel links. Equation (8) is known as the Coopeation gain of a DF system and it gives us an idea of the best pefomance gain we can obtain using DF Coopeation. If the channel between souce and elay is wost than the channel between elay and destination, the Coopeation gain can be educed to (9): b δs,d δ DDF = A (9) On the othe hand, if the channel between souce and elay is much bette than the channel quality between elay and destination, the Coopeation gain can be educed to (3): B. Amplify and Fowad b δs,d δ,d DDF = B (3) imila analysis can be done in the case of Amplify and Fowad scheme. The ymbol Eo Rate is given by (7), combining this with equations (5) and (6) we can obtain (3) and (3), as in []: ( ) DAF Φ (3) / bδ, d s,d δδ δ + δ + 8δ,d DAF = (3) 3/ B 3δ + δ + 8δ, d Equation (3) is the Coopeation gain of a AF scheme and give us an idea of the best pefomance of a system using Amplify and Fowad. Equation (3) shows that AF also gives us a divesity of ode, which is the same as an adaptive DF Coopeation system. If we compae the Coopeation gain of DF to the Coopeation gain of AF, we obtain the ation β, which is given by β = D DF /D AF. Analyzing the thee possible cases of channel quality: Case. ouce-elay channel wost than elay-destination channel (δ² << δ²,d ): B β > (DF pefoms bette than AF) A Case. ouce-elay channel bette than elay-destination channel (δ² >> δ²,d ): β (DF and AF pefoms the same) Case 3. ouce-elay channel equal than elay-destination channel (δ² = δ²,d ): A/B 6 β= A/B By giving the values of A and B fo 8 and 6QAM, we have β. 67 fo 8, and β. 378 fo 6QAM. VI. Chaacteistics of the atellite Channel Model We conside the Hybid satellite/teestial channel to have a diect line of sight (LO) coming fom the satellite and seveal teestial eceives located in an open aea, thus esulting in a popagation model with seveal paths. The satellite LO path is modeled by using a Rician distibution. Rician distibution is a multipath model that is descibed by the facto K, which is the atio of the powe in the diect link to the powe of the multipath links. Typical values fo K ae: 5dB, 7dB, 8dB. The teestial model is descibed as Rayleigh distibution which is a type of distibution whee the LO is nonexisting, thus leaving K=. Rayleigh fading channels affects the signal much moe that Rician fading channels because all the paths that each the eceive ae eflected, diffacted o fom scatteing. Othe vaiables fo descibing these multipath fading channels include: Delay spead and Dopple pead. The maximum Dopple shift can be found by using the following expession: v f dm = f c whee c is the speed of light, v- is the mobile speed, and f o is the fequency. ome chaacteistics of the atellite channel include: 9 ACADEMY UBLIHER

7 JOURNAL OF COMMUNICATION, VOL. 4, NO., NOVEMBER Non-linea distotion intoduced by onboad owe Amplifie - Long ound-tip popagation time 3- Reduced Time Divesity 4- Rain attenuation The High owe Amplifie (HA) is intoduced in the satellite channel. HA opeates nea satuation egion to maximize output powe and efficiency. Unde the mentioned condition, a Non-linea distotion is intoduced inceasing spectal e-gowth and in-band distotions. This can be poblematic if we use highe ode modulations like 8, 6QAM, etc, and damage the channel capacity inceasing adjacent channel intefeence. ome wavefom pe-distote is necessay to tackle these issues. Typically HAs ae Taveling Wave Tube (TWT). Highe ode modulations, such as 8 and 6 QAM, ae the modulation vehicles though which the highe thoughputs that boadcastes and satellite opeatos ae now demanding ae achieved. Howeve, this bings with it challenges that have taditionally not been evident with existing Q modulation. hase noise, highe C/N equiements, and inceased dish sizes at downlink sites to name but a few. In ode to meet this challenge it is necessay to develop a system solution that allows us to use highe modulation schemes ove satellite channels and at the same time compensate fo any distotions in the channel. A satellite dynamic pecoection system will allow maximising the satellite tansponde thoughput, significantly educing downlink eceive antenna sizes and inceasing the link eliability. This dynamic pecoection will compensate fo vitually any linea and non-linea distotion that is likely to be encounteed in a typical satellite tansmission chain. It also compensates fo both eath and satellite distotions. We also need to conside the effects of the satellite High owe Amplifies (HA) when we use OFDM to tansmit high data ates ove the satellite link. OFDM is highly sensitive to the pesence of non-linea distotions and synchonization eos between tansmitted and eceived signals. Digital pe-coection schemes can be applied fo the compensation of the AM/AM and AM/M distotion intoduced by on boad satellite HA. This lineaization can not be ealized successfully unless the path delay intoduced by the analog chain is peviously estimated. This type of non-linea distotion is solely dependent on the modulus of the input signal and appeas at the eceive as a waped symbol constellation theeby degading the bit eo ate (BER), while in fequency domain the distoted signal undegoes spectal e-gowth which geneates intemodulation poducts and adjacent channel intefeence. The time delay intoduced by the analog chain esponsible fo fequency up-convesion to the HA input and fequency down-convesion fom the HA output must be compensated fo befoe estimation of the pecoection coefficients. A time delay estimation module is necessay befoe any adaptive pe-coection scheme is initiated. The time delay estimation algoithm poposed in [7] is an accuate one that can be used in satellite HA. The algoithm is based on the definition of an intelligent coss-coelation between the input and output of the HA signals. They used a aleh Model fo the Tavelling Wave Tube Amplifie (TWTA), as it intoduces moe significant AM/M distotions than the olid tate owe Amplifie (A). The memoy less model of the HA is defined by: α a x A[ x ] = (33) + β x Φ[ x ] = α = β = a a α Φ x (34) + β Φ x β a = Φ π α Φ = 3 If we want to use OFDM ove satellite channels we must guaantee to have divesity gain. By exploiting time divesity we can use OFDM in satellite links as long as the tansmission of two consecutive symbols will take place in a time inteval longe that the satellite coheence time. atellite links have long uplink and downlink paths, making the ound tip vey lage. This affects the accuacy of the channel estimatos that ae used in OFDM teestial links in ode to keep an updated channel condition. We must select a value fo the time between two OFDM symbols that satisfies T > C means that the time between symbols is lage than the channel coheence time C symbols will be un-coelated. CT s CT, it ; and two consecutive VII. elective and Incemental Relaying in atellite / Teestial Coopeation Fom section V we can see that Decode and Fowad pefoms bette in two of the thee cases, and pefoms simila to Amplify and Fowad when the souce-elay channel is bette than the elay-destination channel. The use of highe ode modulations ove satellite links [5] has to be caefully designed and stong eo coection algoithms must be used. Also, as we said at the beginning of ou pape, OFDM is needed to obtain a bette spectal efficiency and to tansmit high data ates. This bings us to conside (fo most cases) that the souce-elay channel may be wose than the elaydestination channel. It is impotant to note that, although the elay-destination channel is modeled as a Rayleigh multipath which is a type of channel with stong fading, the use of OFDM and highe modulation ode is moe eliable hee than in the satellite-elay channel. In satellite links OFDM depends on inceased time divesity, and high ode modulations depend on pe-distotion to make them wok in a suitable way. In this case, we see that the Coopeation gain β >, so Decode and Fowad pefoms bette than Amplify and Fowad. By combining Decode and Fowad with elective and Incemental elaying, we can accomplish a stonge 9 ACADEMY UBLIHER

8 8 JOURNAL OF COMMUNICATION, VOL. 4, NO., NOVEMBER 9 scheme. As shown in Figue 6, the destination node will equest tansmission fom the elay node only if its NR is less than the theshold value meaning that the signal it is not stong enough to obtain the data sent by the souce. If the destination node loses the satellite link fo some easons, the NR will dop below the theshold and it will equest hase II fom the elay node. The elay node will employ elective Relaying and will tansmit the decoded signal to the destination node only if its own NR is above the theshold value. Hence, fo the elay node to tansmit two things must occu: the destination node must equest tansmission and the NR on the elay itself must be above the theshold. Q modulations. The satellite channel is modelled as a Rician Model with a ath Loss Block that simulates the signal attenuation fom the satellite to the eath teminal. The satellite is of Geostationay Obit (GEO atellite). Fig. 8. hase I hysical Laye The paametes of the imulation ae as follows: Fig. 6. Incemental and elective Relaying The elay node will emain idle if it does not eceive a equest fom the destination node and/o its own NR is low. If the elay node does not eceive the equest it will also emain idle even if its NR is high. This is the main diffeence between this poposed scheme and elective elaying in which the elay decodes and fowads the signal as long as its NR is geate than the theshold. The diffeence between ou scheme and Incemental elaying is that when the destination equests a tansmission fom the elay it will occu if the NR in the elay is high. In this case the destination will totally lose the signal if both channels (satellite-destination and satellite- elay) ae unavailable as shown in Figue 7. Bandwidth: 5 MHz Cental Fequency: GHz OFDM ubcaie spacing: f = 5 KHz OFDM IFFT ize: 48 fo 6QAM OFDM IFFT ize: 4 fo Q Tx I = ms OFDM ymbol Time: µs Numbe of OFDM symbols: Cyclic efix duation: 6.67 µs Rician Facto: K = Maximum Dopple hift: 5 Hz atellite-eath tation distance: 35 Km atellite ath Loss: 8 db Figue 9 shows the OFDM spectum of a Q OFDM Uplink and Downlink signals; and the HA Effects on the OFDM spectum. OFDM pectum Fist ymbol HA Effects on the OFDM pectum Uplink ignal Downlink ignal Fig. 7. Both NRs (at destination and elay) ae low VIII. IMULATION REULT The simulation esults ae divided in two pats. The fist pat is elated to hase I tansmission between the atellite and the Relay node. The second pat is elated to the teestial link between Relay node and Destination node. Figue 8 shows the simulation block diagam fo hase I. Fo the simulation we used OFDM 6QAM and OFDM Fequency Fig. 9. Q OFDM atuation Level = db Relative to AM Aveage As can be seen, the Downlink pectum is seveely attenuated. This can be poblematic especially fo highe ode modulation schemes making that the bit eo ate at the eceive pefom pooly. The HA esponse compaed to the satuation level fo the values of Figue 9 is showed in Figue. We can see that the HA Response is vey attenuated compaed to the Refeence Linea value. Also, Figue shows the Q Uplink and Downlink Constellations. 9 ACADEMY UBLIHER

9 JOURNAL OF COMMUNICATION, VOL. 4, NO., NOVEMBER Refeence Response HA Response OFDM Amplitude Refeence Response HA Response OFDM Amplitude Output ignal 3.5 Output ignal Input ignal Fig.. HA Response compaed to Linea Refeence fo Q with atuation Level of db Input ignal Fig. 3. HA Response compaed to Linea Refeence fo Q with atuation Level of 5 db.5 Q Constellation Downlink Uplink.8 Q Constellation Downlink Uplink Q channel -.5 Q channel I channel Fig.. OFDM Q Constellation atuation Level db, K=, ath Loss = 8dB We now incease the atuation Level to 5 db. The effects on the Q OFDM pectum, the HA Response and the Q Constellation ae showed is Figs., 3 and 4, espectively. Note the impovement of the Downlink Q OFDM pectum (in ed) which is close to the Uplink signal (in blue), as well as the impovement of the Constellation and the HA Response. The HA Response with satuation Level of 5 db becomes much close to the Linea Response in the lowe values of the Input ignal axis, making the Downlink pectum and Constellation less distoted and impoving the bit eo ate at the eceive site. The Constellation noise and pectum noise and attenuation ae also due to the Rician Channel and ath Loss Block contibutions I channel Fig. 4. Q OFDM Constellation atuation Level 5 db, K=, ath Loss = 8dB We now pesent the same esults fo the case of OFDM 6QAM satellite link communication with IFFT size of 48. Figs. 5 and 6 and show the simulation esults with atuation Level of db and atellite Channel contibution. Figs. 7 and 8 shows the simulation esults with atuation Level of 5 db and atellite Channel contibution due to the Multipath Rician Model and the ath Loss Block fo 6QAM OFDM. We can note the impovement of the Downlink ignal (in ed) compaed to the Uplink ignal (in Blue) and compaed to the Downlink ignal of Figue HA Effects on the OFDM pectum 4 HA Effects on the OFDM pectum OFDM pectum 5 5 Uplink ignal Downlink ignal OFDM pectum Fist ymbol Uplink ignal Downlink ignal Fequency BW = 5 MHz Fig.. Q OFDM atuation Level 5 db Relative to AM Aveage Fequency Fig. 5. 6QAM OFDM atuation Level db Relative to AM Aveage 9 ACADEMY UBLIHER

10 84 JOURNAL OF COMMUNICATION, VOL. 4, NO., NOVEMBER QAM Constellation.5 Downlink Uplink Q channel I channel -.5 Table. Bit Eo Rate Results Fig. 6. 6QAM OFDM Constellation atuation Level db, K=, ath Loss = 8dB HA Effects on the OFDM ignal OFDM pectum 5 Uplink ignal Fequency BW = 5 MHz.5.5 Fig. 7. 6QAM OFDM atuation Level 5 db Relative to AM Aveage 6QAM Constellation.5 Downlink Uplink.5 Q channel OFDM 6QAM IFFT = 48, BW = 5 MHz atuation Level (db) OFDM Q IFFT = 4, BW = 5 MHz e e e e e..5. e e e 5. e e 6. e e 7. e e. e e. e I channel.5.5 Figues 9 and show the gaphs of Bit Eo Rate fo both Q and 6QAM OFDM ove the satellite channel fo diffeent values of the Rician facto K. Fig. 8. 6QAM OFDM Constellation atuation Level 5 db, K=, ath Loss = 8dB The Table and show the Bit Eo Rate (BER) and Modulation Eo Rate (MER) measues at the eceive as follows. Table. Modulation Eo Rate Results atuation Level (db) OFDM 6QAM IFFT = 48, BW =5 MHz ACADEMY UBLIHER OFDM Q IFFT = 4, BW = 5 MHz Fig. 9. BER values Q OFDM atellite Channel

11 JOURNAL OF COMMUNICATION, VOL. 4, NO., NOVEMBER 9 85 Fig. 3. hase II hysical Laye Fig.. BER values 6QAM OFDM atellite Channel If a Channel Coding Block is added to the imulation the BER esults impove consideably as shown in Figue and. The signal spectum at the output of the elay node and at the input of the destination node shows how the Rayleigh channel affects the oveall fequency distibution. By using a stong eo coection method these impaiments can be ovecome and the esulting (3) is within limits of pefomance. Figue 4 shows the signal spectum and the eye diagam of the tansmitted and eceived signals. The effects of the Rayleigh multipath ove the fequency esponse can be seen at the destination node (in ed). The OFDM signal is attenuated at diffeent fequency components. ince the OFDM signal is composed of seveal individual caies, this uneven attenuation effect is not as destuctive as in a single caie modulation. The individual caies ae theefoe detected ove a small bandwidth. Fig.. BER values Q OFDM using Tubo Code Fig. 4. OFDM spectum and Eye Diagam in hase II E b /N = db Fig.. BER values 6QAM OFDM using Tubo Code The imulation esults fo depend on the E b /N value and the eo coection employed as shown in Figues 5.a and Figue 5.b. As mentioned befoe, hase II will take place when the destination node equests it and NR at the elay is above cetain value. A imulation of hase II consist of the elay node using OFDM 8 and 6QAM, a Rayleigh multipath fading channel and the destination node whee the is measued, as depicted in Figue 3. 9 ACADEMY UBLIHER

12 86 JOURNAL OF COMMUNICATION, VOL. 4, NO., NOVEMBER 9 Fig 5.a. E b /N vs OFDM 8 hase II Fig 5.b. E b /N vs OFDM 6QAM hase II IX. CONCLUION Coopeative atellite Communications will be an impotant pat of futue 4G systems. We need to guaantee a constant tansfe of infomation fom the satellite to the mobile unit, even when the mobile unit tavels into aeas that ae uneachable by the satellite. We think that Decode and Fowad is the best option fo the Coopeating potocol since it is neve outpefomed by the Amplify and Fowad altenative. It is impotant to note hee that even AF is easie to implement than DF. AF does not allow us the flexibility to adapt to bandwidth constains that may be pesent when tansfeing the signal fom the satellite link to the teestial one. OFDM and high modulation techniques such as 8 and 6QAM ae needed in both channels. When the satellitedestination channel is not available, the powe distibution will depend on the channel chaacteistics between the satellite-elay and the elay-destination. To allow bette bandwidth efficiency we think that the combination of elective and Incemental elaying is the best option. As stated in section VI, the tansmission fom the elay to the mobile use will take place only when the mobile use does not eceive the signal fom the satellite, and the signal at the elay node is stong enough. REFERENCE [] V. Mahinthan, H. Rutagemwa, J.W. Mak, and. Xuemin, efomance of Adaptive Relaying chemes in Coopeative Divesity ystems with ARQ, IEEE Global Telecom. Conf. (GLOBECOM), p , Nov. 7. []. Ikki, and M.H. Ahmed, efomance Analysis of Incemental Relaying Coopeative Divesity Netwoks ove Rayleigh Fading Channels, IEEE Wieless Commun. and Netw. Conf. (WCNC 8), pp. 3-35, Apil 8. [3] B. Gui, L. Dai, and L.J. Cimini, elective Relaying in Coopeative OFDM ystems: Two-Hop Random Netwok, IEEE Wieless Commun. and Netwoking Conf. (WCNC 8), pp. 996-, Apil 8. [4] F.A. Onat, A. Adinoyi, Y. Fan, H. Yanikomeoglu, J.. Thompson, and I. D. Masland, Theshold election fo NR-based elective Digital Relaying in Coopeative Wieless Netwoks, IEEE Tansactions on Wieless Com., Vol. 7, Issue, at:, pp , Nov. 8. [5] Y. Wang, and F. Lin, efomance Analysis and owe Allocation fo Amplify-and-Fowad Relay Netwoks, Int. conf. on Wieless Comm. Netw. and Mobile Comp. (WiCom 7), pp , ept. 7. [6] W. u, A.K. adek, and K.J.R. Liu, efomance Analysis and Optimum owe Allocation fo Decode-and- IEEE Fowad Coopeation otocol in Wieless Netwoks, Wieless Communications and Netwoking Conf., Vol., pp , Ma. 5. [7] J.N. Laneman, D.N.C. Tse, and G.W. Wonell, Coopeative Divesity in Wieless Netwoks: Efficient otocols and Outage Behavio, IEEE Tansactions on Info. Theoy, Vol. 5, Issue, pp , Dec. 4. [8].Taasak, H. Minn, and Y.H. Lee, Analysis of Incemental Relaying otocol with RCC in Coopeative Divesity ystems, IEEE 6 nd Vehicula Technology Confeence (VTC 5), Vol. 4, pp , ept. 5. [9] I.Y. Abualhaol, and M.M. Matalgah, Outage obability Analysis in a Coopeative UAVs Netwok Ove Nakagami-m Fading Channels, IEEE 64 th Vehicula Technology Confeence (VTC 6),, pp. -4, ept. 6. [] Ray Liu, K.J., A.K. adek, W. u and A. Kwasinski, Coopeative Communications and Netwoks, Cambidge Univesity ess, IBN , Jan. 8. [] W. u, A.K. adek, and K.J.R. Liu, Coopeative Communications in Wieless Netwoks: efomance Analysis and Optimum owe Allocation, Wieless esonal Comm., Vol. 44, Issue, pp. 8-7, Jan. 8. [] A. Reznik,.R.Kulkani, and. Vedu, Degaded Gaussian Multielay Channel: Capacity and Optimal owe Allocation, IEEE Tansactions on Infomation Theoy, Vol. 5, Issue, pp , Dec. 4. [3] A. endonais, E. Ekip, and B. Aazhang, Use Coopeation Divesity. at I. ystem Desciption, IEEE Tans. on Com., Vol.5, Issue, pp , 3. [4] A. endonais, E. Ekip, and B. Aazhang, Use Coopeation Divesity. at II. Implementation Aspects and efomance Analysis, IEEE Tansactions on Commun., Vol. 5, Issue, pp , Nov. 3. [5] A. Vanelli-Coalli, et al., atellite Communications: Reseach Tends and Open Issues, Int. Wokshop on atellite and pace Com. (IWC), pp. 7-75, ept. 7. [6]. G. Glisic, Advanced Wieless Communications 4G Cognitive and Coopeative Boadband Technology, nd Edition, Wiley, IBN , ept. 7. [7] J. ala, H. Duney, Coase Time Delay Estimation fo e-coection of High owe Amplifies in OFDM Communications, IEEE 56 th Vehicula Technology Confeence (VTC ), Vol. 4, pp ,. 9 ACADEMY UBLIHER

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