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Wireless Personal Communications (2005) 32: 43–57DOI: 10.1007/s11277-005-2378-8 C© Springer 2005

Cross Layer Considerations for an Adaptive OFDM-Based WirelessCommunication System

HERMANN ROHLING and RAINER GRUNHEIDDepartment of Telecommunications, Technical University Hamburg-Harburg, Eißendorfer Str. 40, D-21073Hamburg, GermanyE-mail: [email protected], [email protected]

Abstract. Future applications require high, but variable data rates and different quality of services (QoS) which is areal challenge for the communication system design. Additionally, the broadband radio channel can be assumed tobe frequency selective and time variant, which means the transmission performance varies over time and frequency.The OFDM transmission technique is very flexible in adapting the transmission parameters to the current channelsituation and to the application-specific requirements. This kind of flexibility will be applied to solve the technicaltasks in the design procedures of future communication systems.

Keywords: OFDM, multicarrier, cross layer design, link adaptation, schedulling, MAC

1. Considered Situation

The variety of complex tasks to be performed in mobile communication systems is commonlygrouped in different layers, ranging from the application (highest layer) to the air interface(lowest layer). The well-known ISO/OSI-model [1] consists for good reasons of seven dif-ferent layers and is an example of such an hierarchical approach, which implies that a layercommunicates exclusively with the layers directly above and below it, keeping the effort whichmust be spent for the information exchange between layers to a minimum. Figure 1 shows thelayer structure for the physical (PHY) and data link control (DLC) layer.

It is the main advantage of the conventional approach that the layer-specific protocols canbe developed and optimised independently of each other. However, this approach is limitedin flexibility and the protocol optimisation can only be processed on the basis of availableinformation from adjacent layers. The transmission of high data rates from different applica-tions with heterogeneous and variable quality-of-service (QoS) parameters demands a highflexibility of the involved layers and the communications structure. The OFDM transmissiontechnique is part of the PHY layer, but continuously generates detailed information aboutthe current radio channel conditions, which can be used for a channel prediction and for anadaptation procedure in a cross layer approach.

This requirement of a cross layer consideration is particularly of importance for the PHYs-ical (PHY) and the data link control (DLC-) layer which are considered in the scope of thispaper. It can be expected that in this case a joint design and optimisation of both layers witha larger degree of interaction (see Figure 1, right) will lead to a considerable capacity en-hancement of the overall system (see e.g. [2–4]). In the framework of this paper, technicalpossibilities for the cross layer design of the PHY and DLC layer in an OFDM-based wireless

44 H. Rohling and R. Grunheid

Figure 1. Conventional (left) and cross layer approach for wireless communication systems.

communication system are discussed and analysed in detail. On the physical layer, the use ofthe OFDM transmission technique [1] is assumed where detailed radio channel informationis available.

The concrete technical task comprises the suitable definition of the functionalities of bothinvolved layers. On the one hand, the resulting system should be able to react flexibly to thegiven statistics of the data sources (QoS parameters), while on the other hand, the statistics ofthe time-variant and frequency-selective radio channels need to be taken into account. Thus,these two aspects are highlighted in the following sections. On the basis of this discussion,Section 4 is devoted to the important task of allocating resources to the different mobilestations (MS) in a MAC frame, i.e. the scheduling of packets according to different criteria.The scheduling policies considered in this paper will be introduced and categorized.

The quantitative results for the applied scheduling algorithms are presented and contrastedin Section 5 to stress the significance and advantages of a cross layer design. Section 6 sum-marizes the main aspects with some conclusive remarks.

2. Flexibility and Adaptivity in the System Design

On the one hand, future applications can be described by a variety of different QoS require-ments, such as high and variable data rates, different delay constraints, etc. On the other hand,the transmission medium, i.e. the mobile radio channel, is characterised by a stochastic time-variant and frequency-selective behaviour. Thus, a suitable system design must be tailored tothese given properties of the applications and the transmission channels.

2.1. DATA QUEUES WITH VARIABLE DATA RATES AND QOS

It can be expected that in future wireless communications systems, a mix of different appli-cations (voice, data, video) will be present, each of which is associated with different QoSrequirements. From a user’s point of view, throughput, delay, availability and reliability can

Adaptive OFDM-Based Wireless Communication System 45

Figure 2. Example of a data source with high and variable data rate (video signal).

be regarded as four typical QoS parameters. Real-time services, such as video applications,have strict requirements on the communication system due to a demand for high adaptivitytowards a time-varying transmission quality. In this paper, such data sources with variabledata rates together with frequency-selective and time-variant radio channels are consideredand quantitatively analysed.

More specifically, video signals with high and variable data rates are assumed in this paperfor the different data sources. An example is depicted in Figure 2, which shows the dynamicbehaviour over time with respect to the high and variable data rate of a single source. Thesimulation results presented later are generated using video signals from different sources withthe same average (880 kbit/s), but clearly different momentary (100 kbit/s–2.3 Mbit/s) datarates.

2.2. TI M E-VARIANT AND FREQUENCY-SELECTIVE RADIO CHANNELS

Apart from the stochastic behaviour of the data sources described above, also the stochasticcharacteristics of the radio channel are to be taken into account in the cross layer design.The major effects used in the link budget calculation to typically describe the impact of thetime-variant and frequency-selective channels can be summarized as follows:

Path loss Lpath

The decay of the received power in dependence of the distance d between base station (BS)and mobile station (MS) is assumed to follow the one-slope model:

Lpath = L0 + 10n log(d) (1)

In this equation, L0 denotes the path loss at a distance of 1 m, and n is the so-called slopefactor. Both parameters depend on the propagation scenario.


Slow fading xs

Statistically, the distribution of the amplitude xs as a consequence of the slow fading can bedescribed by a log-normal distribution [7, 8].

Fast fading γ

In a multipath environment, the received signal is composed of an additive superposition ofseveral replica of the transmitted signal, each with different attenuations and delays. Thiseffect can lead to a constructive or destructive superposition. The resulting variation of thesignal envelope over time, i.e. the fast fading, can be characterized by a Rayleigh distribution(non-line-of-sight scenario) or a Ricean distribution (line-of-sight situation).

To summarize, the received power PR at a given transmit power PT is obtained as:

PR[d Bm] = PT [d Bm] − Lpath[d B] − xs[d B] − γ [d B] (2)

The physical layer of an OFDM-based system has all this information available which canbe used for a link adaptation procedure integrated in the DLC layer.

3. MAC Frame Structure and PHY Mode Selection

3.1. MAC FRAME STRUCTURE

In this paper, a TDD based system with a MAC frame structure of length 2 ms is considered(see Figure 3). A header (denoted by H in Figure 3) at the beginning of the MAC frame containsall information about the resources inside the MAC frame for each MS. Using a random accesschannel (RA) at the end of the frame, new or inactive MS can access the system. Each MSreceives and transmits the signals over a MS-specific radio channel. Link adaptation is appliedby selecting a MS-specific PHY mode in accordance to the current radio channel status, asexplained in the following.

3.2. INDICATOR-BASED PHY MODE SELECTION

The general principle of link adaptation is to choose certain physical transmission parameters,such as modulation level, coding rate, transmit power, etc. according to the current radiochannel behaviour. To limit the multitude of possible combinations of these parameters, theyare typically grouped into different sets, such as the “PHY modes” in the HIPERLAN/2 andIEEE802.11a WLAN systems.

Clearly, a table of possible pre-defined PHY modes serves as a basis for the PHY modeselection process. Different criteria are conceivable for selection of an appropriate PHY mode

Figure 3. Considered MAC frame structure, including header (H), downlink/uplink part and random access channel(RA).


for a given radio channel behaviour (delay-oriented/throughput-oriented). Those kinds ofmethods typically require a knowledge of the expected PER for a given instantaneous radiochannel. For PER estimation, conventionally the average received SNR together with theaverage PER performance curves are used. In [6] it was shown that a much better adaptationcan be accomplished if an additional radio channel indicator is developed and applied, takinginto account the instantaneous behaviour of the channel instead of adapting to some averageperformance figures. This means that the additional radio channel indicator is used on theDLC layer to obtain a reliable estimate of the expected PER performance for a given channel,facilitating a much better PHY mode selection.

In the conventional approach, only the average SNR of a radio link is taken into accountin accordance with some resulting average PER performance curves, in order to determine asuitable PHY mode. But this procedure is a real misunderstanding in link adaptation and PHYmode selection [6]. In contrast, the indicator-oriented approach, as proposed in [6], is based oncalculating an indicator out of a more detailed knowledge (e.g. channel transfer factors) of aninstantaneous radio channel. The additional indicator describes roughly the observed dynamicin the channel transfer function, see Figure 4. The radio channel (1) shows a high frequencyselectivity and therefore leads to a large indicator value whereas radio channel (2) shows asmall dynamic and consequently has a small indicator value. With the help of this additionalradio channel indicator, an estimation of the expected PER can be performed for each PHYmode, and SNR thresholds can be derived for the given instantaneous channel in much moredetail. Finally, the PHY mode selection is accomplished using these “instantaneous” SNRthresholds instead of “average” thresholds, leading to a more efficient choice of PHY modesand in particular avoiding a fatal misadaptation.

Figure 4. Principle of indicator-based PHY mode selection.


Figure 5. Gain of indicator-based PHY mode selection compared with conventional approach.

For illustration purposes, Figure 4 contrasts the conventional (“one-dimensional SNR basedonly”) and the indicator-based (“two-dimensional SNR plus indicator”) concept, which maylead to different PHY mode selection results (“blue” mode vs. “green” mode in the exampleof Figure 4). As depicted, the latter concept implies a mapping of a specific radio channel ontoa certain point in the SNR-indicator plane and a PHY mode selection according to the areawhich the point falls into. The boundaries between these areas represent the SNR thresholdcurves which are derived following the PHY mode selection.

The SNR gains that can be achieved by this additional indicator-based method comparedwith the conventional approach are plotted in Figure 5 vs. the bandwidth efficiency. Consid-erable improvements in the performance figures of more than 4 dB in SNR on average can beobserved.

4. Strategies for the Scheduler Design

The allocation procedure of available resources to different MS and thus the mapping of datapackets onto the MAC frame is commonly termed as scheduling and is defined as one importanttask inside the DLC layer.

The general objective of the scheduler is to find a compromise in the resource allocationprocedure and the MAC frame organisation between the statistical behaviour of all sourcesand the statistical behaviour of the MS-specific radio channel. The goal is to maximise thetransmission quality by a proper priority handling based on some fairness aspects.

Consequently, suitable allocation mechanisms have to be developed and realised, whichassign priorities to the data packets and data queues for the order of transmission, making useof some optimisation criterion. A pure analytical solution of this task is non-trivial, even morebecause the formulation of the optimisation criterion in a mobile network (delay, throughput,fairness. . .) is in many cases unclear.

It should be emphasized here that throughout the performed investigations and independentof the scheduling algorithm, it is assumed that the DLC layer is capable of adjusting thetransmission parameters to the momentary radio channel characteristics, i.e. to perform a linkadaptation (LA) technique which chooses the appropriate PHY mode (see also Figure 6) inaccordance with some pre-defined limit for the resulting packet error rate (PER) and the ARQprocedure. The subsequently described scheduling algorithms can be distinguished by the fact


Figure 6. Concept of cross layer design where information from the data sources and the radio channels is takeninto account.

if they take into account the channel state information and/or the requirements from the datasources when fixing priorities for the data packets.

In the scope of this paper, several different scheduling algorithms are described and anal-ysed. For an appropriate categorization, the following types of protocols are distinguished:

Purely data-source oriented protocolsFor the allocation of data packets to MAC frames, only information from the higher layers(QoS parameters of the packets) is used. This approach is in line with the conventionallayered concept, where the information flow is directed from upper to lower layers. Althougha PHY mode selection is applied, the momentary channel states are not taken into accountfor scheduling in these types of protocols.

Purely radio channel oriented protocolsIn this case, exclusively information from the lower layers (PHY layer and momentaryradio channel characteristics) is exploited. Thus, the scheduler tries to adapt the resourceallocation to the dynamic and stochastic channel behaviour, e.g. by giving priorities to a MSwhich currently experiences a favourable radio channel. To apply fairness aspects, it mightbe useful not to choose the MS with the absolutely best, but rather the relatively best radiochannel.

Cross layer oriented protocolsFor the allocation, information from the higher as well as from the lower layers is taken intoaccount. The aim in this case is to consider the channel statistics as well as the statisticsof the data sources for compiling the MAC frame. This concept is illustrated in Figure 6and represents the idea of a cross layer concept which is supported here. The benefit of thisapproach will be presented and discussed in Section 5.

Reference protocolFor comparison, the well-known Round Robin (RR-) protocol is used as a reference. In thisprotocol, no information at all from other layers is exploited. Instead, a certain number ofpackets from each data source is mapped onto the MAC frame in a fixed cyclic fashion.

In the following, typical examples for the first three protocol categories are presented, whichwill later also be considered in the quantitative analysis. The criteria for resource allocation


are described, and according to the focus of this paper, an emphasis is put on the cross layerprotocols.

4.1. PURELY DATA-SOURCE ORIENTED PROTOCOLS: SHORTEST QUEUE (SQ)

In this DLC protocol, the number of packets of the different data sources, which are organizedin queues, plays a central role. The shortest queue is served first by mapping all the packetsof this queue onto the MAC frames. This strategy reduces the resulting time delay. In the nextstep, the packets of the second shortest queue are mapped, etc. With this priority handlingstrategy, the characteristics of the radio channels have no impact on the resource allocation.However, LA procedures for PHY mode selection are still applied, as described above.

4.2. PURELY RADIO CHANNEL ORIENTED PROTOCOLS: RELATIVE BEST

CHANNEL (RBCH)

In the RBCH protocol, exclusively the momentary radio channel characteristics are used forresource allocation. By a comparison of “relative” channel states, the MS experiencing the best“relative” channel (with respect to a measured average channel) is served first. This strategycontains already a fairness idea: not the absolutely best but the relatively best channel getsthe highest priority. This relative channel state phyRelai for MS i is expressed in a quantisedform by the number of the currently selected PHY mode phyi and the average PHY modephyAveragei (averaged over the past MAC frames) as follows:

phy Relai = phyi − phyAveragei (3)

Considering the relative instead of the absolute channel qualities has the advantage that anMS with a low average PHY mode (e.g. an MS far away from the BS) is given a much betterchance of allocation if the channel state temporarily improves compared with the average one.A strategy based on the absolute channel state is generally feasible and would optimise thedata throughput, but would on the other hand give priority to MS close to the BS (high SNR).Thus, for reasons of fairness the RBCH protocol is considered here.

4.3. CR OSS LAYER ORIENTED PROTOCOLS

As already pointed out, the cross layer concept is the central aspect in this paper. Consequently,the corresponding cross layer protocols are described in more detail for three examples, whichare also taken into account in the quantitative analysis.

4.3.1. Shortest Transmission Time (STT)As the name of this protocol implies, the time needed for the transmission of all data packetsfor one MS is calculated, and these times are sorted in ascending order. This transmission timeis influenced by the number of packets inside the queue and additionally by the momentarychannel state. Priority is given to the MS whose packets require the shortest transmission time.The transmission time ti for MS i is calculated as:

ti = li LDPDU

phyi(4)


Figure 7. Considered scenario inside a radio cell with 3 MS.

Here, li denotes the number of packets (=DPDUs) inside queue i, LDPDU stands for the(fixed) packet size and phyi represents the currently selected PHY mode, according to themomentary channel conditions. From this equation, it becomes apparent that the resourceallocation takes into account the requirements of higher layers (number of packets) as well asthe stochastic behaviour of the radio channel (chosen PHY mode). The resulting transmissiontime can directly be observed in Figure 7 where the MS-specific time duration inside the MACframe is coded by colour.

4.3.2. Advanced Relative Best Channel Shortest Transmission Time (aRBCHTT)In some cases, it seems favourable to use more than one criterion for the scheduling algo-rithm, in order to consider different requirements and measures from upper or lower layers inaccordance with the cross layer approach. The aRBCHTT protocol presented here proceedshierarchically in two steps:

1. The order of the MS to be served is chosen such that the MS with the best “relative” radiochannel is prioritised. Compared to Section 4.2 the definition of the relative channel stateis modified as follows:

phyRelai = 2.phyi − phyAveragei (5)

2. In case that two MS have the same phyRelai , the MS with the smaller transmission time ti(see Section 4.3.1) is prioritised.

4.3.3. Total RankingAs a generalisation of the above-mentioned concepts, the method of total ranking (TR) grantsthe possibility to use several criteria simultaneously for resource allocation, where eachcriterion is to be weighted with an appropriate factor. The approach presented here comprisesthree parameters and derives a priority value Pi :

Pi = aphyRelai − β RTi − γ ti (6)


As an additional measure, the remaining time (RTi ) comes into play which quantifies thetime that a packet may remain in the queue until its deadline is reached and it will have to bediscarded.

MS with the highest Pi are given priority for resource allocation. Clearly, the choice of theweighting factors α, β, and γ has a strong impact on the system performance and hence thesefactors must be chosen carefully.

5. Quantitative Analysis

In order to facilitate an interpretation of the quantitative results, the simulation set-up and thebasic assumptions are summarized in Section 5.1, before the simulation results are presentedand discussed (Section 5.2).

5.1. SIM ULATION SET-UP

A single-cell scenario is considered with one BS in the centre of the cell (see Figure 7) andseveral MS. All MS communicate with the BS, whose range determines the cell size. At thestart of the simulation, the start/end positions of the MS are uniformly distributed inside thecell and the MS are assumed to be moving linearly between these points with a constantvelocity v. The radio channels between the BS and the MS are measured in every MACframe, and based on this measurement an LA technique (delay-oriented PHY mode selection)is applied. This information is in some cases also passed on to the scheduler, depending onthe scheduling algorithm. The basic assumptions concerning data sources, radio channel, andsystem parameters are summarized in Tables 1 and 2.

5.2. SIM ULATION RESULTS

On the basis of the assumptions and system parameters listed above, the different schedulingalgorithms are analysed in detail and for a variable number of MS. Selected characteristicresults are depicted in the following figures.

For a comparative assessment of the concepts, it is a viable approach to measure the qualityof service (QoS) that is experienced by the MS when different scheduling policies are applied.

Table 1. Used parameters concerning parameters of data sources and radio channels

Data source

Data stream Identical but statistically independent type of data stream for all MS:video files with the same average data rate of 880 kbit/s

Radio channel

Transmit power 10 dBm

Thermal noise −100 dBm

Path loss L0 = 46.7, n = 2.4 (Large open space)

Slow fading Lognormal distribution, σ = 5.8 dB

Fast fading Rayleigh-fading on each subcarrier, σ 2 = 0.07


Table 2. System parameters used in the simulations

OFDM parameters

Carrier frequency 5 GHz

FFT length 64

Number of subcarriers (data + pilots) 52

Number of pilot carriers 4

OFDM symbol duration 4 µs

Modulation scheme BPSK, QPSK, 16-QAM, 64-QAM

Coding scheme Convolutional codes, R = 1/2, 3/4, 9/16

PHY modes According to HIPERLAN/2 standard

DPDU packet size 432 Bits

MAC frame structure

Duplex scheme TDD

Multiple access TDMA

MAC frame duration 2 ms = 500 OFDM symbols, 350 for downlink,150 for uplink (not simulated)

Overhead 6 OFDM symbols for uplink(7+3 ∗ No. of MS) OFDM symbols for downlink

Cell parameters

Shape, size quadratic, 400 × 400 m

BS Centred inside the cell

MS Uniformly distributed in x- and y-direction inside the cell

Speed of the MS 1 m/s

Link Adaptation (LA) Delay-oriented, PHY mode selection according to maximal PER of 10−2

On the DLC layer, the transmission of complete segments – delivered by the network layer– is of importance. Thus, the measurements in the simulation refer to the following QoSparameters:

• Segment discarding rate: Number of incompletely received segments, relative to all trans-mitted segments. A segment loss occurs if the delay of the packets in the queue is largerthan 40 ms.

• Segment throughput: Rate of completely received segments in Mbit/s.• Segment delay: Time difference between the reception time at the receiver and the arrival

time of that segment at the transmitter site.

These measured values are depicted in Figures 8–10 for all six considered scheduling pro-tocols. A variable system load is realised by changing the number of MS in the considered cell.

Generally, it can be observed that the system performance depends on the system load,i.e. the number of MS. A higher number leads to more packet losses, decreasing throughputand increasing delay, since the system capacity is in many cases not sufficient to transmit allqueued packets in time.

Furthermore, it becomes apparent that the protocols have different performance with respectto the given QoS parameters. For a qualitative assessment of the scheduling policies, Table 3gives an overview of the behaviour of the measured parameters, where the relative performance


Figure 8. Segment discarding rate for different scheduling protocols.

Figure 9. Segment throughput for different scheduling protocols.

is characterised roughly by symbols/colours in Table 3 (“very good” to “very poor”, “++” to“− −”).

As expected, the reference protocol (RR, Round Robin) shows the worst results, since inthis protocol, neither information from the data sources nor from the radio channels are used,instead resources are allocated according to a fixed scheme and on a packet-by-packet basis.


Table 3. Qualitative comparison of the considered scheduling protocols

Figure 10. Segment delay for different scheduling protocols.

The shortest queue (SQ) and particularly the shortest transmission time (STT) protocollead to short delays, because shorter segments are given priority. In contrast, the discardingrate is rather high since many longer segments are dropped.

The purely channel-based RBCH protocol already yields a good performance. In thisconcept, the fairness aspect has been taken into account, i.e. a fair resource assignment forall MS is ensured. If the relative channel quality is applied as the only or the main criterion(RBCH or aRBCHTT), for two of the three QoS parameters, discarding rate and throughput,very good results can be achieved, whereas the delays are larger than in the case of SQ and STT.It should be mentioned that the selection of the relatively best channel implicitly considers afairness aspect. By a suitable weighting of the different values in the total ranking concept,


the performance regarding the QoS parameters can be controlled flexibly, which in the givenexample results in a similar behaviour as in the aRBCHTT protocol.

Hence, in principle a suitable scheduling policy can be selected, depending on the QoS re-quirements of the considered application. Incorporating information from both the data sourcesand the radio channel characteristics (STT/aRBCHTT/TR), in general a superior system per-formance is obtained.

As an example, a segment discarding rate of 5% is considered. In this case, approx. 20 MScan be served in the assumed system, the total throughput amounts to 17 Mbit/s and the delayis 15 ms. If 25 MS are to be served by this system, an increase to 10% is observed in discardingrate, while the increase in delay (to 20 ms) can be regarded as low.

For a different combination of data sources, the quantitative results may differ, still thegeneral advantage of cross layer design for scheduling protocols is clearly expected to remaina key issue.

6. Conclusions

In this paper, the topic of a cross layer approach for the two lowest layers of an OFDM-based communication system has been addressed in detail. It has been shown that a properjoint design of the DLC and PHY layer turns out to be beneficial in terms of the systemperformance. In particular, the issues of improved link adaptation concepts and channel-aware scheduling policies have been stressed and quantitatively analysed. To summarize, byexploiting the detailed channel knowledge, available at the OFDM-based PHY layer, for thetasks of link adaptation and scheduling, the different QoS requirements of the applications canbe accommodated in a much better way.

References

1. H. Rohling, T. May, K. Bruninghaus, and R. Grunheid, “Broad-Band OFDM Radio Transmission for Multi-media Applications”, in Proceedings of the IEEE, No. 10, October 1999.

2. M. Zorzi, R.R. Rao, and L.B. Milstein, “Error Statistic in Data Transmission Over Fading Channels”, IEEETrans. Comm. pp. 1468–77, November 1998.

3. Y. Wingho, N. Heung Lee, and A. Timothy, “Simple but Effective Cross-Layer Networking System for MobileAd hoc Networks”, in Proceedings of the IEEE PIMRC, Lisbon, 2002.

4. Wanghong Yuan, Klara Nahrstedt, V. Sarita Adve, L. Douglas Jones, and H. Robin Kravets, “Design andEvaluation of a Cross-Layer Adaptation Framework for Mobile Multimedia Systems”, in Proceedings of theSPIE Conference on Multimedia Computing and Networking, Santa Clara, California, January 2003.

5. H.P. Frank Fitzek and Martin Reisslein, “MPEG-4 and H.263 Video Traces for Network Performance Evalu-ation”, Technical Report, KN-00-06, 2000.

6. M. Lampe, H. Rohling, and W. Zirwas, “Misunderstandings About Link Adaptation for Frequency SelectiveFading Channels”, in Proceedings of IEE PIMRC, Lisbon, 2002.

7. F. Ayman Naguib, Adaptive Antennas for CDMA Wireless Networks, Dissertation, Stanford University,1996.

8. J. Arogyaswami Paulraj and B. Constantinos Papadias, “Space-Time Processing for Wireless Communica-tions”, IEEE Signal Processing Magazine, pp. 49–83, 1997.

9. C. William Jakes, “Microwave Mobile Communication”, The Institute of Electrical and Electronics Engineers,INC, New York, 1995.

10. Raymond Steele, Mobile Radio Communication, 3rd edn., New York, IEE Press, 1995.


Prof. Hermann Rohling received the Diplom Mathematiker degree from the Technical Uni-versity Stuttgart, Germany in 1977 and the PhD from the Faculty of Electrical Engineering,Rheinisch-Westfälischen Technischen Hochschule (RWTH) Aachen, Germany in 1984. From1977 to 1988 he was with the AEG Research Institute, Ulm as a researcher working in the areaof digital signal processing for radar and communications applications. From 1988 to 1999 hewas a Professor of Communications Engineering at the Technical University Braunschweig(TUBS). Since 1999, Professor Rohling is with the Technical University in Hamburg-Harburg(TUHH), Germany. His research interests include Wideband Mobile Communications espe-cially based on Multicarrier Transmission Techniques (OFDM) for future broadband systems(4G), Wireless Local Loops, Multiple Access and channel coding schemes, Digital RadarSignal Processing especially for automotive radar applications, differential GPS for high pre-cision navigation. Prof. Rohling is a member of ITG, DGON and a senior member of IEEE.

Dr. Rainer Grunheid studied Electrical Engineering at the Technical University Braunschweig(TUBS), Germany, from 1989–1994. After receiving his Diploma degree, he pursued his Ph.D.at the Technical University Hamburg-Harburg (TUHH), Germany, until 2000. Currently, heis working as a research assistant at the Department of Telecommunications at TUHH. Hisresearch interests include mobile communications and multicarrier systems (OFDM), with aspecial emphasis on multiple access schemes, MAC protocols, link adaptation techniques andcross-layer design.

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