A Multi-frequency TDMA/TDM System for a VSATTerminal Network Operating in Ka Band

Nedo Celandroni and Erina Ferro

Abstract: FH-JFDA (Frequency Hopping-Jitter Free Demand As-signment) is a satellite access scheme suitable for VSAT systemsand designed for the Ka band (20–30 GHz). It uses the FH-TDMA(Frequency Hopping-Time Division Multiple Access) technique onthe in-bound link towards the hub station, and the TDM (TimeDivision Multiplexing) technique on the out-bound link, from thehub towards the traffic stations. Each traffic station is allowed totransmit to the hub in temporal slots assigned by the hub itself, gen-erally hopping among different TDMA carriers at each data bursttransmission. The hub station retransmits the data received by us-ing two TDM carriers modulated at different bit rates. It tries tomaintain the same temporal position of the real time data packetswhen it switches between the two carriers in order not to intro-duce any further jitter on data. The fade countermeasure systemadopted is based on the up-link power control and on data bit ratechanging.

This paper describes the FH-JFDA MAC protocol, the algo-rithm for assigning the transmission temporal slots, the algorithmadopted by the hub station for mapping the TDMA transmissionsonto the TDM links, and the fade countermeasure technique used.The results of the investigation on one particular parameter of thesystem, which influences the choice of the TDMA assignment pro-cedure, are also presented. A possible realization of the system us-ing two transponders of the Italsat National coverage payload isalso included in the paper, and the performance of the fade coun-termeasure system is evaluated.

Index Terms: FDMA/TDMA satellite allocation scheme, multime-dia traffic, jitter control, rain fade countermeasure.

I. INTRODUCTION

The FH-JFDA (Frequency Hopping-Jitter Free Demand As-signment) is designed for a network of VSAT terminals,equipped with SSPA (Solid State Power Amplifier) technology,which in TDMA access a bent pipe geostationary satellite link.A more powerful hub station retransmits in TDM the packetsreceived from the TDMA channels. In fact, in a traditionalVSAT network the traffic stations cannot actually communicatewith each other directly; this is only possible via a more pow-erful hub station towards which the data transmissions from thetraffic stations are concentrated and then redistributed (doublehop). We look at a terrestrial hub station which acts as a chan-nel dispatcher. However, the design of the access scheme is

Manuscript received October 25, 2000; approved for publication by HyungJin Choi, Division II Editor, May 23, 2001.

The authors are with CNUCE Institute of National Research Council (C.N.R.),Via Moruzzi 1, 56010 Pisa, Italy, e-mail: nedo.celandroni@cnuce.cnr.it,erina.ferro@cnuce.cnr.it.

This work was supported by the Italian National Research Council (C.N.R.)under the 5% Multimedia Programme.

inbound links

TDMA carriers

trafficstation #1A

LAN

outbound linksTDM carriers

HUBstation

A

A

trafficstation #n

traffic

statio

n #2

LAN

LAN

A

A= local application

Fig. 1. Network configuration.

also suitable for an on-board hub, which would reduce the end-to-end delay by a round trip time (an RTT is equal to about aquarter of second). FH-JFDA is based on an FH-TDMA (Fre-quency Hopping-Time Division Multiple Access) technique onthe in-bound link from the traffic stations towards the hub (burstmode transmissions), and on a TDM (Time Division Multiplex-ing) technique on the out-bound link, from the hub towards thetraffic stations (continuous mode transmission). A traffic sta-tion is assumed to aggregate different kinds of traffic generatedby real-time (stream) and non real-time (datagram) applicationsdirectly running on the traffic station itself or on hosts of an at-tached LAN. The hub itself, other than working as a repeater ofthe data received on the TDMA links, can work as a traffic sta-tion as well. The hub differs from the other stations because itdoes not use any TDMA link for the transmissions. It uses twoTDM carriers: the normal carrier, for sending data to stationsin clear sky conditions, and the assisting carrier, at a bit ratelower than the normal carrier bit rate, for sending data to thosestations which are experiencing a severe rain attenuation on theout-bound down-link. Fig. 1 depicts the network configuration.

The multimedia data transmissions towards the hub occur intemporal slots assigned by the hub, each one generally on differ-ent TDMA carriers (frequencies). The frequency hopping fea-ture allows us to divide the system capacity into a number of

channels, so that the traffic stations can be downsized with re-spect to a pure TDMA system. A traffic station cannot, how-ever, transmit on different TDMA carriers in the same temporalslot because it is assumed to have only one modulator. Once apattern of temporal slots has been assigned on certain TDMAfrequencies to a traffic station for its real-time transmissions,this pattern must be maintained until the transmission session iscompleted, thus avoiding the introduction of data jitter on thein-bound link. On the out-bound link, the hub must maintainthe temporal positions of the real-time allocations even whenswitching between the two TDM carriers, which occurs depend-ing on the fade level of the destination station. This jitter-freetransmission of the real-time data, which reduces the overallend-to-end delay of the real-time applications, is the main char-acteristic of the FH-JFDA algorithm. On the other hand, as far asthe non real-time transmissions are concerned such as the trans-missions of unspecified bit rate (UBR) and available bit rate(ABR) traffic, the hub assigns the temporal slots where possi-ble, on any TDMA slot/frequency, after the assignment for thereal-time transmissions is completed. This generally leads to avariable delay.

The FH-TDMA system is being developed on a modem andcontroller provided by Marconi Communications in Italy. It willbe employed on the Italsat satellite in the framework of the 5%Multimedia Program funded by the Italian National ResearchCouncil (C.N.R.). The current implementation is limited dueto several hardware constraints which are not considered in thegeneral algorithm presented here.

The paper is organized as follows. Section II describes howto access and quit the satellite network. Section III describes thetemporal assignments on the TDMA links and presents the mostsignificant simulation results with regard to the choice betweentwo different allocation procedures. Sections IV and V describethe TDMA/TDM mapping, the signaling and the fade counter-measure technique adopted, particularly important when trans-missions occur in the Ka band. Section VI presents a case studyof a possible realization of the system by using two transpon-ders of the Italsat national coverage payload. Section VII sum-marizes the conclusions of the work done.

II. ACCESSING/QUITTING THESATELLITE NETWORK

In order to access the satellite network to begin a transmissionsession, a traffic station sends a session entry message (SEM) tothe hub which contains the first stream and/or datagram request.In each TDMA frame, or multiple of it, only a few slots (entryslots) on a channel are available to open a transmission session.Each entry slot is divided into m sub-slots, which are accessedby the traffic stations in S-Aloha mode. The time position andfrequency of the entry slots are known by the traffic stationsby listening to a reference burst (RB) sent by the hub on boththe TDM carriers at constant time intervals. We assume thatthe length of the time frame used for the TDMA transmissionsis also used by the hub as a time interval for sending both thereference burst and all the data received during a TDMA frame.Fig. 2 shows the time relationship between the TDMA frameand the TDM frame.

RTT HUB

TRAFFIC STATION

time

timeTDMA frame length

TDM frame length

RB RB

TX

RX

1 2 3

1 2 3

A B C

TX

A B C

TX

D E F

RX

B CX Y

X Y

D E F

RX

RTT RTT

TX TX

RX RX

A B C

ARB

RB RB RB

Fig. 2. TDMA and TDM frames.

After obtaining the first allocation, any further allocation re-quest is made by using the relevant fields in the satellite headerthat precedes each data transmission.

The hub deallocates the TDMA slots assigned to a specifiedapplication of a station in two ways:

1. Explicit declaration: The traffic station explicitly sends adeallocation request for a specific application.

2. Timeout: The hub does not receive any data from a trafficstation for a certain period of time. In this instance, allthe slots assigned to all the applications of that station aredeallocated.

In both cases, the slave station must repeat the “first access”procedure in order to restart any further data transmissions.

III. THE TDMA LINKS

A. Real-time Traffic Requests

A portion � of the TDMA system capacity Ctot is devoted totransmitting real-time traffic, where Ctot � ks [slot/frame], k isthe number of TDMA carriers in the system, and s the numberof slots in each TDMA frame. The TDMA frame is a repeatedinterval of time, where s temporal slots can be accommodated ineach carrier; the slot is the temporal interval needed to transmita data burst, which generally contains a packet fragment (codeddata + overhead). In one slot per frame (spf) the basic capacityunit (BCU, equal to b bps) for the constant bit rate (CBR) real-time traffic can be transmitted. Each request must be a multipleof b. Variable bit rate (VBR) traffic can be supported by adopt-ing a bandwidth booking option such as the one described in [1].The constraint on the jitter avoidance implies that each stationsends one allocation request to the hub for each real-time ap-plication. In fact, each application must have its individual slotpattern in order not to cause a shift in time to the other applica-tions when it stops.

Each real-time allocation request sent by a traffic station ito the hub is expressed as a number of slots per frame: R i ��Ci�b��, where Ci is the capacity needed by the station i, and�x�� indicates the integer that rounds up the value of x. Thehub organizes the real-time requests in a FIFO queue, like thedatagram requests, but the real-time requests queue is analyzedand served first. If the request for a real-time transmission isaccepted, the allocation is equal to the request itself. A real-time allocation request is accepted provided that the followingcall admission control (CAC) procedure is satisfied:

1. Station capacity: The sum of real-time requests comingfrom a station must not exceed the capacity of a wholechannel, i.e., s slots per frame,

XRi � s� (1)

2. Space availability: The current request (Rc), when addedto all the previous real-time assignments (Aj � j �

�� � � � � c��), must not exceed the fraction (�) of the totalsystem capacity devoted to real-time traffic:

Rc � �Ctot �

c��X

j��

Aj � (2)

3. Allocatability: Rc available slots must be allocated with-out a time overlap with any other slot already allocated tothe requesting station. The satisfaction of this conditiondepends on the allocation algorithm used. In the follow-ing, two different algorithms are proposed.

For each request coming from station i, that satisfies condi-tions (1) and (2), the hub checks whether or not all the slots toallocate are usable. A slot y is defined as usable by station i ona TDMA channel CZ if both the following conditions are veri-fied: a) no station occupies the slot y on CZ; b) station i doesnot occupy the slot y on any other channel.

If the number of usable slots is lower than the number of slotsrequested, the request is refused; if it is equal, the requested us-able slots are allocated on the relevant channel; if it is greater,the usable slots are allocated according to the assignment al-gorithm. The following two variations of the assignment algo-rithm have been simulated for various values of the � parameterin order to find the most efficient one in terms of call blockingprobability of the real-time applications:

1. Minimum occupancy (MO): The slots that are occupiedthe least by other stations are allocated to station i. Thisprocedure maximizes the allocation probability for anysubsequent request; nevertheless, when the parameters in-volved are big (i.e., the number of stations, slots, chan-nels), this operation can take up considerable computa-tional time.

2. Random allocation (RA): The slots are assigned startingfrom the last completed assignment, which currently hasa random position. This procedure is faster than the MOfrom a computational point of view, but it generally re-duces the allocation probability.

B. MO and RA Comparison & Simulation Results

In all the simulations done, both inter-arrival time and du-ration of the connections are exponentially distributed. Fig. 3shows the probability of violating the space availability condi-tion (no space line), and of the allocatability condition (impos-sible line), together with the total blocking probability, which isthe sum of the first two probabilities, for both the MO and theRA algorithms, for � � �, and � � ���, respectively. The num-ber of slots per frame of each request is uniformly distributedbetween 1 and 16. The impossible curves are generally higherfor the RA algorithm and the difference from the MO algorithm

0.0001

0.001

0.01

0.1

Blo

ckin

g P

roba

bilit

y

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Request [Slots/frame]

= 1load = 324 [Erlang]16 spf24 channelsuniform distribution

total block MO

impossible MO

no space MO

total block RA

impossible RA

no space RA

α

(a)

0.0001

0.001

0.01

0.1B

lock

ing

Pro

babi

lity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Request [Slots/frame]

total block RA

impossible RA

no space RA

total block MO(no space)

α=0.9load=295 [Erlang]16 spf24 channelsuniform distribution

(b)

Fig. 3. No space, impossible allocation, and total blocking probabilitiesversus request capacity (requests with uniform distribution), for � �

� (a) and � � ��� (b). Up to 16 spf requested.

decreases with �. The total blocking probability is lower for asmall number of slots per frame and higher for a large number ofslots per frame in the RA case than in the MO algorithm. We re-peated the tests by using a distribution of the requests’ frequencyinversely proportional to the value of the request itself (Fig. 4).We also extended the dynamics of the request’s values up to 32slots per frame with requests’ uniform distribution (Fig. 5). In

0.0001

0.001

0.01

0.1

Blo

ckin

g P

roba

bilit

y

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Request [Slots/frame]

α = 1load = 346 [Erlang]16 spf24 channels1/n distribution

total block RA

impossible RA

no space RA

total block MO

impossible MO

no space MO

Fig. 4. No space, impossible allocation, and total blocking probabilitiesversus request capacity (requests with 1/n distribution), for � � �.

0.0001

0.001

0.01

0.1

Blo

ck in

g Pr

obab

ility

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32

Request [Slots/frame]

α = 1load = 324 [Erlang]32 spf12 channelsuniform distribution

total block MO

impossible MO

no space MO

total block RA

impossible RA

no space RA

Fig. 5. No space, impossible allocation, and total blocking probabilitiesversus request capacity (requests with uniform distribution), for � ��. Up to 32 spf requested.

all the cases (here only reported for � � �), it is evident that theresults are not substantially different from those shown in Fig. 3.

In any case, there is virtually no difference in performance

10

100

ρ

0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1

α

β=0.85 MO

β=0.85 RA

β=0.8 MO

β=0.8 RA

Fig. 6. P����block

�P���block

�� �) versus � for different loading conditions (�).

0.0001

0.001

0.01

0.1

Blo

ckin

g P

roba

bilit

y

225 250 275 300 325 350

Load [Erlang]

16 spf; α=1

16 spf; α=0.93

16 spf; α=0.85

16 spf; α=0.77

1 spf; α=1

1 spf; α=0.93

1 spf; α=0.85

1 spf; α=0.77

MO algorithm

Fig. 7. Blocking probability versus network offered load for various val-ues of � when the MO algorithm is used.

between the two algorithms when the coefficient � is below athreshold which can easily be estimated from Fig. 6, obtainedusing a uniform distribution of the requests from 1 up to 16 slotsper frame. Here the ratio � between the blocking probabilitieswhen a capacity of 16 and a capacity of 1 slots per frame arerequested (� � P

����block

�P���block

), is reported as a function of �for both algorithms and for two different system loading condi-tions. The traffic load (�) is normalized to the �Ctot capacity.Regardless of the loading conditions, it is evident that when us-ing a value of � below about 0.8, it is not convenient to adoptthe MO algorithm, which requires a higher computation in thehub station.

The call blocking probability can be obtained by looking atFig. 7, when the MO algorithm is used and the frequency of therequests is uniformly distributed. The one slot per frame and16 slots per frame curves for various values of � are reported asfunctions of the system loading conditions.

C. Non-real Time Requests

After completion of the real-time assignments in a certainframe, the hub analyses the queue of the non real-time requests.This scanning process ends when the allocation matrix, preparedby the hub station, is full or when there are no more usable slots

for the remaining stations in this queue. When the assignmentprocess is over, the hub prepares the Burst Time Plan (BTP) forthe TDMA transmissions, which is sent in the Reference Burst(RB) at each TDM frame. The RB, other than to broadcast in-formation to all the traffic stations, is sent for synchronizationpurposes. In the preparation of the next allocation matrix thescanning of non real-time requests is resumed from the last posi-tion analyzed in the current frame. The non real-time allocationsgenerally vary on a frame basis.

IV. THE TDM TRANSMISSIONS

In addition to the fully meshed network service of our sys-tem, we also considered using part of the system capacity tosuperpose a star-wise network topology. This double featurenotably extends the possibilities offered by the system. For ex-ample, we can attach the hub to an Internet server, thus allowingdata exchanges among any Internet user and the traffic stations,hereafter also called remote users. Data which transit betweenremote and Internet users via the hub station do not need to beretransmitted; so they are not penalized by a double hop satellitedelay.

At regular intervals of time, equal to the TDMA frame length,the hub transmits the reference burst on both the TDM carriers;therefore, the total TDM capacity must be such to guarantee theretransmission of all the packets received from the traffic sta-tions on the TDMA links (which are ks packets, at maximum),other than the signaling information. When the hub itself is thefinal destination of some data, packets sent on the TDMA chan-nels by the traffic stations do not need to be retransmitted, andtherefore the relevant TDM capacity is left free and can be usedby the hub to send its own data (e.g., Internet traffic) to remoteusers.

The following points show the criteria adopted by the hub formapping packets from TDMA to TDM carriers as the best wayto prevent jitter on the real-time data transmitted by the traf-fic stations. When the hub receives an allocation request for aconnection-oriented real-time transmission, the destination sta-tion is well known. According to the procedure that all the trafficstations must follow to allow the hub to monitor their fade sta-tus (as described in Section V), the hub knows whether or notdata are addressed to a station in fade. If it is, the hub assignsthe requested number of slots to the transmitting traffic station(if the request has been accepted) in the range �k � a � �� k�of the TDMA carriers, thus reserving the range ��� k � a� forthe allocations for stream data addressed to non faded stations.Data received on TDMA channels are scanned after each timeslot in an ascending order of channels, i.e., (ch�, s�), (ch�, s�),� � � (chk, s�), (ch�, s�), (ch�, s�), � � � (chk, s�), � � � (ch�, ss),(ch�, ss), � � � (chk, ss). The real-time packets received on the��� k � a� TDMA frequencies are located directly onto the nor-mal TDM carrier, in the same order as received on the TDMA,and the ones received on the �k � a � �� k� frequencies are puton the TDM assisting carrier (Fig. 8).

This procedure is not applicable for best effort traffic, becausein this case the hub assigns the frequencies and the time slots inthe holes left after the real-time assignments, regardless of thedata destination as it is not known. Thus, a stream packet is

............

............

............

............

...........................................

............

...........................................

............... ... ... ...

... ... ... ...............

S1 S2 S3 Ss

C1

C2

Ck-a

Ck-a+1

Ck

TDMnormal

TDMassisting

frame

TDMA

NORMAL

ASSISTING

Fig. 8. Real-time traffic mapping from TDMA capacity to TDM capacity.

directly routed on the appropriate TDM carrier at the TDMAreceptions’ scanning time, while a datagram packet is queuedby the hub in one of the two datagram queues, one for the nor-mal and one for the assisting transmissions, independently ofthe channel number assigned to it in the BTP, but according tothe fade level of the destination address, which is now known 1.The first packet of one of the datagram queues is then put inthe TDM slot which corresponds to the TDMA position of thepacket just received, according to the type of the TDM carrier(normal or assisting) which is being filled.

In order not to introduce jitter, the hub replaces missing dataon the TDMA links with dummy packets in the appropriate tem-poral positions on the appropriate TDM carrier, if the normaldatagram queue is empty.

In this way, a jitter which is less than one slot may affectthose stream transmissions that are moved back and forth fromthe ��� k � a� and �k � a � �� k� range of TDMA carriers, i.e.,when they are moved to/from normal from/to the assisting car-rier. This is true when the assisting carrier has enough spacein the slots corresponding to the stream applications that needassistance. When the corresponding slots are already occupied,the hub can choose to move packets to different positions, thusintroducing a higher jitter, or to drop the connection, accordingto the quality of service initially required by the user.

In the following, a TDMA/TDM mapping example is re-ported:

Let us assume that 1–4 frequencies are the TDMA channelswhich will be mapped on the normal TDM carrier, and 5, 6 arethe TDMA channels which will be mapped on the assisting car-rier. After the temporal slot 1 has elapsed, the hub has receivedthe following packets:

�If the packet must be broadcast, it is put in the assisting datagram queue if atleast one of the destination stations is in fade.

C1: stream1; C2: stream2; C3: datagram1;

C4: stream3; C5:stream4; C6: empty1 (no data).

The hub performs:

1. Stream1 is put in the first position of the normal carrier.2. Stream2 is put in the second position of the normal carrier.3. Datagram1 is put in the normal datagram queue or in the

assisting datagram queue, according to the fade level ofthe destination station.

4. The first packet of the datagram normal queue is put in thethird position of the normal carrier. If there is no packetin the datagram normal queue, a dummy packet is put inthat position.

5. Stream3 is put in the forth position of the normal carrier.6. Stream4 is put in the first position of the assisting carrier.7. The first packet in the datagram assisting queue is put in

the second position of the assisting carrier. If this queueis empty, the first packet of the datagram normal queue isput there. If this queue is empty too, a dummy packet isput in the second position of the assisting carrier.

8. The same procedure is followed after the temporal slot 2has elapsed, and so on.

In order to ensure an amount of capacity for the hub traf-fic generated by Internet applications without starving remoteusers, there are h s slots per frame on the TDM normal carrierwhich exceed the TDMA capacity.

Data originated by non real-time applications attached to thehub are put in one of the two datagram queues of the hub, ac-cording to the fade level of the destination station, and transmit-ted on the appropriate TDM carrier following the rules for theretransmission of datagram data.

Data originated by real-time applications attached to the hubare put in positions ��k � a � ��� �k � a � h�� of the TDMnormal carrier if transmitted toward stations which do not needassistance. On the other hand, if necessary, they compete withTDMA stream data to occupy the TDM assisting carrier. Avirtual TDMA allocation matrix �h � s� is maintained by thehub, which is scanned by rows for the virtual allocations and isscanned by columns for the routing on the TDM carriers. Thisallows us to reduce the data burstiness which is defined by thepeak-to-mean packet rate ratio. The hub maintains a trace ofthe allocations on the assisting carrier in a �a � s� matrix forits own traffic as well, in order not to allocate TDMA slots onthe assisting carrier in positions already occupied by stream dataoriginated by applications attached to the hub. TDMA data ad-dressed to the hub produce holes on TDM carriers, which arefilled by datagram or dummy packets.

Fig. 9 shows the information transmitted in a TDM frame,where UW is the TDM unique word, frn is the frame number,the out-bound signaling channel contains information explainedin sub-section A, and pkt pad and frame pad are the packet andframe padding, respectively, that allow the alignments with thecorrect timing. The ENTRIES field contains the list of the sta-tions that have started a transmission session. As the hub trans-mits the same RB on both carriers, the traffic stations must beprovided with two demodulators.

UW Reference Burst data data

frn Burst Time Plan(BTP)

ALLOCATIONMATRIX

ENTRIES

.......Frame_pad

SHdataHDR

userdata

Pkt_pad

Pkt_pad

TDM frame length

cks

cks=data checksum

outboundsignalling

Fig. 9. Information transmitted in a TDM frame.

A. Signaling

Signaling is required both by the hub and the traffic stationsfor capacity requests/allocations, and to control the system.� In-bound Signaling (traffic stations � hub): Signaling is

carried out via the satellite header as far as sending re-quests is concerned. When data have to be sent as a replyto the hub’s explicit requests, one or more datagram pack-ets are generated by the specified traffic station or by allthe traffic stations. These packets are transmitted at thefirst opportunity for transmitting datagrams, with a higherpriority given to the packets in the queue of the datagramdata. The hub allocates datagram slots with a priority tothose stations that have to reply to information requestedby the hub itself (via the out-bound signaling channel).

� Out-bound Signaling (hub� traffic stations): Signaling isperformed via the out-bound fields in the reference burst.Possible information of this type concerns requests aboutthe traffic station status, requests to switch off or to re-sume transmissions, the delay correction to compensatefor the satellite shift, the frequency shift of the station,and when to transmit at a lower bit rate on the assignedTDMA carrier.

V. THE FADE COUNTERMEASURE TECHNIQUE

The reference burst also contains information on how data arereceived by each single traffic station in terms of signal qualityand power level. Each traffic station uses this information toadjust its HPA output, in order to reach the transponder with apower that is as close to the optimum level as possible. Whenthe power required for the fade compensation goes beyond theHPA limit, the station has to reduce its transmission bit rate bya certain amount, e.g., four times for a 6 dB gain. This allowsthe station to survive deeper fades, but it is only possible withABR/UBR traffic or with real time applications that can tem-porarily reduce the data bit rate needed. Traffic stations use thenormal or the lower bit rate on the assigned TDMA carrier/slotaccording to the hub indication contained in the out-bound sig-naling field of the reference burst.

Traffic stations also monitor the quality of data received andsend it to the hub using a field of the satellite header, which isin front of each packet. The hub uses this information to switchfrom the normal to the assisting carrier when sending data to thestations that go over a certain fade level and vice versa. If morethan one satellite transponder can be used, it is preferable to al-

Table 1. System parameters.

TDMA Burst overhead (ObTDMA) = carrier and bittiming acquisition (200 bits) + UW (32 bits)+ guard time (32 bits)

264 [bit]

TDM Burst overhead (ObTDM) = UW 32 [bit]Packet overhead (Op) = satellite header (11 bytes)+ IP/UDP header (32 bytes) + cyclic redundancy check(2 bytes) (same for TDM and TDMA)

45 [byte]

Packet information length 192 [byte]Basic Capacity Unit (b) 8000 [bps]

Data coding rate on TDMA link (CTDMA)2/3Convolutional-Viterbi

Data coding rate on TDM link (CTDM)1/2Convolutional-Viterbi

Frame length (f ) (same for TDMA and TDM) 192 [ms]Reference burst length 790 [byte]Burst Time Plan length (BTPl) 768 [byte]

locate the TDMA low speed carriers and the TDM higher speedcarriers on different transponders in order to avoid carriers withvery unbalanced power levels on the same transponder, whichwould increase the interference due to intermodulation.

Various solutions can be adopted for measuring the quality ofthe signal received [2]–[4], and for estimating the level of thepower received. To measure the quality of the signal received,the ratio Eb��N� � I�� is estimated, i.e., the ratio between theuseful energy contained in a bit and power spectral density of thethermal and interference noise. Eb depends on the attenuationof the signal, which can be compensated for by regulating thepower of the transmitter. I� depends on the interference level,and thus on the back-off level at the transponder. Therefore, itwould be wrong to increase the transmitted power on the basisof the signal quality alone. This is because if I� is too high, byincreasing the power transmitted by a station, the transponderreaches lower levels of back-off, and there is a general increasein intermodulation noise. It is very important to estimate boththe Eb��N� � I�� ratio and the power level received in orderto ascertain whether the transponder is working at the optimalpoint. If conditions differ from the optimal ones, the hub oper-ates a tune-up by obliging all the transmitting stations to mod-ify their transmitting power. This can be done simply by alter-ing the indications relevant to the levels of power received byall stations, which are indicated in the BTP. With regard to theswitching of a station from the normal to the assisting condition,a threshold mechanism with hysteresis can be assumed, in orderto reduce the number of switches between the two conditions.The threshold should be based on the quality of the signal re-ceived, not on the power level, in order to take into account boththe intermodulation noise and the variation (with the attenuationlevel) in N�, even if small.

VI. THE ITALSAT CASE STUDY

We present a possible realization of the FH-JFDA system byusing the transponders 1 and 2 of the Italsat national coveredpayload which operate in the 20/30 GHz band.

Table 1 reports the most important system parameters. As inthe simulation runs for the RA and MO algorithms that gave the

results presented in Section III, we considered 24 carriers QPSKmodulated at 256 kbps for the TDMA links (k). From 1 up to16 slots per frame (s) can be assigned to each traffic station fora Ctot of 384 slots per frame.

Table 2 shows the link budget relevant to transponder #1 (in-bound in TDMA) and to transponder #2 (out-bound in TDM) fortwo different cases (Case 1 and Case 2). The two cases differ interms of the bit rate partition of the total system capacity (whichis the same in both cases) between the normal and the assistingcarriers. Both cases are compared with the case of a uniqueTDM carrier whose bit rate is the sum of the two carriers, inorder to evaluate if it would be better to adopt two TDM carriersinstead of one. The data used in Table 2 are taken from [5].

The hub station must be able to transmit an amount of in-formation on the TDM links which is not lower than the totalamount transmitted on the TDMA links, i.e., it must be:

ksb � CTDM

�BnTDM �BaTDM �

�

f�ksOp � �BTPl�

�

�ks� �

fObTDM � (3)

where b, CTDM , Op, ObTDM , BTPl, and f are explained inTable 1, BnTDM and BaTDM are the normal and the assistingTDM bit rates, respectively. In non-faded conditions, the hubuses the normal carrier for sending real-time traffic, while theassisting carrier is used for non real-time data. The jitter-freecondition on the real-time traffic must be maintained as long aspossible, thus the hub station does not transmit any real-timetraffic, which does not require assistance, on the assisting car-rier. Only the transmissions towards faded stations are moved tothe assisting carrier, by trying to maintain the same temporal po-sitions. In this situation, in order to avoid any time overlap withthe non real-time transmissions, the last ones are shifted in timeon the same carrier, if possible. If the shift is not possible, theyare moved to the normal carrier as long as they are not faded ortemporarily removed until the fade is not critical. The value of� which allows us to fill the assisting carrier with non real-timetraffic is given by:

� �CTDM �BnTDMf � �BTPl��ObTDM

ks�bf � �CTDMOp �ObTDM �� (4)

A. Collision Probability When Sending the SEM

The following numerical considerations allow us to choosethe number m of sub-slots per frame dedicated to accessingthe system in S-Aloha mode by sending the session entry mes-sage. Let us consider N stations in the system with an averageaccess rate � expressed in (sub-slot)��, and a retransmissionprobability, after the first collision, �. We have � � f��mTa�,where Ta is the average access time of each station and f is theframe duration. If one slot per frame is devoted to the SEMtraffic (m � �) and Ta � �� s (12 minutes), � is equal to�� ���. Assuming a Poisson distribution for the SEM traf-fic, the probability � that a station sends a message in a sub-slotis � � � � e�� �� � for very small �. A value of � equal to0.008 means that, after a collision, a station must draw and wait

Table 2. ITALSAT link budget for national covered payload transponders 1 and 2.

TDM [out-bound link] Transponder #2 TDMA [in-bound link]

Transponder #1 Case 1 Case 2

Hub antenna diameter 3 m

Traffic station antenna diameter 1 m

Bit rate [QPSK modulation scheme]

256 Kbps (BnTDMA, NORMALbit rate)64 Kbps (BaTDMA, REDUCEDbit rate)

8,192 Kbps (BnTDM)[on normal carrier]2,048 Kbps (BaTDM)[on assisting carrier]

9,216 Kbps (BnTDM)[on normal carrier]1,024 Kbps (BaTDM)[on assisting carrier]

UP-LINK Traffic station Satellite Hub Satellite

Earth stat. TX saturation power 3 dBW 20 dBW

Earth stat. power control range 10 dB 12 dB

Satellite G/T 5.9 dB/oK [in orbit] 5.2 dB/oK [on ground]

Up-link Eb/No [up-link atten.

within power control range]

6.5 dB [normal bit rate]

12.5 dB [reduced bit rate]

10.8 dB [normal carrier]

16.8 dB [assist. carrier]

10.2 dB [normal carrier]

19.8 dB [assist. carrier]

DOWN-LINK Satellite Hub Satellite Traffic station

Transponder EIRP 48 dBW [in orbit] 48.4 dBW [in orbit]

Numb. of carriers /transponder 24 2

Output backoff 3 dB 1 dB

Earth station G/T 20.2 dB/oK 29.8 dB/ oK

Down-link Eb/No [clear sky] 23 dB11.3 dB [normal]

17.3 dB [assisting]

10.8 dB [normal]

20.3 dB[assisting]

Modem implementat. margin 1 dB

NORMAL CARRIER

7 dB [clear sky] 3.2 dB [6 dB attenuat.]

6.5 dB [clear sky]3.2 dB [5.3 dB atten..]

ASSISTING CARRIER

Net Eb/No at decoder input

NORMAL bit rate (up to 10 dBof up-link attenuation) &REDUCED bit rate (up to 16 dBof up-link attenuation)

>5 dB (up to 6.5 dB of down-linkattenuation)

3.2 dB [12.8 dB atten.] 3.2 dB [16 dB atten.]

Coding rate [convolut./Viterbi] 2/3 (CTDMA) 1/2 (C TDM)

Bit error rate (BER) [6] 10-8 [Eb/No=5dB] 10 -8 [Eb/No=3dB]

Total BER (TDMA +TDM links) 2*10 -8

for a number between 1 and 125 sub-slots before retrying thetransmission, thus waiting for an average time of 2 s. Consider-ing these figures, it can be seen [6] that the average number ofstations that are retrying the transmission after the first collisionis far lower than the unity and thus negligible with respect to N .The probability of the first collision, pc, can then be computed

by solving the equation pc � ��

���

���pc

�N, which gives a

value of about 0.011. Subsequent collisions have a probabilityof about pc�� (�0.019). These probabilities must be divided bythe number of slots per frame devoted to the SEM traffic. Suchfigures make the choice of only one slot per frame sufficient togive reasonable access conditions.

B. System Availability

Let us now make some considerations about the system avail-ability in terms of percentage of time in a year. The ITU-R in-terpolation formula [7]

Ap � A��������p������������� log p (5)

estimates the attenuation exceeded for a percentage p of an av-erage year, when the attenuation A���� exceeded for 0.01% of ayear is known for a given site.

We consider the values of A���� taken from the frequencyscaled attenuation distributions resulting from five years of Sirioexperiments at Fucino and Lario [8], two Italian sites with very

Table 3. System availability for Fucino and Lario like stations.

Traffic stations availability (in-bound link)A001[dB]

(up-link)

A001 [dB]

(down-link)

Hub station

Availability [%] power contr. alone Power contr.+1/4 bit rate red.

Fucino like

stations22.5 12

99.95

2 hubs: 99.999999.93% 99.98% (BER = 10-7)

Lario like

stations52.3 28.4

99.71

2 hubs: 99.99699.59% 99.86% (BER = 10-7)

different climatic characteristics, in the center and in the northof Italy, respectively.

From data in Table 2 and by inverting the relation (5), wecompute the availability figures reported in Table 3.

In order to evaluate the availability on the out-bound link ofa traffic station, we refer to [8],[9], where the data of the Sirioexperiment were employed to estimate the joint attenuation dis-tributions using three stations: Lario, Spino d’Adda and Fucino.In a region that excludes very low and large attenuation levels,[8] and [9] show that the measured joint distributions of eachpair of stations differ approximately by a multiplicative factor hfrom the joint distributions that would have been obtained fromthe marginal distributions in the case of statistical independence.For the tested frequency (11.6 GHz), the factor h was found tobe equal to 20 for the Lario-Spino (85 km) pair, and equal to 4and 2 for Spino-Fucino (510 km) and Lario-Fucino (570 km),respectively. Let us now extend these results to a large numberof stations operating at 20 GHz, and consider cases in whichthe attenuation probability curves and the h parameters are the

0.01

0.1

Una

vaila

bilit

y[%

]

10 20 30 40 50 60 70 80 90 100

h

2 Mbit/s Fucino

1 Mbit/s Fucino

1 Mbit/s Lario

2 Mbit/s Lario

no countermeasure Fucino

no countermeasure Lario

Fig. 10. Single station unavailabilty versus h, for the assisting carrier at1 and 2 Mbit/s and for two different class of sites. The coefficient �is unitary and the load of each station is 8 spf.

same for all the stations. This last hypothesis is not realistic, be-cause it is practically impossible that many stations all have thesame dependence on each other. However we need to expresssystem performance as a function of a single parameter whichshould then be considered as an average value, or a maximumexperimental value, if a conservative result is preferred.

Denoting byAa andA� the attenuation thresholds that delimitthe need of assistance of a station, by p� the probability that theattenuation at a station is higher than A�, by pa the probabilitythat the attenuation is in the range Aa � A�, and by paf theprobability that the assisting carrier is already full when a stationneeds assistance, the probability pua that a station is unavailableis [9]

pua � p� � papaf � (6)

where

paf �

n��X

i�a

�n� �

i

��hpa�

i��� hpa�n���i

�i� �� a

i� �

��

(7)

The last term in summation (7) gives the probability that thestation considered is not assisted when i more stations need as-sistance and only a stations are assisted.

We can easily plot relation (6) versus various values of h,under the restriction that all stations have the same capacity re-served for stream applications. The results are shown in Fig. 10for the two cases reported in Table 2 which offer the same totalcapacity, for two classes of sites with different climatic charac-teristics, and for a load per station of 8 slots per frame.

The coefficient � has been assumed as unitary, i.e., both nor-mal and assisting carriers’ capacity is devoted to stream appli-cations which is the worst case scenario. In the figure, the lines

3

4

5

6

7

8

Gai

n [d

B]

10 20 30 40 50 60 70 80 90 100

h

2 Mbit/s

1 Mbit/s

16 spf α=1

16 spf α=0.88 spf α=1

8 spf α=0.8

1 spf α=11 spf α=0.8

Fucino like stations

(a)

-2

-1

0

1

2

3

4

5

6

7

8

Gai

n [d

B]

10 20 30 40 50 60 70 80 90 100

h

1 Mbit/s

2 Mbit/s1 spfα =0.8

8 spf α =1

16 spf α =1

8 spfα =1

16 spf α=0.8

1 spf α=1

1 spf α=0.8

1 spfα

=1

8 spf α =0.8

16 spf α=0.8

16 spfα=1

8 spf α=1

Lario like stations

(b)

Fig. 11. Gain of the fade countermeasure system versus h, for the as-sisting carrier at 1 and 2 Mbit/s and for different values of each sta-tion’s and system’s load of stream capacity: (a) Fucino like stations;(b) Lario like stations.

labeled with no countermeasure refer to the employment of onlyone carrier with the same capacity as the sum of the two carriers.

The probability pua given by relation (6) can be convertedinto percentage terms and put in (5) in order to get the equiv-alent attenuation Ae that is exceeded with that probability. Ae

can be compared with the attenuation Anc that causes the out-age of a station in a system that has no fade countermeasure,i.e., a system which has only one TDM carrier, in order to getthe gainG of the countermeasure itself. Anc, relevant to the car-rier modulated at 10.148 Mbit/s, turns out to be 8.5 dB. The gain

G�h� � Ae�h��Anc is reported in Fig. 11 for both cases of Ta-ble 2, for the two different classes of sites, and for the loads of 1,8, and 16 slots per frame of each station. Other than unitary, thecoefficient � has also been assumed to be equal to 0.8 which ismore reasonable because it satisfies inequality (4). The curvesrelating to 16 slots per frame can also be assumed as the jitter-free curves relative to 1 slot per frame load. We can see that, forsystems with Fucino like stations, Case 1 (1 Mbit/s assisting car-rier) is more convenient than Case 2 even for high dependencylevels of the stations’ rain conditions. For Lario-like stations,the choice of the best case depends on the h coefficient; Case 2is more convenient for high dependency values. Note that Case1 may even produce a negative gain for high values of h whichmeans that it is not suitable to adopt this fade countermeasuremethod. We think that a gain of at least 3 dB is necessary tojustify the employment of such a complex fade countermeasuretechnique.

The figures concerning the traffic stations’ availability do notmatch with the ones of the hub, because the latter should have amuch higher availability. Two possible solutions to this problemare: using a larger and more powerful hub or adopting a sec-ond hub, working in stand-by mode, placed at a distance fromthe first in order to guarantee a sufficient independence of thefading conditions (site diversity countermeasure). The latter so-lution seems to be the most convenient one. Table 3 gives theavailability of the hub function, considering two hub stationswith an h coefficient of 5. The resulting figures are satisfactoryeven for Lario-like stations.

VII. CONCLUSION

This paper has presented FH-JFDA, a VSAT system designedfor the Ka band, which uses fade countermeasures based on theup-link power control and on bit rate changing, both on the in-bound link in TDMA and on the out-bound link in TDM. Someuseful criteria, in terms of the choice of system parameters, havebeen given. A case study based on the employment of two trans-parent transponders of the Italsat satellite has also been showntogether with the fade countermeasure system performance. Wehave shown the quantitative dependence of the performance onthe level of spatial correlation of the rain events among the sta-tions, taking the results of the Sirio satellite experiment. Weconclude that the convenience in adopting one of the two casespresented with different bit rates of the TDM carriers depends onthe climatic conditions of the stations in the system. In the casestudy the bit rates of the traffic stations seem to be relatively lowin comparison with today’s requirements for multimedia appli-cations. The reason for this is that we considered a national cov-erage payload, transparent transponders, and double-hop links.Figures would significantly change if on-board processing (hubon satellite) and multi-spot antennas were adopted.

REFERENCES[1] N. Celandroni, E. Ferro, and F. Potortı̀, “Performance evaluation of a

multi-level allocation algorithm for VBR traffic over a geostationary satel-lite,” in Proc. SPECTS’00, Vancouver, Canada, July 2000, pp. 452–457.

[2] N. Celandroni and S. T. Rizzo, “Detection of errors recovered by de-coders for signal quality estimation on rain faded AWGN satellite chan-nels,” IEEE Trans. Commun., vol. 46, no. 4, pp. 446–449, Apr. 1998.

[3] N. Celandroni, E. Ferro, and F. Potortı̀, “Quality estimation of PSK modu-lated signals,” IEEE Commun. Mag., vol. 35, no. 7, pp. 50–55, July 1997.

[4] N. Celandroni and F. Potortı̀, “Fade countermeasure using signal degrada-tion estimation for demand-assignment satellite systems,” J. Commun. andNetworks, vol. 2, no. 3, pp. 230–238, Sept. 2000.

[5] F. Carducci and M. Francesi, “The Italsat satellite system,” J. SatelliteCommun., vol. 13, no. 1, pp. 49–81, Jan./Feb. 1995.

[6] J. F. Hayes, “Modeling and analysis of computer communication net-works,” Series ed. R. W. Lucky, Plenum Press, New York, 1986.

[7] ITU-R, “Propagation data and prediction methods required for earth-spacetelecommunication systems,” Rep. 564-4, 1990.

[8] F. Carassa, E. Matricciani, and G. Tartara, “Frequency diversity and itsapplication,” J. Satellite Commun., vol. 6 no. 3, pp. 313–322, July/Sept.1988.

[9] F. Carassa, “Adaptive methods to counteract rain attenuation effects inthe 20/30 GHz band,” Space Commun. and Broadcasting 2, pp. 253–269,1984.

[10] N. Celandroni, E. Ferro, N. James, and F. Potortı̀, “FODA/IBEA: A flex-ible fade countermeasure system in user oriented networks,” J. SatelliteCommun., vol. 10, no. 6, pp. 309–323, Nov./Dec. 1992.

Nedo Celandroni received his Dr. Ing. degree inElectronic Engineering from the University of Pisa,Italy, in 1973. Since 1976 he has been a researcherwith the CNUCE Institute of the Italian National Re-search Council (C.N.R.). He worked for the realiza-tion of the Flight Dynamic System of the SIRIO satel-lite. Since 1979 he has been involved in the field ofdigital satellite communications. He participated inseveral projects in this area, such as: STELLA I/II(Satellite Transmission Experiment Linking Labora-tories), FODA (Fifo Ordered Demand Assignment),

FODA/IBEA (Information Bit Energy Adaptive), Progetto Finalizzato Teleco-municazioni, and Experiments on the satellites Olympus and Italsat. He holdstwo patents on satellite systems design. His interest includes rain fade counter-measure systems, data quality estimation, design and performance evaluation ofGEO, MEO, and LEO satellite networks for mobile telephony and multimediasystems. He reviewed a number of international journals and congress papers.Dr. Celandroni is a member of the IEEE Communications Society. He authoredabout hundred journal and congress papers and technical notes.

Erina Ferro received her M.S. degree with distinc-tion in Computer Science from the University of Pisa(Italy) in 1974. Since 1976 Dr. Ferro is with CNUCE,an Institute of the Italian National Research Council(C.N.R.), where she is currently working as a seniorresearcher in the digital transmissions via satellite.She participated to several international and nationalprojects where some systems designed by her teamwere implemented and used on the Eutelsat, Olympusand Italsat satellites. Working in the TDMA satelliteaccess schemes field she obtained two patents, in 1989

and 1996, respectively. She has also participated to several COST actions of theEuropean Community. Her research activity covers the fields of satellite accessschemes for multimedia traffic, fade countermeasure techniques, and controlaccess schemes for GEO, MEO, and LEO constellations of satellites for multi-media systems with guaranteed quality of service. She has served as a reviewerfor a number of international journals and IEEE congresses. Dr. Ferro authoredover seventy publications in the above fields, in international scientific journalsand conference proceedings.