analysis of sub-carrier multiplexed radio over fiber … personal communications (2005) 33: 1–20...

Wireless Personal Communications (2005) 33: 1–20DOI: 10.1007/s11277-005-8314-0 C© Springer 2005

Analysis of Sub-Carrier Multiplexed Radio Over Fiber Link for theSimultaneous Support of WLAN and WCDMA Systems

ROLAND YUEN AND XAVIER N. FERNANDORyerson University, Toronto, Ontario, CanadaE-mail: [email protected]; [email protected]

Abstract. The present third generation (3G) wireless technology can provide data oriented applications. However,the bit rate is limited to around 2 Mbps with limited mobility. Today, more applications demand high data rate andreasonable mobility. Therefore, by integrating 3G cellular system and wireless local area network (WLAN), thereis a potential to push the data rate higher. This integration means 3G cellular users can enjoy high data rate at alocation that is within WLAN coverage area. Similarly, WLAN users also can have data services as long as they areunder the coverage of the 3G cellular system. The 3G cellular system has a much larger coverage than the WLAN.In this paper, we present the first step toward an integration of the two systems. This paper presents a fiber-wirelessarchitecture that simultaneously supports the wideband code division multiple access (WCDMA) system and theIEEE 802.11b WLAN. Our approach uses sub-carrier multiplexed (SCM) architecture to combine and transmit 2.4GHz WLAN and 1.9 GHz WCDMA signals through an optical fiber from a central base station (CBS) to a radioaccess point (RAP, single antenna unit). After the fiber, the signals continue to propagate through the air interfaceto respective mobile stations. The WLAN access point is also located at the CBS. For the SCM architecture, weinvestigate three areas: i) the signal to noise ratio of the uplink and the downlink, ii) the cell coverage area for theWCDMA and WLAN systems, and iii) the throughput of the IEEE 802.11b WLAN. Our results show that with upto 2.5 km cell radius, better than 18 dB SNR is possible with 5 km fiber link for WLAN system. Simultaneously,the WCDMA system has at least 18 dB SNR for a cell coverage radius of 8 km. These numbers depend on therelative RF power of each system in the fiber.

Keywords: sub-carrier multiplexed, radio over fiber link, wideband code division multiple access, wireless localnetwork, IEEE 802.11

1. Introduction

The sub-carrier multiplexed (SCM) architecture integrates both the 3G wideband code divisionmultiple access (WCDMA) system and the IEEE 802.11b wireless local area network (WLAN,Figure 1). It uses the SCM technique to carry RF signals of both systems through optical fibersbetween a central base station (CBS) and a radio access point (RAP). The optical fiber thatsupports this communication is called the radio over fiber (ROF) link. The RAP operates like anextended antenna from the base station. It transmits and receives the signals of mobile stations(MSs). For the WCDMA system, an MS can be a cellular phone. For the WLAN system,the MS can be a laptop with IEEE 802.11b interface. The SCM architecture is illustrated inFigure 2.

The integration of the two systems is responding to the demands for high data rate appli-cations and reasonable mobility. The employment of the ROF link in the SCM architectureallows reduction in cell size that increases the frequency reuse, thus improves the spectrumefficiency. The RAP with relatively simple functions not only is inexpensive, and its compact

2 R. Yuen and X.N. Fernando

Figure 1. Microcellular architecture that employs radio over fiber link.

Figure 2. The sub-carrier multiplexed architecture.

size can save estate cost. A centralized network can be constructed using ROF links to connectmultiple RAPs to a single CBS. This is illustrated in Figure 1. The advantages of this networkare: i) flexible radio spectrum management, ii) flexible data flow control, iii) sharing the costof the CBS deployment and operation and iv) an efficient handover across systems.

There is significant work done in the wireless access using the ROF link by many authors.Tonguz et al. [1] has investigated the personal communications access networks using SCMROF link. They have derived the carrier to noise ratio of the link that includes the optical noiseand the nonlinear distortion in the link. Moreover, they also investigated the cell coverage ofthe link. Fernando and Anpalagan [2] also studied the ROF link, and they have derived thecumulative SNR for the combination of the ROF link and the air interface. They also studiedthe relationship between the cumulative SNR and the SNR after the optical link. Walker et al.[3] presents the criteria of optimizing the carrier-to-noise ratio of the sub-carrier multiplexedoptical network. The criterion is the optimal choice of an optical modulation index that is usedto modulate a laser. This optical modulation index is dependent on the number of channelsand the nonlinearity of the entire optical link. The relative intensity noise in the ROF link isfurther improved by Fernando in [4] and this improved expression gives a more accurate modelfor the ROF link. In the paper by Kim and Chung [5], they have investigated several passiveoptical networks that support narrow band CDMA signal in microcellular communicationsystem. One of the passive optical networks they investigated employs the SCM technique,and they derived the carrier to noise and distortion ratio for the network. Fan et al., in [6]investigated employment of ROF link in microcellular personal communication system. Theyhave included the fading and the co-channel interference of the air interface to improve theirmodel. They also compared the performance between the uplink and the downlink.

Analysis of Sub-Carrier Multiplexed Radio 3

The performance of WLAN is also dependent on the throughput of IEEE 802.11b. Thethroughput is a medium access control (MAC) layer issue. The throughput might be lower inthe proposed architecture; because of the extra distance (in the fiber) the radio signal has totravel. Many authors have worked to develop an accurate model for the throughput of the IEEE802.11b standard. Bianchi in [7] developed the model using a Markov chain, and assumedunlimited packet retransmissions. Wu et al. in [8] improved Bianchi’s work to include packetretransmission limit. Chatzimisos et al. in [9] further improved the model by including the biterror rate of the channel.

In this paper, we analyse the quality of the sub-carrier multiplexed (SCM) architecturethat provides the third generation WCDMA and the WLAN IEEE 802.11b services as wellas the throughput of the WLAN IEEE 802.11b in this architecture. In the analysis, we derivethe SNR expressions for both the uplink and the downlink at the optical receiver, the RAP,or the MS (see Figure 2). This analysis allows the investigation into the quality of the twosystems in the SCM architecture. It also provides system design parameters: the cell coverageradius and the length of the ROF link for both systems. The throughput analysis focuses onmodifying the parameters in Chatzimisos’s throughput model [9] to suit the SCM architecture.

In section two of this paper, we derive the SNR expressions for both the uplink and thedownlink of the SCM architecture. The SNR expressions include two parameters that areimportant for the system design. These parameters are the cell coverage radius and the lengthof the ROF link. In this section, we also investigate the throughput of the WLAN IEEE802.11b in the SCM architecture. Section three of this provides the numerical results of theuplink SCM architecture. The results include the SNR of the stand-alone ROF link. The threeresults that generated for both systems are: i) the coverage area with respect to the cumulativeSNR of the SCM architecture, ii) and the effect of one system on the other. The last resultis the throughput efficiency of the WLAN IEEE 802.11b in both the SCM architecture andthe normal architecture. We discuss the results in section four. The discussion includes thebehavior of the ROF link, the reason for only considering the uplink, the effect of powerdistribution that affects the systems, and the throughput efficiency of the SCM architecture.

2. Sub-Carrier Multiplexed Architecture

The SCM architecture analyzed in this paper employs a single mode ROF link. The WCDMAand the WLAN IEEE 802.11b signals are transmitted through the ROF link simultaneouslyand provide services to many mobile stations. Figure 1 illustrates the basic architecture ofmicrocellular system that employs the ROF link. Here, a CBS with centralized processing islinked to many remote base stations, also known as the RAP, via fiber. The RAP serves asan extended antenna and provides wireless access to mobile stations. Each RAP can provide(low bit rate) service for large coverage area through the WCDMA interface and high-speedservices for smaller coverage area through the WLAN interface. The RAP consists of simpledevices and its main function is to convert received electrical signals to optical signals andvice versa.

To study the performance of the SCM architecture, we derive the SNR at various locations(see Figure 2) in this section. This section also investigates the cell coverage aea and the lengthof the ROF link of both systems, and the throughput efficiency of the WLAN IEEE 802.11bsystem. The characteristics of the WLAN IEEE 802.11b and the WCDMA signals are listedin Table 1.


Table 1. The characteristics of the WLAN IEEE 802.11b signal and the WCDMA signal

IEEE 802.11b WCDMA

Transmission technology DSSS, FHSS DSSS

Data rate 1, 2, 5.5, 11 Mbps 2 Mbps

Modulation scheme DBPSK, DQPSK, 2GFSK, 4GFSK QPSK

Bandwidth 22 MHz 5 MHz

Frequency range 2.4–2.4835 GHz 2.11–2.17 GHz, 1.92–1.98 GHz

2.1. UPLINK

The uplink starts with transmitted signals from mobile stations (MSs) propagate through theair interface, and then the RAP receives the signals. In the WCDMA system, there are n usersdenoted by i = 1, 2, . . . , n simultaneously sharing the same bandwidth. In the WLAN IEEE802.11b system, there are many MSs trying to access the system. However, the IEEE 802.11bis designed to allow only one user to access at a time. Hence, the whole bandwidth is consumedby one user at a time in IEEE 802.11b. The SNR at the RAP of a i th WCDMA user is given as:

SNR1up, wcdma = Pti /Lwl(ri )

nwl Bwcdma(1)

and the SNR of a WLAN IEEE 802.11b user is given as:

SNR1up, wlan = Pt,wlan/Lwl(rwlan)

nwl Bwlan(2)

where, Pti and Lwl(ri ) are the transmitted RF power and the RF power loss in the air interfaceof the i th WCDMA user respectively, Bwcdma is the WCDMA system bandwidth. Similarly,Pt, wlan, Lwl(rwlan), and Bwlan denote the WLAN user transmitted RF power, RF power loss inthe air interface, and the system bandwidth respectively. The term nwl in the two SNR expres-sions (1) and (2) is the noise and the interference power per unit bandwidth in the air interface,and nwl is different for the WCDMA and the WLAN systems. The interference power of theWCDMA system increases with the number of active users, while the interference power ofthe WLAN system does not increase because only one user transmits at a time. The asyn-chronous transmission of the MSs causes interference among WCDMA users in the uplink.This nature of transmission leads to the near far effect. To avoid the near-far effect powercontrol is assumed, so the power at the RAP,

PtiLwl(ri )

is the same for all n users.The RF power loss in the air interface is expressed as a function of the distance ri or rwlan

between the MS and the RAP; it is modeled by a large-scale propagation model. We modifiedthe expression (13) for the received signal power in [1] to express it in terms of the RF powerloss and the 90% confidence coverage radius that accounts for the statistical power fluctuationin the air interface. The modified expression is given as:

Lwl(R90) = 1

S

(4π R90

λ10(−0.13σ/γ )

)γ

(3)

where, S is the parameter that reflects the shadowing effect, γ is the path loss exponent, λ

is the wavelength of the transmitted signal, and R90 is the 90% confidence coverage radius.


The term 10( −0.13σγ

) comes from the relationship between the 90% confidence coverage radiusR90 and the average coverage radius r [10], where σ and γ are the standard deviation of thechannel loss and the path loss exponent respectively.

At the RAP point, the WCDMA signals are amplified with a power gain of Gup, wcdma tocompensate for the loss in the air interface. Similarly, the WLAN signals are amplified with apower gain of Gup, wlan. Then, the combined signal of both systems modulates the laser. TheRF power of the n WCDMA users that modulates the laser is given as:

PRF, wcdna = Gup, wcdma

n∑i=1

Pti

Lwl(ri )

= m2wcdma P2

o

2(4)

Similarly, the RF power of the WLAN user is given as:

PRF, wlan = Gup, wlan · Pt,wlan

Lwl(rwlan)

= m2wlan P2

o

2(5)

where, mwcdma and mwlan are the system optical modulation indices for the WCDMA andthe WLAN systems respectively, and Po is the average optical power output from the laser.In expressions (4) and (5), there are a factor of 1

2 . This factor comes from the power of thesinusoidal carrier, cos ωct . The total RF power including the biased power that modulates thelaser is given as:

PRF, laser = P2o + m2

wcdma P2o

2+ m2

wlan P2o

2(6)

The mean optical power Po is the same for the two systems because the same bias currentis used for the laser. However, the system optical modulation indices for both systems aredifferent. The square of each system optical modulation index is proportional to the RF powerof the system that modulates the laser. Therefore, the two optical modulation indices (mwcdma

and mwlan) can be used to compare the RF power of both systems in the ROF link. Therelationship between the system optical modulation index and the RF amplifier power gain forthe WCDMA system is given as:

Gup, wcdma = m2wcdma P2

o

2

n∑i=1

Lwl(ri )

Pti

(7)

Where, n indicates this power gain is applied to the whole WCDMA system with n users.Similarly, the relationship between the system optical modulation index and the RF amplifierpower gain for the WLAN system is given as:

Gup, wlan = m2wlan P2

o

2· Lwl(rwlan)

Pt,wlan(8)


The RF amplifier power gain Gwcdma and Gwlan should bring the n WCDMA users and theWLAN user signals received at the RAP up to the optimal laser input level.

The performance of the ROF link is determined by the optimal optical modulation indexmopt that is optimized considering the optical noise and the nonlinear distortion in the ROFlink. The optimal optical modulation index affects the level of total RF power that modulatesthe laser. The total RF power that modulates the laser not including the biased power is equalto the sum of the WCDMA and the WLAN RF powers:

m2opt P

2o

2= m2

wcdma P2o

2+ m2

wlan P2o

2(9)

The relationship of the optimal optical modulation index to the two system optical modulationindices is given as:

mopt =√

m2wcdma + m2

wlan (10)

When the sum of the m2wcdma and m2

wlan is larger than m2opt that means the total power input to

the laser exceed the optimal level, and the nonlinear distortion in the ROF link will becomedominant and limit the overall performance. On the other hand, when the summation is smallerthan m2

opt, then the optical noise in the ROF link is the limiting factor.Finally, the performance of the entire uplink depends on the cumulative SNR at the output

of an optical receiver. For a i th WCDMA user, the cumulative SNR is given as:

SNR2up, wcdma = m2wcdma P2

o /(2Lnop)(nwl

(Gup, wcdma

Lop

) + nop)Bwcdma + nNLD

(11)

This cumulative SNR is divided by n, number of WCDMA users, because the power of anindividual user is the RF power of the whole WCDMA system m2

wcdma P2o

2 divided by the numberof users. The cumulative SNR for the WLAN user is given as:

SNR2up, wlan = m2wlan P2

o /(2Lop)(nwl

(Gup, wlan

Lop

) + nop)Bwlan + nNLD

(12)

where, nop is the optical noise per unit bandwidth, nNLD is the nonlinear distortion, and Lop isthe RF power loss in the ROF link and it had been derived in [2]. The RF power loss in theROF link is given as:

Lop,d B = −20 log(Gm�) + 2(nclc + αd)

Lop = 10(Lop,d B/10) (13)

This expression assumes perfect impedance matching in electronics. It also accounts for fouroptical parameters: the modulation gain Gm of the laser, the responsivity � of a photodetector,nc connectors each has loss lc, and the fiber attenuation α and the fiber length d in km.

The noise in the ROF link are shot noise, thermal noise, and relative intensity noise [4];the nonlinear distortion includes clipping distortion and third order intermodulation distortion.The shot noise and thermal noise are independent of the optical modulation index mopt. The


relative intensity noise originates from the laser is dependent on the square of the photocurrent�2 P2

o and the optical modulation index mopt [4]. The clipping distortion and the third orderintermodulation disortion also depend othe optical modulation index mopt [1]. The square ofthe optical modulation index mopt is proportional to the total RF power that modulates thelaser.

The cell coverage radius for the WCDMA and the WLAN systems in the uplink aredetermined from the cumulative SNR expressions (11) and (12) respectively. The cell coverageradius is the distance ri or rwlan in the cumulative SNR expressions that yields the requiredminimum SNR for the corresponding systems.

2.2. DOWNLI NK

To complete the SNR evaluation of the SCM architecture, we now consider the downlink. Thedownlink is also illustrated in Figure 2. In the downlink, the signals for the WCDMA andthe WLAN systems are first transmitted from the CBS to the RAP, then to the MSs. In orderto simplify the expressions, we assume that the transmitted RF power of n WCDMA usersare the same. The SNR at the optical receiver for a i th WCDMA user is given as:

SNR1down, wcdma = m2wcdma P2

o /(2nLop)

nop Bwcdma + nNLD(14)

and for a WLAN user is given as:

SNR1down, wlan = m2wlan P2

o /(2Lop)

nop Bwlan + nNLD(15)

where, n is the number of WCDMA users, mwcdma is the system optical modulation index ofthe WCDMA system and mwlan is the system optical modulation index of the WLAN system.The term Lop, nop and nNLD are the same parameters used in the cumulative SNR expressions(11) and (12) for the uplink. For the optimal performance, the sum of m2

wcdma and m2wlan should

not exceed the square of the optimal optical modulation index m2opt.

The performance of the entire downlink depends on the cumulative SNR at the MS. TheSNR of a i th WCDMA user is given as:

SNR2down, wcdma =(m2

wcdma P2o /(2nLop)

)(Gdown, wcdma

Lwl(ri )

)(nROF

(Gdown, wcdma

Lwl(ri )

) + nwl)Bwcdma

(16)

and the SNR of a WLAN user is given as:

SNR2down, wlan =(m2

wlan P2o /(2nLop)

)(Gdown, wlan

Lwl(rwlan)

)(nROF

(Gdown, wlan

Lwl(rwlan)

) + nwl)Bwlan

(17)

where, the total noise and distortion power in the ROF link nROF per unit bandwidth isnop + nNLD

Bwcdmafor the WCDMA and nop + nNLD

Bwlanfor the WLAN, Gdown, wcdma and Gdown, wlan

are the RF amplifier power gain at the RAP for the WCDMA and the WLAN systems respec-tively, Lwl(ri ) and Lwl(rwlan) are the RF power loss in the air interface of the i th WCDMA


user and the WLAN user respectively. The RF power loss in the air interface is expressedin expression (3). In the air interface, the interference power of the WCDMA system in thedownlink does not increase with the number of users because the synchronized transmissionkeeps the signals orthogonal. On the other hand, the WLAN is assumed to have only oneuser consuming the channel at a time. Therefore, multiple access interference does not con-tribute to the interference power. However, the co-channel interference does exist for bothsystems.

The downlink cell coverage radius for the WCDMA and the WLAN systems are determinedfrom the cumulative SNR expressions (16) and (17). The cell coverage radius is the distanceri or rwlan in the cumulative SNR expressions that yields the required minimum SNR for thecorresponding system.

2.3. THROUGHPUT AND PACKET DELAY FOR IEEE 802.11B

The IEEE 802.11b standard employs a distributed coordination function (DCF) for the mediumaccess control. The DCF is based on carrier sense multiple access with collision avoidance(CSMA/CA) that provides asynchronous access to the medium with exponential backoff. Here,we briefly describe the DCF basic access method and if readers who want more details shouldrefer to chapter 9 of [11].

When a packet is ready to be transmitted, the station listens to the channel for idle untilthe duration equals a distributed interframe space (DIFS). If the channel is sensed busy beforethe end of the DIFS duration, it will not transmit until the channel is idle for another DIFSduration. After a DIFS, the station backs off for few slot times before transmission. A slottime is defined as the time for a station to detect a transmission from any other station. Thenumber of backoff slot times is uniformly chosen from (0, CW-1) where CW is the currentcontention window. In the first attempt, the current contention window size is at its minimumand it doubles for every unsuccessful transmission until the specified maximum is reached.During the backoff period if the channel is busy, the backoff counter will stop decrementinguntil the channel becomes idle for a DIFS. When the backoff counter reaches zero, the stationwill transmit the packet.

When the receiving station receives a packet, it will transmit a positive acknowledgement(ACK) after a short interframe space (SIFS). The received station can transmit the ACKwithout collision because the immediate waiting time, SIFS, allows the station to access themedium faster. A SIFS time together with the channel propagation time is shorter than a DIFStime. That means the station can grab the medium faster than any other station. A successfultransmission is when the ACK received within the ACK timeout period. An unsuccessfultransmission is when the transmitted station does not receive the ACK or it received a packet.Then, the transmitted station needs to retransmit.

The two interframe spaces, DIFS and SIFS, mentioned above are the time intervals betweeneach frame. They are independent of the bit rate of the station. The duration of interframe spacesare determined by the characteristics of the physical layer. A different modulation implies adifferent physical layer. For a different physical layer, there is a different set of value for theinterframe spaces. For IEEE 802.11 specified in [11], the durations of the interframe spacesare specified at Table 57a for frequency-hopping and Table 59 for direct sequence. The tablesdoes not list the durations for DIFS because DIFS can be determined from a timing relationdefined in [11]. In section 9.2.10 of [11], the timing relations for a slot time and a DIFS time


is given as follows:

aSlotTime = aCCATime + aRxTxTurnaroundTime + aAirPropagationTime

+ aMACProcessingDelay (18)

TDIFS = TSIFS + 2 × aSlotTime (19)

where the values for aCCATime, aRxTxTurnaroundTime, aAirPropagationTime, aMACPro-cessingDelay, TSIFS, and aSlotTime are listed in the two tables mentioned above.

In the SCM architecture, the physical layer has an additional ROF link. This additionalROF link distance increases the propagation time of the signals, so the timing expression for aslot time should be modified to adjust for the change. To modify the expression, a propagationtime through the ROF link is introduced. The propagation time in terms of the fiber core indexof refraction n and the distance of the ROF link d is as follows:

aRofPropagationTime = n

cd (20)

where, c is the speed of light. The new expression for a slot time is given as follows:

aSlotTime = aCCATime + aRxTxTurnaroundTime + δ + aMACProcessingDelay (21)

and

δ = aAirPropagationTime + aRofPropagationTime (22)

where, δ is the total propagation time including the air and ROF link propagation times.Expression (19) for the DIFS time depends on the slot time. For the SCM architecture, theDIFS time is also increased as the slot time is increased. Since the slot time and the DIFS timeare dependent on the distance of the ROF link, for different ROF link distance there would bedifferent values for the DIFS time and the slot time. Therefore, it is possible to assign a set ofDIFS time and slot time values for a range of ROF link distances. It will be shown later thatwhen the slot time is larger than the minimum required, the throughput only slightly reduces.

The saturated throughput model has been derived in [7], then Chatzimisios et al. [9] im-proved it to a more accurate model that includes the bit error rate of the channel and the limitedpacket retransmission. The saturated throughput refers to the system condition where n con-tenting stations always have a packet ready to transmit. The saturated throughput efficiency,S, is given as:

S = Ptr Psl

E[slot]= Ptr Psl

(1 − Ptr)σ + Ptr Ps Ts + Ptr PcTc + Ptr PerTer(23)

Where, Ptr is the probability that at least one transmission occurs in a randomly chosen slottime, Ps is the conditional probability that this transmission is successful, and l is the packetsize. The E[slot] is the average length of a slot time that includes the average duration of anempty slot (1− Ptr)σ where σ is a duration of an empty slot, and the average time that a stationsenses for a successful transmission, a collision, or a transmission error. The probability Pc andPer are the probability of collision in transmission and error in received packet respectively.The time Ts, Tc and Ter are the average time that a station sense busy due to a successful


transmission, a collision, and an error in received packet respectfully. In [9], the authors didnot define those times. However, the authors include [7] as their reference which defines Ts

and Tc. The time Ts and Tc are given as follows:

Ts = TDIFS + H + P + 2δ + TSIFS + TACK (24)

Tc = TDIFS + H + P + δ (25)

where, TDIFS, TSIFS and δ are the time for a DIFS, a SIFS, and a propagation delay respectively,H is the duration of the physical layer header and the MAC layer header, P is the duration ofthe packet payload, and TACK is the duration of a positive acknowledgment. The time Ter canbe derived according to the scenario where an error in received packet. When an error occursin a receive packet, the sending station simply will not receive an acknowledgement and willretransmit after an acknowledgement timeout. Therefore, the time Ter can be given as:

Ter = TDIFS + H + P + TACKTimeout (26)

where TACKTimeout is the acknowledgement timeout.Chatzimisios et al. [9] presents the average packet delay E[D] as E[D] = E[X ]E[slot]

where, E[X ] is the average number of slot times required for a successful transmission.The WLAN signal in the SCM architecture travels the additional distance through the ROF

link. This leads to 25 µs and 50 µs increase in the propagation delay for a 5 km and a 10 kmROF links respectively. This increase does not affect the accuracy of the saturated throughputand the average packet delay expressions. However, the duration for a DIFS and a slot timeshould be determined from the modified slot time expression (21) that is suitable for the SCMarchitecture. With the increase in the DIFS time and the slot time, we can expect a roll backin the throughput and an increase in the average packet delay.

3. Numerical Results

In our calculation, we only consider uplink of the SCM architecture and the reason for thiswill be explained in the discussion section. The values of all parameters that are used in thecalulation are listed in Table 2. The RF power loss in the air interface is calculated fromexpression (3). The RF power loss in the ROF link is calculated from expression (13). The RFoptical noise power nop is calculated from the expressions found in [4], and the expressionsare given as:

nshot = 2q�Po (27)

nth = 4F KbTo

RL(28)

nRIN = RIN �2 P2o

(1 + m2

rms/2)

(29)

where, nshot is the shot noise, nth the thermal noise and nRIN the relative intensity noise. Inexpression (27), q is the electron charge, � is the responsivity of the photodiode, and Po is theaverage optical power at the photodiode. In expression (28), F is the noise figure of the opticalreceiver, Kb is Boltzmann’s constant and RL is the load resistance of the optical receiver. In


Table 2. Parameters used in the numerical results

Pti , Pt,wlan MS transmission power 10 dBm

nwl Noise and interference power 10−25 W/Hz

γ Path loss exponent 2

σ Standard deviation of channel gain 4dB

S Shadowing effect parameter 0.01

λ Average radio signal wavelength 0.15 m

Bwcdma Bandwidth of WCDMA 5 MHz

Bwlan Bandwidth of WLAN 22 MHz

Po Laser mean optical power 1 mW

mopt Optimal optical modulation index 0.107, 0.124, and 0.150optimized for 1 km, 5 km, and 10 km

Gm Laser modulation gain 0.12 A/W

� Photo diode responsivity 0.75 W/A

nc Number of optical connectors 2

lc Optical connector loss 1 dB

α Fiber attenuation 0.5 dB/km

RIN Relative intensity noise parameter −155 dB/Hz

To Optical receiver temperature 275 K

RL Receiver load resistance 50 �

F Receiver amplifier noise factor 1

a3 Third-order nonlinearity parameter 0.2

expression (29), RIN is the relative intensity noise parameter. The root means square (RMS)optical modulation index mrms in expression (29) is the RMS of all the individual system opticalmodulation indices. Under optimal condition mrms in expressions (30)–(32) is the same as mopt

in (10). The clipping distortion ncl and the third order intermodulation distortion n3OI in theROF link together give the total nonlinear distortion nNLD. The two distortions in [1] are givenas:

ncl = 1

10

√2

π�2 P2

o m5rms exp

(− 1

2 m2rms

)(30)

n3OI = 243

16�2 P2

o a23m6

rms (31)

Figure 3 shows the RMS optical modulation index mrms versus the SNR of the ROF linkalone. This figure is generated using the following expression:

SNR = m2rms P2

o /(2Lop)

nop(Bwcdma + Bwlan) + nNLD(32)

where, mrms is the RMS optical modulation index that varies from 0.01 to 1, and Lop is the RFpower loss in the ROF link that depends on the length of the ROF link. The three SNR curvesare generated for 1, 5 and 10 km ROF links.


Figure 3. Optical modulation index versus the SNR of 1, 5, and 10 km ROF links.

Figure 4. Cumulative SNR of the uplink and downlink versus the received power at the RAP (uplink) or the MS(downlink). In the uplink, the ROF link is optimized for a received power of −113.6 dBm. In the downlink, theMS is expected to receive −113.6 dBm of power.

Figure 4 illustrates the difference in quality requirement for the uplink and the downlink ofthe SCM architecture. This figure shows the cumulative SNR of the uplink and the downlinkversus the received power at the RAP and the MS respectively. Expressions (11) and (16) areused to generate the uplink and the downlink cumulative SNRs respectively. The calculationassumed the ROF link is 5 km and the cell coverage radius is 1 km. In the uplink, the ROFlink is optimized for a received power of −113.6 dBm. In the downlink, the MS is expectedto receive −113.6 dBm of power.

Figure 5 shows various uplink SNR of a i th WCDMA user versus the cell coverage radius.There are ten users assumed in the WCDMA system, the same is also assumed for Figures 7–10.The four SNR curves are the SNR at the RAP, and the cumulative SNR for 1 km, 5 km and10 km ROF links. Figure 6 shows the same set of SNR curves versus the cell coverage radius,but the curves are the SNR of the WLAN user. Both systems modulate the laser with the same


Figure 5. Cell coverage radius versus various uplink SNR of the i th WCDMA user. The WCDMA system modulatesthe laser with 50% of the optimal RF power (0.5(m2

opt P2o /2)). So, the system optical modulation index of WCDMA

is mwcdma =√

0.5 m2opt.

Figure 6. Cell coverage radius versus various uplink SNR of the WLAN user. The WLAN system modulates thelaser with 50% of the optimal RF power (0.5(m2

opt P2o /2)). So, the system optical modulation index of WLAN is

mwlan =√

0.5 m2opt.

RF power. The RF power is 50% of the optimal RF power (0.5(m2opt P

2o /2)), so the system

optical modulation indices are mwcdma =√

0.5 m2opt and mwlan =

√0.5 m2

opt. The SNR at theRAP for the i th WCDMA user and the WLAN user are generated from expressions (1) and (2)respectively. The cumulative SNR for the i th WCDMA user and the WLAN user are generatedfrom expressions (11) and (12) respectively. Figures 5 and 6 are generated with the same scalefor the ease of comparison across systems.

Figure 7 illustrates the SNR curves from a different perspective. It shows the length of ROFlink versus the uplink cumulative SNR. This figure has six SNR curves where three of themare the cumulative SNR of a i th WCDMA user and the other three are the cumulative SNRof the WLAN user. Both systems modulate the laser with same RF power. The RF power is50% of the optimal RF power (0.5(m2

opt P2o /2)). The set of three SNR are generated for a cell

coverage radius of 100 m, 500 m, and 1 km using the expressions (11) and (12).


Figure 7. Length of ROF link versus three different uplink cumulative SNR of the cell coverage radius: 100 m, 500m, and 1 km. Both systems modulate the laser with the same RF power. The RF power is 50% of the optimal RF

power (0.5(m2opt P

2o /2)), so the system optical modulation indexes are mwcdma =

√0.5 m2

opt and mwlan =√

0.5 m2opt.

Figure 8. Cell coverage radius versus cumulative SNR. The SNR are generated for ROF link distance of 1, 5, and10 km. The WCDMA and the WLAN systems modulate the laser with 90% and 10% of the optimal RF power

respectively. The system optical modulation index for the WCDMA is mwcdma =√

0.9 m2opt and the WLAN is

mwlan =√

0.1 m2opt.

Figure 8 illustrates the changes to the cumulative SNR when the WCDMA and the WLANsystems modulate the laser with 90% and 10% of the optimal RF power respectively. Inthis figure, there are six cumulative SNR curves versus the cell coverage radius. Three ofthe cumulative SNR curves are the cumulative SNR of a i th WCDMA, and other three arethe cumulative SNR of a WLAN user. The set of three cumulative SNR curves are gener-ated for 1, 5, and 10 km ROF links. The system optical modulation index for the WCDMAis mwcdma =

√0.9m2

opt and the WLAN is mwlan = 0.1 m2opt. The cumulative SNR curves are

generated using expressions (11) and (12).Figures 9 and 10 illustrate how the RF power that modulates the laser of one system can

affect the other system. The figures show the cumulative SNR to the RMS optical modulation


Figure 9. Cumulative SNR to the RMS optical modulation index mrms. The WLAN system modulates the laserwith 50% of the optimal power, while the WCDMA system varies its RF power from 10–200% of the optimal

power. mrms is the RMS of the individual system optical modulation index. It is given as√

0.5 m2opt + Pwcdmam2

opt

where Pwcdma ε [0.1, 2.0] and mopt is 0.124.

Figure 10. Cumulative SNR to the RMS optical modulation index mrms. The WCDMA system modulates the laserwith 50% of the optimal power, while the WLAN system varies its RF power from 10–200% of the optimal power.

mrms is the RMS of the individual system optical modulation index. It is given as√

0.5 m2opt + Pwlanm2

opt wherePwlan ε [0.1, 2.0] and mopt is 0.124.

index mrms. The cumulative SNR curves are calculated for a 100 m cell coverage radiusand a 5 km ROF link. In Figure 9, the WLAN system modulates the laser with 50% ofthe optimal RF power, while the WLAN system varies its RF power from 10% to 200%

of the optimal RF power. The RMS optical modulation index mrms is√

0.5m2opt + Pwcdmam2

opt

where Pwcdma ε [0.1, 2.0]. Figure 10 shows the opposite by keeping the modulated RF power ofthe WCDMA system constant at 50% of the optimal RF power and the WLAN system variesthe RF power from 10% to 200% of the optimal RF power. The RMS optical modulation

index mrms in Figure 10 is√

0.5m2opt + Pwlanm2

opt where Pwlan ε [0.1, 2.0]. The cumulativeSNR curves are generated using expressions (11) and (12). Figures 9 and 10 are generatedwith the same scale for the ease of comparison.


Figure 11. Saturated throughput efficiency of the SCM architecture and the normal IEEE 802.11b architecture.The SCM architecture has a 10 km ROF link.

Figure 11 shows the saturated throughput efficiency of the WLAN IEEE 802.11b versus thenumber of contenting stations. The throughput effciency is the throughput bit rate normalizedby the bit rate that is provided by the physical layer. The two throughput curves in Figure 11 arethe WLAN IEEE 802.11b throughput in the SCM architecture and in the normal WLAN archi-tecture that only has an air interface. According to Table 59 of [11] for direct sequence physicallayer characteristics, the normal WLAN architecture has a slot time of 20 µs and a SIFS time of10 µs. Using timing relation in (19), the DIFS time is 50 µs. For a SCM architecture with a 10km ROF link, the SIFS time stays the same, while the slot time is calculated using the modifiedexpression (21). The corresponding slot time and DIFS time are 70 µs and 150 µs respectively.The throughput curves are generated first by solving two nonlinear expressions (1) and (2) in[7] numerically, then uses expressions (23)–(26) to calculate the throughput efficiency.

4. Discussion

In this section, we discuss the role of the RMS optical modulation index, the quality of theuplink and the downlink, the effect of one system on the other, and the saturated throughputof the IEEE 802.11b in the SCM architecture.

The quality of the ROF link is closely related to the RMS optical modulation index. Thisoptical modulation index is the RMS of the sum of individual system optical modulationindices. The RMS optical modulation index limits the amount of RF power that modulatesthe laser. When only considering the optical noise, a large SNR can be achieved simply byincreasing the RMS optical modulation index. However, as this optical modulation indexpasses its optimal level, the nonlinear distortion becomes dominant and degrades the SNR.Figure 3 illustrates such behavior. This figure also shows there is an optimal optical modulationindex mopt for each SNR curves that gives the highest SNR. These optimal optical modulationindices are 10.7%, 12.4% and 15.0% for 1 km, 5 km and 10 km ROF links respectively. Theoptimal optical modulation index increases with the length of the ROF link, which means morepower can be modulated the laser for a longer link distance. However, the SNR decreases withthe length of the ROF link, even with the optimal optical modulation index. This is becauseof fiber attenuation.


The reason for only considering the uplink of the architecture is due to the quality of theuplink is more demanding than the downlink. Generally, the quality of the SCM architectureis limited by the lower SNR of the two interfaces; the ROF link and the air interface. However,depending on which interface goes first, the quality can be different. In the downlink, theWCDMA and the WLAN systems modulate their signals to the optimal level of the ROF link,and then signals propagate through the air interface to MSs. The quality of the downlink isdetermined from the cumulative SNR at the MS. In the uplink, signals first propagate throughthe air interface, then through the ROF link. The quality of the uplink is determined from thecumulative SNR at the optical receiver. The air interface has large power fluctuation because ofthe shadowing effect and the fading effect. We can observe from Figure 4 that the cumulativeSNR of the uplink exhibits the same shape as the SNR of Figure 3. This SNR curve shows thatif the received power is larger or smaller than the optimized received power of −113.6 dBm,it results in a lower SNR. The SNR curves of the downlink is very much proportional to thereceived power at the MS. Comparing the uplink and the downlink SNR curves, there is only asmall received power range from −114.8 dBm to −113.6 dBm where the uplink SNR is betterthan the downlink SNR, and everywhere else the downlink SNR is better than the uplink.The range of received power in Figure 4 is well within the typical power fluctuation of theair interface. Therefore, the uplink, where the air interface follows by the ROF link is moredemanding than the downlink. We can conclude that the quality of the uplink limits the qualityof the whole SCM architecture, thus most of the calculations consider only the uplink.

The performance of the SCM architecture for the WCDMA and the WLAN systems can beobserved from the SNR curves of the uplink in Figures 5–8. In Figure 5, the three cumulativeSNR curves (SNR2up, wcdma) follow the shape of the SNR at the RAP (SNR1up, wcdma) after 4km of the cell coverage radius. That means when the cell coverage radius is larger than 4 km,the SNR of the air interface is the limiting factor. When the cell coverage radius smaller than4 km, the SNR of the ROF link is the limiting factor. The similar trend is even more clearlyshown in Figure 6 where the cumulative SNR (SNR2up, wlan) is closely resemble the SNR atthe RAP (SNR1up, wlan) for the cell coverage radius greater than 3 km. Beyond the 3 km cellcoverage radius, the length of the ROF link does not have much effect on the SNR. That meansthe SNR of the air interface is the limiting factor. When comparing the two systems, the SNRof a i th WCDMA user is better than the SNR of a WLAN user for the cell coverage radiuslarger than 4 km. This is because the i th WCDMA user has a smaller bandwidth, so the noiseand the interference power is also smaller and results in a better SNR than the WLAN user. Inother words, the WCDMA system can provide a larger coverage radius in this case.

Figure 7 gives insights to the WCDMA and the WLAN system performances versus thelength of the ROF link as well as the performance versus the power distribution of systems.From this figure, we observe a general decline of the cumulative SNR curves as the lengthof the ROF link increases. Although the power that modulates the laser for both systems areequal, Figure 7 shows the SNR of the WCDMA user is 3 to 5 dB lower than the SNR of theWLAN user. This is because there are ten users in the WCDMA system who share the availablepower that modulates the laser, while the WLAN system has only one user at a time in thesystem. Figure 8 shows a fairly even power distribution among all the users in both systems.The WCDMA system with ten users modulates the laser with 90% of the total optimal RFpower and the WLAN system modulates 10% of the total optimal RF power. For the cellcoverage radius within 1 km, the SNR of the WCDMA user is about 2 dB better than the SNRof the WLAN user. As the cell coverage radius increases, the margin between the SNR ofthe WCDMA user and the SNR of the WLAN user also increases. For a large cell coverage


radius, the SNR of the WLAN user always has a lower SNR because the larger bandwidth ofthe WLAN user significantly reduces its SNR in the air interface. If a system needs a largercoverage area, then a larger RF power needs to be transmitted.

In Figure 9, the SNR for the WCDMA user peaks at around 13% of the RMS optimalmodulation index which is closed to the optimal optical modulation index of 12.4% for a 5 kmROF link. The cumulative SNR curve of the WLAN user is always decreasing because thepower that modulates the laser keeps constant while the noise and distortion power increasesdue to the increase of the total power that modulates the laser. The rate of the WLAN SNRdecline is larger when the RMS optimal modulation index passes the optimal optical modu-lation index of 12.4%. The increase in the decline rate is due to the nonlinear distortion ofthe ROF link. In Figure 10 where the WCDMA system RF power that modulates the laser isconstant, there is a much sharper decline in the SNR of the WCDMA user. The sharp declinein the SNR is because the power of individual WCDMA user is small compares to the noiseand the distortion power brought by the power of the WLAN system. When the signal powerof a user is small, the power of the other system can have large effect on the SNR of that userjust as the WCDMA user in Figure 10. Moreover, the best SNR of the WLAN user does nothappen at the optimal optical modulation index, and the SNR curve is relatively flat comparedto the SNR curve of the WCDMA user in Figure 9. This is mostly due to the larger power ofthe WLAN user.

The IEEE 802.11b router is located at the CBS, and the additional distance to the RAPincreases the collision detection time, and Figure 11 shows the reduction in the throughputof the WLAN IEEE 802.11b system is about 0.01 of saturated throughput efficiency. This isvery low even for significantly long (10 km) fiber. Therefore, reduction in throughput is not aconcern with this scheme.

5. Conclusion

In this paper we have analyzed a SCM architecture that supports both WLAN IEEE 802.11band WCDMA services. This architecture employs ROF links with centralized processing inthe CBS. SCM refers to the transmission of RF signals from both systems through the fiberin frequency multiplexed manner. The main advantage of this architecture is having relativelysimple single antenna RAP that translates to lower deployment cost. Our numerical analysisindicates that this system has the potential to support both WLAN and WCDMA signals fromreasonably long fibers into reasonable size cells. For example considering 5 km ROF link, it ispossible to have better than 18 dB SNR within 2.5 km cell radius for WLAN system and, 8 kmcell radius for WCDMA system simultaneously. The cell sizes can be varied by changing therelative RF power through the fiber.

In the SCM scenario, WLAN system throughput should also be analyzed because of theadditional delay involved in the ROF link. However, the reduction in the throughput is notseemed significant from our analysis. The reduction is about 0.01 in saturated throughputefficiency for 10 km ROF link. This is because the effect of propagation delay on the distributedinterframes space (DIFS) is very small.

References

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4. X.N. Fernando, “An Improved Expression for Dynamic Relative Intensity Noise in Radio over Fiber Applica-tions,” Under review to be published in IEEE Transactions on Communications, 2004.

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Roland M.C. Yuen received a Bachelor of Electrical Engineering degree in 2003 from RyersonUniversity, Toronto, Canada. He is pursuing a Master of Applied Science degree at RyersonUniversity. He has a conference paper. His research interests are in the area of optical andwireless communications. Currently, he works on unique fiber based architecture to extendthe capability of cellular networks and support wireless LANs simultaneously.

Xavier N. Fernando (http://www.ee.ryerson.ca/∼fernando) obtained B.Sc. Eng. (First ClassHonors) degree from Sri Lanka, where he was first out of 250 students. He got Master’s degree


from the Asian Institute of Technology (Bangkok) Ph.D. from the University of Calgary,Canada in affiliation with TRLabs. He has worked for AT&T for three years as an R&DEngineer. Currently he is an Assistant Professor at Ryerson University, Toronto, Canada.

Dr. Fernando one US patent and about 38 peer reviewed publications in journals andconference proceedings. His research focuses on signal processing for cost-effective broadbandmultimedia delivery via optical wireless networks. Dr. Fernando’s work won the best researchpaper award in the Canadian Conference of Electrical and Computer Engineering for theyear 2001. His student projects won both the first and second prize at Opto Canada – theSPIE regional conference in Ottawa in 2002. He is a senior member of IEEE, member ofSPIE, Chair of the IEEE Communications Society Toronto Chapter and licensed ProfessionalEngineer in Ontario, Canada. He has many research grants including Canadian Foundationof Innovations (CFI), Ontario Innovations Trust (OIT) and Natural Sciences and EngineeringResearch Council (NSERC) of Canada.

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