Performance analysis of resource allocation in wireless multihop networks

Computer Communications 29 (2006) 983–993 www.elsevier.com/locate/comcom

Performance analysis of resource allocation in wireless multihop networks Matthias Lotta,*, Martin Weckerlea, Matthias Siebertb a

Siemens AG, Communications, St.-Martin-Str. 76, 81541 Munich, Germany Aachen University, Chair of Communication Networks, Aachen, Germany

b

Received 7 June 2005; accepted 7 June 2005 Available online 1 August 2005

Abstract Mobile radio systems beyond 3G will comprise one-hop and multihop communication. Analytically derived performance results indicate a potential capacity gain when introducing multihop communication. In this paper a novel resource allocation scheme is presented that beneficially exploits these potentials of relaying by means of concurrent resource scheduling under control of a central station. However, whether multihop communication can be advantageously deployed strongly depends on parameters like noise power, interference, selected modulation and coding scheme, attenuation, and distance between source and destination. These dependencies will be demonstrated by means of analytical performance evaluations taking into account link-level simulations. q 2005 Elsevier B.V. All rights reserved. Keywords: Resource allocation; Smart direct link; Multihop; Oligohop; System design

1. Introduction MultiHop (MH) communication is one interesting research area [1], especially in wireless self-organizing networks, often also referred to as ad hoc networks, where no long term planned infrastructure exists. If partially connected nodes want to communicate, MH links are a logical consequence and nodes with relaying capability are required. Nevertheless, even in fully meshed scenarios the introduction of MH communication with reduced transmit power and/or better link quality might increase the reliability of the link and exploit the scarce radio resource more efficiently. Due to different network topologies the benefit of MH communication will result in different performance figures. It has been shown that with a node density increasing to infinity the aggregated capacity for pffiffiffi n nodes becomes Oð nÞ bit-meters per second for a pointto-point coding model [2]. It has been further shown that the capacity reaches O(log n) when allowing arbitrary * Corresponding author. Tel.: C49 89 636 45136; fax: C49 89 636 45591. E-mail addresses: [email protected] (M. Lott), [email protected] (M. Weckerle), [email protected] (M. Siebert).

0140-3664/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.comcom.2005.06.019

cooperation between nodes, e.g. multiple access and broadcast, compared to the former assumption in [2], where only a point-to-point coding model has been assumed and where simultaneous transmissions are considered as purely noise [3]. However, in both cases the individual endto-end data rate for an individual link approaches zero when the number of stations increases to infinity. Allowing for unbounded delay and using only one-hop relaying and taking into account mobility, the capacity is limited by O(n). In the latter case, end-to-end throughput stays constant for an increasing number of stations. However, the delay assumption makes this approach not practical for any delaybounded data transfer and seems to be unacceptable for users. Hence, it cannot be expected that MH communication is the ultimate solution to the increasing penetration of wireless terminals and the demands for higher user data rates. From a different point of view, where an existing topology and transmission technology is assumed, MH communication opens the potential for a more efficient usage of the radio resource and the solution to increase the capacity within the network under investigation. Examples are given in [4], where planned relaying nodes, so-called extension points, and directional antennas are introduced to extend the coverage for high-data rates. In [5], performance gains for specific scenarios and especially for large distances between a source and destination for data relaying have been

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presented. Within the MIND project [6] the potential of MH communication has been investigated by the authors. In this paper, exemplary MH scenarios are presented and their benefits compared to conventional one-hop communication are highlighted. The basic idea is twofold: first, one link is broken down in several smaller links with higher data rates. At the same time scheduling is introduced to support simultaneous transmissions under control of a central station. HIPERLAN/2 (H/2) [7] has been selected as reference system to show the applicability of the proposed concepts for relaying. Specifically, the required changes to apply sophisticated scheduling in H/2 are explained in Section 2. Different topologies where the novel scheduling can be deployed are presented in Section 3. In Section 4, the expected performance with respect to the link-budget and the bit/packet error ratio (BER/PER) for one-hop and two-hop communication is compared. The results show that sophisticated central scheduling in H/2 allows efficient exploitation of communication over several links. It is also shown that under specific circumstances the introduction of MH communication can increase the spectral efficiency. However, relaying will not increase efficiency in all scenarios even for optimal geographical placement of the relaying nodes considering constant overhead for relaying on each hop. This result is also validated by theoretical analysis based on the Shannon theory in Section 3.

Fig. 1. Two non-interfering transmissions using the same DiL-resource in parallel, different length.

MAC frame and the phases it consists of are shown in Fig. 1. At the beginning of each frame, general cell related information is transmitted, starting with the Broadcast CHannel (BCH), the Frame CHannel (FCH) and the Access Feedback CHannel (ACH). The BCH contains general announcements whereas the FCH carries information about the structure and contents of the upcoming frame, i.e. the DL, DiL, and UL phase. The ACH informs about (non-) successful access requests transmitted on the Random Channel (RCH) of terminals in the previous Random Access phase (RA). The DiL phase is used to directly transmit user data from one terminal to another without prior reception and forwarding by the AP. The DiL is of special interest for this paper. 2.1. Smart direct-link

2. Direct-link communication The support of direct communication between MTs is a useful feature for allowing peer-to-peer communication. Moreover, the direct-link can be used for multihop communication to economically exploit the frequency spectrum, as explained in the sequel. In addition, under the control of a central instance the allocation of resources can be most efficiently managed. In the following the standard H/2 is introduced as reference system inheriting the aforementioned features, and hence, representing exemplarily a system concept for the proposed new concept of intelligent resource allocation in multihop networks. In H/2 central instances are used for the organization of the assignment of radio resources. If a binding exists to a fixed network, the respective instance is called Access Point (AP). If no infrastructure is present, a so-called Central Controller (CC) takes over the tasks of the AP. In the following, we will refer to CC synonymously for AP. The H/2 Medium Access Control (MAC) protocol applies a centralized concept. The CC dynamically assigns the radio resources for the uplink (UL), direct link (DiL), i.e. the direct communication between MTs using the direct mode, and downlink (DL) phase to the individual MTs in its decoding range [8]. It thus realizes a load-adaptive TDMA/TDD (Time Division Multiple Access/Time Division Duplex) scheme. The basic structure of a H/2

One aspect within the context of MH transmission is the multiple grant of resources. This means, the CC, which is aware of resource requests of associated terminals, could allocate the same part of the MAC frame several times. For direct link communications this means that e.g. MT A1, which is directly sending to MT A2, should be outside the reception range of another communication couple MT B1 and MT B2, in order to avoid harmful interference. For this, the CC needs to estimate the interference situation each of its MTs would face, if several MTs were assigned same resources. The required information can be extrapolated by the CC considering measurement reports from the MTs. For this, the CC requests each MT to determine and report its attainable neighbors, i.e. by evaluation of the pathloss between the MT and other MTs in the cell. On reporting this information the CC possesses the knowledge, which MTs are in the coverage area of each other and could communicate directly, respectively, which MTs would disturb each other if the same resource were used simultaneously. To determine the mentioned pathloss dedicated interference measurements may be exploited. In the H/2 standard respective measurement procedures are defined [9]. Another concept to gather this information is proposed in [10]. The proposed concept referred to as Hybrid Information System allows inquiry of locationrelated link-state information on which the scheduling decision can take place.

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The multiple allocation of DiL resources can easily be realized within H/2. The Frame Channel (FCH), which informs MTs on their assigned transmission start time, simply needs to indicate the same slot to different mobiles. Thus, the appropriate transmissions in the DiL phase take place at the same time. This sophisticated resource allocation autonomously performed by the CC will be referred to as Smart Direct Link, SDiL, in the sequel. Fig. 1 exemplary shows the structure of a MAC frame for SDiL scheduling. The concurrent transmissions between two anticipated non-interfering MT couples, e.g. MT A1 and MT A2 as well as MT B1 and B2 apply simultaneously. Note that the respective assignment of concurrent resources was based on the perception that these two transmissions would not interfere each other. The duration of the simultaneous transmissions not necessarily needs to be of the same length, as indicated in Fig. 1. For example the two DiL transmissions start at the same time but the communication between the MTs A1 and A2 ends earlier than the communication between B1 and B2. It is even possible to start with a DiL phase before the DL-phase has ended, or to extend the DiL phase into the UL-phase (these two possibilities are not shown in Fig. 1). However, if the duration of a parallel resource usage of different communication partners should be of the same length, further sophisticated approaches like the combination of power control and link adaptation can be used for adjusting the transmission time periods within the parallel scheduling phase [11]. 2.2. Related work 2.2.1. Resource allocation Unlike for SDiL, systems with centralized scheduling usually grant an exclusive transmission right, regardless whether this is for a direct MT-to-MT communication or a transmission between the MT and the central instance. Of course, owing to frequency-reuse the same resource will be simultaneously assigned several times at an acceptable distance and, hence, interference level. Resource in this context means frequency or frequency and code in a TDMA, respectively TDMA/CDMA-based system. However, the same resource usually is not assigned multiple times by one and the same central instance. An exception to this may apply to systems that integrate several transceivers per cell and sectorization. One access technique that explores these system features is Space Division Multiple Access (SDMA) [12]. However, those approaches rely on smart antenna technology and beamforming. The same resource (frequency) can be allocated for systems using SDMA schemes by exploiting orthogonality in the space domain, but this is not directly applicable to central instances using omnidirectional antennas, as regarded in this paper. Furthermore, these approaches do not consider mutual interference from communicating MTs using the direct-mode.

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The parallel usage of same resources during the transmission of data between different pairs of communication partners generally leads to interference and/or to mutual disturbances of the respective transmissions. In Spatial Time Division Multiple Access (STDMA), the Multiple Access Interference (MAI) in a slotted and framed system is managed in a distributed way [13,14]. STDMA is a generalization of the TDMA protocol for MH networks where slots are allocated to a set of non-interfering transmissions. Based on a so-called compatibility matrix, which indicates links that can be simultaneously enabled without causing a collision at either of their respective destinations in the network, a schedule is defined. This approach provides a general methodology to assign resources in a slotted system. Nevertheless, each node requires a lot of information because of the distributed STDMA character. Furthermore, a synchronized system is assumed because of the slot and frame structure. In contrast to STDMA and its distributed assignment of resources, the SDiL appliance envisages that resources in self-organizing radio systems with direct-link communication between mobile terminals are assigned, supervised, and controlled by one single central instance. Moreover, no slotted operation is required, since the synchronization of the assignment is guaranteed by the central instance. 2.2.2. Multihop communication The benefit of MH communication is investigated in [5] and [15], which we refer to as classical MH communication. Instead of direct communication of an MT with the CC a further relaying node is introduced. With classical MH connections the required transmit power can be considerably reduced since several low-power links are established to connect arbitrary end-user-nodes with each other over several intermediate nodes [15]. At the same time interference is reduced, which in turn provides potentials for capacity increase. For example, it has been demonstrated for H/2 that under special constellations capacity can be increased with MH connections [5] in both, an isolated cell and a cellular system. Potential gains for relaying under special conditions, e.g. in CDMA-based systems, are referenced in [16]. In contrast to these results, it has been shown in [15] for a CDMA-based scheme that capacity decreases considerably as a result of the relaying option. For the direct link to the base station (BS) only a single resource is needed, whereas the use of relaying requires several resources, which are related to the number of individual links. Hence, it is worth mentioning that relaying requires more transmissions and receptions, which might result in higher interference, respectively power consumption. Moreover, it introduces additional delays. A capacity bottleneck occurs especially near the BS, where often only a few nodes are used to relay all packets in the cell. However, in [15] no link adaptation has been considered and the authors expect to increase capacity with MH when a reuse of resources within a cell is taken into account. This is the case in

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the approach investigated in this paper and is another distinguishing aspect of the SDiL from former research work. Other research efforts concentrate on the investigation of MH communication for enhanced coverage [15,17,18]. These approaches assume that MH communication over mobile terminals with relay capabilities mitigates typical problems of cellular networks, i.e. the existence of deadspot locations, e.g. due to shadowing of buildings. Different from the concept of mobile relay stations, which can no longer guarantee a full coverage of the area [17], in [19] it is proposed to extend the coverage by means of fixed multihop nodes, so called Extension Points. They are installed at certain locations and serve to extend the infrastructure in a flexible manner.

3. Oligohop communication Since every transmission between nodes needs resources, introduces additional delay, and requires appropriate scheduling, the maximum number of hops should be kept very low. Due to this restriction to a very limited number of hops the term ‘oligohop’ is introduced to reflect this characteristic of an end-to-end connection. It is assumed that an oligohop connection consists of up to four single links, respectively hops. The value of four hops is a compromise of potential benefits of relaying on the one hand, which is expected to increase with the possible number of hops, and complexity on the other hand, which increases considerably, too, not forgetting the end-to-end delay and protocol overhead that rises with each hop and transmission for an end-to-end connection. The value of four hops will be justified by the following general analytical investigations. Connections with more than four hops will be called MH connections. 3.1. Shannon capacity estimation To come to a more general view of the potential gains of MH communication the Shannon capacity can be used as upper bound for the achievable throughput on a single link for a given signal-to-noise ratio (SNR) and for a given bandwidth, W. The capacity, CS, over an additive white Gaussian noise (AWGN) channel becomes [22]   P CS Z W log2 1 C rx : N

(1)

with the received signal power, Prx, and average noise power, N at the receiver. On each link of an MH connection with n hops the data rate has to be n times higher than the data rate of the corresponding one-hop connection to end-up in the same end-to-end throughput. The required SNR for

the MH connection becomes      n Prx Prx Z 1C K1: N MH N one

(2)

On one hand, the required transmit power has to be increased to achieve the same data rate, but on the other hand the distance on the individual links decreases with an increasing number of hops. To incorporate this dependency in the capacity calculation, the receive power is substituted by the transmit power, Ptx, and the distance between source and destination, d, based on a simple one-slope pathloss model Prx Z Ptx C c0 K10g logðdÞ

(3)

with a distance-power gradient (attenuation exponent), g, and a constant c0 taking into account the attenuation at 1 m. With this substitution in Eq. (2) the Transmit-power-toNoise Ratio (TNR) for the MH connection as function of the TNR of the corresponding one-hop connection becomes    g      g n Ptx Ptx 1 d c c Z 1C K1 : n N 0 MH N 0 one d (4) 3.1.1. Shannon capacity with frequency re-use In the previous derivation it has been assumed that transmissions cannot take place at the same time to preserve orthogonality of the received signals in the time domain. However, concurrent transmissions increase the interference but can be acceptable if the required signal-tointerference-and-noise ratio (SINR) is sufficiently high. Furthermore, different interference suppression or cancellation techniques can be adopted to decrease the interference, respectively increase the SINR. In addition intelligent combination of all transmitted signals at the relays and final destination can increase the received signal power, exploiting the diversity of the radio channel, see also [20,21]. To consider the frequency reuse to come up with a higher spectral efficiency, the Shannon equation is further exploited to take into account the interference from simultaneous transmissions. The new parameter that is introduced is the reuse distance, dreuse, measured in meters. If we assume equidistant spacing of relays the reuse distance can take values equal to id/n, with i2[1; nK1]. An example line topology with relays equidistantly introduced between source and destination with five MTs and frequency reuse is shown in Fig. 2. As indicated in Fig. 2(B), co-channel interference at MT 2, respectively at MT 3, will occur when the first MT 1 transmits to the second MT 2 while the fourth MT 4 transmits to the third MT 3. The reuse distance for nZ4 in this example is three hops, i.e. dreuseZ3d/4. In the next transmission cycle MT 2 and MT 5 are simultaneously transmitting. In the following two cycles the data transfer takes part in the opposite direction to realize a bi-directional communication. This ends up in four

M. Lott et al. / Computer Communications 29 (2006) 983–993 t

MT1

MT2

MT3

MT4

MT5

0

d/4

d/2

3d/4

d

MT1

MT2

MT3

0

d/4

d/2

MT4

MT5

t1 t2 t3 t4 t5 t6 t7 t8 x

3d/4

d

x

A without frequency reuse

B with frequency reuse

Fig. 2. Frequency reuse in wireless multihop networks.

transmission cycles compared to eight transmission cycles for the MH communication without frequency reuse. Considering the interference constellations and the same end-to-end throughput as for a one-hop connection, the average SINR for multihop communication for a four hop connection with frequency reuse becomes 

Ptx c N 0

reuse MH

h

i     g ðn=2Þ g 1 C PNtx c0 one d1 K1 dn h i  : Z    g ðn=2Þ g 1K 1 C PNtx c0 one d1 K1 1k (5)

In this equation the separation of simultaneous transmission is incorporated in the variable k. A value of kZ2 corresponds to no interference cancellation. The more interference is suppressed the larger k becomes, unless the whole denominator in Eq. (5) becomes 1. In this case the power needed for the four-hop connection becomes equal to the power needed for two hops only. Hence, for an n-hop connection with even values n the capacity is doubled since two transmissions can take part at the same time, which is reflected in the exponent n in Eq. (5) that is divided by 2 compared to Eq. (4). In Fig. 3 the resulting transmit-power-to-noise ratio (TNR) for the MH connection with and without frequency

Fig. 3. Required transmit-power-to-noise ratio for four-hop connections as function of the TNR for a one-hop connection with and without frequency reuse at 5 GHz for dZ100, NZK80 dBm.

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reuse over the respective TNR for a one-hop connection is shown. The capacity breakeven is indicated by the line through the origin with gradient one. At the respective points both approaches, MH and one-hop, require the same transmit power. Curves above this line indicate that one-hop connections do need less power than MH connections, whereas curves below indicate scenarios in which MH connections should be favored. It becomes evident that the gain for a MH connection strongly depends on the transmit power, Ptx, and the attenuation exponent, g. The smaller the transmit power, and the stronger the attenuation, the more attractive becomes the introduction of intermediate hops. It can further be seen that with increasing interference reduction, i.e. with increasing value k, the breakeven point is shifted to larger TNRs when introducing frequency reuse. Comparing the gain with and without frequency reuse, there is a constant offset of about 3 dB as long as the TNR is low enough. Beyond some points the required transmit power rapidly increases and there cannot be achieved any gains in increasing the transmit power even more because of self-interference. Nevertheless, high gains can be obtained under high attenuation, e.g. approx. 10 dB (20 dB) for gZ2 (gZ4) and nZ4 hops. 3.1.2. Shannon capacity comprising overhead For realistic and fair capacity estimation the protocol, respectively physical overhead, is taken into account in the next step. The latter impact can be assessed by the pessimistic assumption that each transmission requires some fixed overhead, ovh, and n transmissions need n-times that overhead. In this case the exponent n in Eq. (4) becomes n(n$ovhC1)/(ovhC1). The relation of the TNR for the MH connection to the TNR for the one-hop connection as function of the introduced number of hops without considering frequency reuse is shown in Fig. 4. For values below 0 dB the MH connection needs less power than the one-hop connection, whereas values above

Fig. 4. Relation of transmit-power-to-noise ratio for MH and one-hop connections as function of the number of hops, dZ100 m.

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indicate scenarios in which a one-hop connection should be favored. It can be recognized that the gain for a MH connection also depends on the distance between source and destination, d, and the number of hops, denoted by n, besides the transmit power, Ptx, and the attenuation exponent, g. The larger the distance the more attractive becomes the introduction of intermediate hops. Typical transmit powers of PtxZ20 dBm (23 dBm), a distance of 100 m, attenuation exponents of gZ2.4 (3), and different values for the overhead between 0 and 30% have been chosen. It can be seen that with increasing number of hops the gain for relaying increases up to a point where the overhead dominates. For example, for PtxZ20 dBm and gZ3 the highest gains can be obtained for a number of five and four hops for overhead values of 20 and 30%, respectively. For better link conditions, respectively higher transmit power (PtxZ23 dBm) the highest gain can be achieved for three hops only taking into account an overhead of 30%. However, the most dominant factor is the attenuation exponent, which is expected to decrease to the free-space value with decreasing distance on the one-hop connection. In this case even for no overhead no benefits are expected from relaying for small transmit power of PtxZ20 dBm and gZ2.4. All the aforementioned facts and results lead us to the conclusion that a number of four hops should not be exceeded to benefit from relaying, which stems the definition for the oligohop connection from Section 3. 3.1.3. Shannon capacity as random variable and MIMO So far, it has been assumed that the channel state can be modeled as stationary ergodic process, i.e. that the packet transmission is much longer than the channel coherence time. Furthermore, we have assumed that we have only one transmit and receive antenna. The generalized Shannon bound on capacity for nT transmit and nR receive antennas for a multiple-input multiple-output (MIMO) system can be expressed as [23]    r CS Z W log2 det I C HH
; (6) nT where I is the identity matrix and r is the average received signal to noise ratio (SNR) assuming identically distributed noise at each receiver and transmit power equally distributed among the transmit antennas; H is the normalized channel transfer matrix and ‘*’ corresponds to the conjugate transpose. This can be simplified for only one transmit antenna to ! nR X 2 CS Z W log2 1 C r jHi j : (7)

difference is the factor before the SNR comprising the summation of the entries of the channel transfer matrix. This increases the TNR by the same amount and shifts the operation point towards higher values, i.e. better link conditions. Consequently, one can conclude that MH communication, i.e. breaking down the distance between two communicating points into smaller segments, becomes less attractive when introducing multiple antennas at the transmitter and receiver. The dimension rules from the previous section remain valid and MH communication should be favored for bad links, e.g. high noise or low reception power. But with MIMO the link performance is improved and it becomes more likely that operation over single-hop connections is more attractive than over MH connections. These generic and theoretical results are reflected and validated in typical scenarios with SingleInput Single-Output (SISO) channels for oligohop communication, which are presented in the next section. 3.2. Scenarios In this section different simple and straight-forward constellations of source, intermediate, and destination nodes for typical oligohop scenarios are displayed. It is assumed that the traffic from the terminals is directed to the CC, or from the CC to the MT. Thus, applications with Internet access, like file downloads or e-mail exchange, are envisaged. The direction of message transfer (download or upload) does not matter as long as the resulting topologies are symmetric. In the following figures, the different possibilities for traffic scheduling in simplified scenarios are depicted. In Fig. 5, the SDiL is applied and it is assumed that one DiL phase and one UL phase are simultaneously scheduled (DiL: MT1/MT2/UL: MT3/CC and DiL: MT4/MT3/ UL: MT2/CC). It is assumed that the two terminals MT1 and MT4 with distance d10Zd40Z2d to the CC have apply for data transmission (see Fig. 6). In case of the SDiL scheduling, the intermediate terminals MT2 and MT3 with distance d20Zd30Zd to the CC can relay the traffic (see Fig. 5). For comparison with the SDiL, a reference scheduling (RS) scenario without intermediate terminals is introduced, see Fig. 6. Identical to the SDiL scenario, the duration needed to transfer data in the reference scenario equals two time units, e.g. two MAC frames. This is the consequence of using the conventional scheduling approach, which is based on

iZ0

From this result one can directly conclude that the same derivation for the TNR can be used as for the ergodic Shannon capacity for single and MH connections. The only

Fig. 5. Smart Direct-Link Scheduling (SDiL).

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Table 1 Simulation parameters

Fig. 6. Reference Scheduling (RS).

the assignment of exclusive resources, e.g. time slots, for transmission between MTs and/or MTs and the CC. Hence, both scenarios result in the same throughput. 3.3. Reliability With oligohop connections, the packet can be lost several times. Hence, the residual end-to-end packet error ratio, which is one basic measure for the quality of the connection, is likely to increase with each additional hop. For the purpose of comparison of an oligohop connection with i hops and a one-hop connection, the resulting probability of success has to be the same. This results in a packet error ratio for each link of the oligohop (ol) connection PPER_ol,i as function of the packet error ratio for the one-hop (single-hop, s) connection, PPER_s. pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi PPER_o1;i Z 1K i 1KPPER_S : (8) For a single-hop PER of PPER_sZ10 each hop of a two-hop oligohop connection has to guarantee a PER of approx. PPER_ol,2Z0.5!10K3, which is half of the value allowed for a one-hop connection. K3

Parameter

Variable

Value

Max. transmit power Noise power Antenna gain Attenuation exponent

Ptx_max N G g

23 dBm K90 dBm/K80 dBm 0 dBi 2/2.4/4

the reference scenario with one-hop transmission the resulting C/N for a noise power of K90 dBm at 5 GHz can be calculated by the following equation, based on the logarithmic presentation for the reception power of Eq. (3): C=N Z 23 dBmK46; 42 dBK10g logð2dÞ KðK90 dBmÞ Z 66; 58 dBK10g logð2dÞ

(9)

For the SDiL and from the perspective of MT2, MT1 is transmitting to MT2 with distance d12Zd, while MT3 with distance (d30Cd20)Z2d is causing interference. In this case the resulting C/(ICN) can be calculated by the following equation: C=ðI C NÞ Z ð23 dBmK46; 42 dBK10g logðd12 ÞÞ Kð10 logð10ð2;3 dBmK4;642 dBKg logðd30Cd20 ÞÞ C 10K9 mWÞÞ (10) and neglecting the noise power this results in

4. Performance evaluation of the SDiL For a realistic and fair comparison of one-hop and oligohop communication, the channel and its impact on the transmission scheme has to be modeled in an appropriate manner. Furthermore, the impacts on the protocol overhead and the single and multiple transmissions have to be comparable. Therefore, the same reliability with respect to the residual packet error ratio is demanded. In the following, performance results of the previously introduced scenarios are investigated. Based on different propagation conditions and channel models the generated interference for the different scheduling approaches is determined. For all scenarios the same residual PER is envisaged. The general simulation assumptions are summarized in Table 1. 4.1. Receive power budget In this section the receive power budget, C/(ICN), for H/2 as function of the distance between the MT and CC and for the SDiL appliance is determined. All calculations assume a maximum transmit power of 23 dBm. Because of the concurrent transmission in the SDiL appliance, the same end-to-end data rate as in the reference scenario adjusts. For

C=I Z 10g logð2Þ Z 3g dB:

(11)

One can directly follow that the possible throughput for the SDiL increases with the attenuation exponent and is independent of the distance as long as the noise can be neglected compared to the interference. Due to the symmetrical nature of the SDiL scenario in Fig. 5 Eq. (8) holds for all receiving stations at any time step. Typical values for the C/(ICN) for the reference (RS) and SDiL scenario are listed in the following Table 2. From these results it becomes clear that the attenuation exponent has a strong impact on the performance of the SDiL and the one-hop transmission. Moreover, it can be recognized that the noise power has a direct impact on the one-hop connection. This is not the case for the SDiL appliance, where most of the times the interference is the limiting factor and the noise power has minor impact. Hence, for these cases it can be expected that the transmission power of 23 dBm can be reduced considerable without impacting the C/(ICN) and thus, the PER performance. Only for large distances and high attenuation the noise power is dominating. Though the performance of the SDiL degrades, it outperforms the one-hop transmission under these conditions. It is therefore expected that benefits for the SDiL can be achieved.

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Table 2 Example receive power budget for one-hop and SDiL appliance gZ2 Distance of communication partners (d) RS (C/N) (dB) SDiL (C)/(ICN) (dB) RS (C/N) (dB) SDiL (C/(ICN)) (dB)

100 m

gZ2.4 200 m

Noise power NZK90 dBm 20.56 14.54 5.98 5.87 Noise power NZK80 dBm 10.56 4.54 5.65 4.71

(a) with respect to interference generation for good onehop connections by reducing the transmit power close to the noise power, and (b) with respect to throughput for bad one-hop connections (insufficient link budget/too large distance) by increasing the transmission rate compared to a one-hop transmission. 4.2. Impact of neighbour cells and interference It is worth mentioning that an increase of the interference, e.g. from neighbor cells or foreign systems, will have a similar impact as an increase of the noise power, i.e. the link performance decreases. Hereby, it is assumed that frequency selection is performed by means of dynamic frequency selection (DFS) procedure as, e.g. defined for H/2. In this case, neighbor cells will not use the same frequency but autonomously select an idle frequency channel. The variance of the interference stemming from different terminals re-using the same frequency will thereby be reduced as the variance in the distance between interfering terminals and disturbed receiver becomes smaller. Furthermore, higher overall system capacity can be achieved for H/2 in a cellular structure if the transmission is evenly distributed over the whole frame generating a homogeneous ’interference floor’ [11]. With increasing the transmission duration and selecting a more robust coding and modulation scheme the required transmit power can be reduced, and as result, the overall capacity increases. With increasing noise-like interference the probability increases that the oligohop connection outperforms the single-hop connection as the link quality degrades. However, higher interference might be a drawback for the SDiL when introducing an interference margin (higher transmit power to combat interference). This extra power cannot be exploited in the case of the SDiL because of selfinterference. In contrast to the SDiL, one-hop connections directly benefit from extra power budget, expressed by enhanced link reliability. Summarizing this section, it is expected that the same tendency of performance results for a single-cell environment will be found in a typical cellular structure

100 m

200 m

11.36 6.92

4.13 5.81

1.36 4.84

K5.87 0.36

with inter-cell interference. The same interrelation has been stated in [5]. 4.3. BER performance Up to now only the signal power and noise/interference has been taken into account to determine the performance of oligohop and one-hop connections. A mapping to the throughput, and hence, to the system performance, is only possible by the BER, respectively PER. In Fig. 7 the BER as function of the transmit power, Ptx, is derived for quadrature phase-shift keying (QPSK) modulation and additive white Gaussian noise (AWGN). A noise power of K80 dBm and a distance for the reference scenario between MT and CC of 40 m are assumed. To consider the reduced reliability of oligohop connections (the higher probability of a packet error owing to multiple transmissions, see Section 3.3), a higher BER is allowed for the one-hop connection in comparison to the BER on a link of an oligohop connection. In this case the end-to-end BER is the same for the RS and SDiL scenario. At this point it is worth mentioning that the PER, which depends on the BER, packet size, code rate, and channel conditions, has to be considered for the reliability of an oligohop connection. However, it can be assumed that for

Fig. 7. BER versus transmission power for the RS and SDiL scenarios considering AWGN and QPSK; NZK90 dBm; dZ20 m.

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reasonable operation points of BER values below 10K2, the negative gradient of the coded PER curve is comparable to the one of the BER curve. Hence, the same increase in transmit power is needed on the oligohop connection to reduce the PER by the required amount compared to the BER, and the following results can serve as a typical scenario under real conditions. This will also be manifested by the results with PER values presented in the following Section 4.4, taking into account the coding and modulation as used in H/2. The respective curves for the reference scenario are denoted by RScomp according to PBER_ol,2, see Eq. (8). The curves without considering this difference in reliability are denoted by RS (PBER_s). The latter curves only reflect the receive power budget as introduced in Section 4.1. From the results it can be derived that the larger the attenuation becomes (larger g), the better is the performance of the SDiL in comparison to the one-hop connection for medium transmit power values, e.g. between 0 and 10 dBm the oligohop connection outperforms the one-hop connections for gZ3. With increasing transmit power the BER for the SDiL cannot be further reduced and results in an errorfloor. The reason for this behavior is the self-interference of the SDiL because of simultaneous transmitting MTs influencing each other. For these transmit power values the self-interference dominates the noise power. For low transmit powers or high BER values, the one-hop connection which considers the reliability (RScomp) is always better since it needs to support only half of the BER that a link of the oligohop connection has to guarantee. In these cases the noise power on the oligohop connection dominates the BER performance. However, because of the smaller distance the SDiL outperforms the one-hop connection when the reliability is neglected (RS). Moreover, the operation point of a link will not be at such high BER values. Realistic operation points are at BER values below 10K1. In summary, the SDiL is attractive before saturation (error-floor) is reached and where self-interference dominates the BER performance. Furthermore, SDiL is attractive for high attenuation coefficients where high performance gains with respect to the required transmit power can be obtained, e.g. gains of up to 3 dB for gZ3 and 8 dB for gZ4. Since the SDiL is under control of a central instance it can be expected that the optimal operation point is likely to be adjusted.

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Fig. 8. PER versus transmission power for the scenarios RS and SDiL considering H/2; BRAIN channel model D; BPSK 1/2; 6 Mbit/s; NZK90 dBm; dZ20 m.

the receiver and the chosen modulation and coding scheme (PHY mode). Based on the C/I at the receiver, that has to be higher for the oligohop connection to achieve a lower PER on each link and, consequently, to result in the same end-toend PER as for the one-hop connection, the required transmission power at the transmitter is calculated. The results in Figs. 8 and 9 give the required transmission power at MT2 in the oligohop case with SDiL (see Fig. 5) and MT1 in the one-hop case (see Fig. 6). Similar to Fig. 7, curves for comparison are denoted by RScomp (with PPER_ol,2), whereas the curves without considering the reliability are denoted by RS. The curves for the one-hop case differ in the PER by the amount that PPER_ol,2 differs from PPER_s. From the results it can be seen that in the case of steep PER curves valid for one-hop transmissions, significant

4.4. SDiL performance in HIPERLAN/2 In this section a more realistic determination of the SDiL appliance is carried out by considering a H/2 system and by using the BRAIN channel model D, which models large open space/outdoor environments with line-of-sight (LOS) conditions and with an root-mean square (rms) delay-spread of 138 ns [24]. From link-level simulations [24], the PER for one-hop transmission is given depending on the C/I at

Fig. 9. PER versus transmission power for the scenarios RS and SDiL considering H/2; BRAIN channel model D; QPSK 3/4; 18 Mbit/s; NZK90 dBm; dZ20 m.

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performance improvements can be obtained by SDiL appliance (Fig. 8). For example, for BPSK and free-space propagation (gZ2), the required transmit power for the oligohop connection is reduced by 4 dB compared to the one-hop connection in order to achieve a PER of 10K2, cf. Fig. 8. This difference becomes even larger when the attenuation increases, e.g. 10 dB for gZ4. For certain PHY modes the PER curves are less steep, which means that the transmission power has to be increased by a larger amount in order to achieve lower PER, cf. Fig. 9. When increasing the transmission power the self-interference is also increased by SDiL appliance, which ends up in an error-floor. This effect particularly occurs when using higher PHY modes. The results show that for the assumed noise N, distance d and attenuation exponents, g!4 SDiL appliance cannot beneficially be used for high PHY modes where the PER curves are less steep e.g. QPSK. Consequently, the use of coding schemes, like turbo coding that yields to extremely steep PER curves, is beneficial for the SDiL. 4.5. Multihop communication without frequency re-use In the previous sections, the SDiL appliance was compared with a reference scenario based on one-hop transmission. Due to the scheduling principles the same data rate can be used on each link of the one-hop and the oligohop transmission in order to achieve the same throughput, e.g. 6 Mbit/s. In general, for multihop transmission without frequency re-use it is proposed to transmit in more time slots (e.g. twice as much time slots for a twohop connection) and with respectively higher data rates (e.g. doubling the data rate from 6 to 12 Mbit/s) in order to achieve the same throughput as for the one-hop transmission, see Fig. 10. The curves in Fig. 11 show the PER valid for the RS, SDiL and multihop communication without frequency reuse with two-hops considering the same throughput of 6 Mbit/s at the receiver. Hence, the data rate on the multihop connection without diversity has to be 12 Mbit/s. From the results it becomes obvious that SDiL appliance outperforms multihop transmission without frequency reuse. The required increase of transmit power to double the data rate is larger than the increase of power needed to overcome the noise and self-interference on the SDiL. In case of the multihop connection without frequency reuse the higher data rate has to be realized by means of a higher PHY

Fig. 10. Multihop scheduling without frequency re-use (Classical).

Fig. 11. PER versus transmission power for the scenarios RS (6 Mbit/s), SDIL (6 Mbit/s) and classical multihop (12 Mbit/s) considering H/2; BRAIN channel model D; NZK90 dBm; dZ20 m.

mode that provides less steep PER curves. Consequently, more additional power is needed to reduce the PER because of the required reliability on the end-to-end connection. From these and the previous results one can conclude that only a limited number of hops seems to be profitable, for both, SDiL and multihop communication without frequency reuse. Especially, for the latter it has to be taken into account that an increase in data rate often requires more transmit power since the PER curves become less steep for higher PHY modes. Moreover, with increasing number of intermediate relays it can be expected that the attenuation exponent does not decrease any more below the value of free-space, and hence, the gain does not increase. Consequently, forwarding for capacity increase should be restricted to oligohop communication, i.e. to no more than four hops.

5. Conclusion MH communication is currently a research topic of high interest since it opens the potential of capacity increase. Furthermore, it is expected that this kind of communication will be part of future system architectures. Within this paper the potential gains of MH connections for a given constellation of terminals with limited number of hops, which are called oligohop connections, have been investigated. Based on the HIPERLAN/2 standard a new resource allocation approach has been introduced, which allows to simultaneously grant resources under the control of the central instance and exploits oligohop communication. This smart direct link (SDiL), which relies on parallel scheduling during the direct-link phase of HIPERLAN/2, has been compared with a sequentially scheduled one-hop connection. Based on Shannon capacity for MH communication and analytical equations the performance with respect to receive power budget and BER, respectively PER has been derived.

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It has been shown that under several conditions the SDiL outperforms the conventional one-hop connection. However, an oligohop connection does not always perform better than a one-hop connection. Specifically, for good links, i.e. small distances between source and destination and low attenuation coefficients, a one-hop connection should be selected. This is different for high attenuation coefficients above 2.4 and large distances, where the noise power is the dominating factor on a one-hop connection. Under these conditions oligohop connections can mitigate the attenuation by reducing the distance on an individual link. In addition, the frequency-reuse can be beneficially exploited by the SDiL appliance. Further investigations in this field aim at a combination of the SDiL appliance with power control and link adaptation. Since the communication partners are closely positioned to each other, a reduction of transmission power may be triggered, resulting in an overall improved interference situation for other stations. Finally, consideration of link adaptation offers an additional degree of freedom for the smart multiple grant of the same resource.

Acknowledgements A major part of this work has been performed in the framework of the IST project IST-2000-28584 MIND, which is partly funded by the European Union. The authors would like to acknowledge the contributions of their colleagues from Siemens AG, British Telecommunications PLC, Agora Systems S.A., Ericsson AB, France Te´le´com S.A., King’s College London, Nokia Corporation, NTT DoCoMo Inc, Sony International (Europe) GmbH, T-Systems Nova GmbH, University of Madrid, and Infineon Technologies AG.

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