Busy Bursts for Trading off Throughput and Fairness in Cellular OFDMA-TDD
© Birendra Ghimire et al. 2009
Received: 1 July 2008
Accepted: 8 December 2008
Published: 12 January 2009
Decentralised interference management for orthogonal frequency division multiple access (OFDMA) operating in time division duplex (TDD) cellular systems is addressed. Interference aware allocation of time-frequency slots is accomplished by letting receivers transmit a busy burst (BB) in a time-multiplexed minislot, upon successful reception of data. Exploiting TDD channel reciprocity, an exclusion region around a victim receiver is established, whose size is determined by a threshold parameter, known at the entire network. By adjusting this threshold parameter, the amount of cochannel interference (CCI) caused to active receivers in neighbouring cells is dynamically controlled. It is demonstrated that by tuning the interference threshold parameter, system throughput can be traded off for improving user throughput at the cell boundary, which in turn enhances fairness. Moreover, a variable BB power is proposed that allows an individual link to signal the maximum CCI it can tolerate whilst satisfying a certain quality-of-service constraint. The variable BB power variant not only alleviates the need to optimise the interference threshold parameter, but also achieves a favourable tradeoff between system throughput and fairness. Finally, link adaptation conveyed by BB signalling is proposed, where the transmission format is matched to the instantaneous channel conditions.
Orthogonal frequency division multiplexing (OFDM) has been selected as a radio access technology for a number of wireless communication systems, for instance, the wireless local area network (WLAN) standard IEEE 802.11 , the European terrestrial video broadcasting standard DVB-T , and for beyond 3rd generation (B3G) mobile communication systems . In OFDMA, the available resources are partitioned into time-frequency slots, also referred to as chunks, which can be flexibly distributed among a number of users who share the wireless medium. Provided that channel knowledge is available at the transmitter, resources can be assigned to users with favourable channel conditions, giving rise to multiuser diversity .
Interference management is one of the major challenges for cellular wireless systems, as transmissions in a given cell cause cochannel interference (CCI) to the neighbouring cells. Full-frequency reuse where the transmitters are allowed an unrestricted access to all resources causes high CCI, which particularly impacts the cell-edge users [5–7]. Although CCI can be mitigated by traditional frequency planning, this potentially results in a loss in bandwidth efficiency due to insufficient spatial reuse of radio resources. Fractional frequency reuse (FFR) [4–6, 8] addresses this issue by realising that in the cellular networks CCI predominantly affects users near the cell boundary. FFR typically involves a subband with full-frequency reuse that is exempt from any slot assignment restrictions. The allocation of the remaining subbands is coordinated among neighbouring cells, in a way that the users in the given cell are denied access to subbands assigned to the cell-edge users in the adjacent cells. To this end, in  a user is classified as a cell-edge user based on the path loss to the desired base station (BS). This approach ignores the fact that the channel attenuation of the desired and the interfering signals is uncorrelated, and therefore fails to exploit interference diversity. Moreover, frequency planning results in a hard spatial reuse of the available resources. As a result, it cannot cater for the dynamic traffic and load across different sites. Furthermore, in systems where BSs are dynamically added in an uncoordinated manner, such as home base stations , reconfigurable frequency reuse planning may prove to be increasingly cumbersome.
The busy-signal concept [10–16] has been identified as an enabler for decentralised and interference aware slot assignment. Receiver feedback informs a potential transmitter about the instantaneous CCI it causes to the "victim" receivers, which enables the transmitter to take appropriate steps to avoid interference, such as deferring its own transmission to another chunk. Early works [10, 11] rely on dedicated frequency-multiplexed channels that carry narrowband busy tones for channel reservation. As the protocol requires the transceivers to listen to the out-of-band busy tones whilst transmitting, complex RF units are required due to additional filters and duplexers involved. This drawback is avoided in [12–14], where time-multiplexed busy bursts (BBs) serve as a reservation indicator for a reservation-based medium access control (MAC) protocol. By transmitting an in-band BB in an associated minislot following a successful transmission, two important goals are accomplished [13, 14]. First, the transmitter of its own link is informed whether or not the data is successfully received. Second, at the same time potential transmitters of the competing links are notified about ongoing transmissions, so that these transmitters can take appropriate steps to avoid interference. Therefore, both slot reservation and channel sensing tasks are accomplished. Furthermore, interference diversity is exploited, in the way that competing links may spatially reuse the same slot, given the interfering links are sufficiently attenuated.
None of the busy tone-based MAC protocols [11–14] allow for dynamic resource allocation where multiple users share a set of parallel frequency slots of a broadband frequency-selective radio channel, such as the 100 MHz channel of the WINNER (Wireless world Initiative New Radio, http://www.ist-winner.org) TDD mode . By extending the BB concept to OFDMA [15, 16], the channel reciprocity of TDD  is exploited for decentralised interference management such that the chunks can be dynamically assigned on a short-term basis thereby ensuring a soft spatial reuse of chunks among cells. This concept termed BB-OFDMA works in a completely decentralised fashion and is therefore applicable to self-organising networks, which may consist of cellular as well as ad hoc network topologies.
The attainable system throughput of BB-OFDMA is sensitive to the selected interference threshold [15, 16]. In this paper, it is demonstrated how the interference threshold can be tuned to tradeoff system throughput to enhance cell-edge user throughput, thereby enhancing fairness. Moreover, by using a variable BB power that takes into account the quality of the intended link, a favourable tradeoff between system throughput and fairness is achieved. A variable BB power exhibits the further advantage that the sensitivity of the selected interference threshold on the performance is mitigated. Finally, BB-OFDMA with variable BB power is the basis for a novel receiver-driven link adaptation algorithm. System-level simulations demonstrate a significant improvement both in terms of fairness and total system throughput of BB-OFDMA, compared to the system with full-frequency reuse, where attempts to avoid interference are not made.
The remainder of the paper is arranged as follows. Section 2 describes the air interface of WINNER-TDD. The allocation of radio resources among the competing user population is discussed in Section 3. Section 4 introduces the BB signalling mechanism and its variants as well as the proposed link adaptation algorithm. The considered Manhattan grid deployment scenario and the system level simulator are introduced in Section 5, and the simulation results are discussed in Section 6. Finally, the conclusions are drawn in Section 7.
2. System Model
The transmitted signal of chunk propagates through a mobile radio channel. The corresponding channel gain comprises radio effects such as distance-dependent path loss, log-normal shadowing as well as channel variations due to frequency-selective fading and user mobility . While channel variations of between adjacent chunks in time and frequency are taken into account, fluctuations within a chunk are neglected. This approximation is justified as long as the chunk dimensions are significantly smaller than the coherence time and frequency .
3. Multiuser Resource Allocation
where the Boolean operator is set to 1 or 0 when the condition is true or false, respectively. The parameter indicates whether or not user is granted access to chunk . For interference aware and reservation-based MAC protocols such as BB-OFDMA (see Section 4.4), setting ensures that user in cell is denied access to chunk . This effectively avoids radiation of CCI from cell to any neighbouring cells that use the same chunk .
represent the set of successfully allocated chunks of user , denoted by .
For BB-OFDMA chunks with are reserved and protected from interference at the next frame by setting the reservation indicator to in (4). When the SINR target is not met, , the reservation indicator is reset to . These chunks are released in a way that user is prohibited access in the next slot by setting . Subsequently, chunk is assigned to other users by (4).
In a cellular OFDMA system without interference protection, there is no restriction for accessing any chunks, so in (3) for all users in the cell. Moreover, no reservation indicator is set, in (4), irrespective of in (5).
4. Busy Burst Signalling
Transmitters are prohibited to access chunks where a BB is detected above a given threshold.
The resulting BB signalling overhead amounts to 6.7%, as 2 OFDM symbols out of 30 OFDM symbols per frame are used for BB signalling. However, instead of dismissing BB signalling as overhead, the BB minislots may be utilised to convey the feedback and control information. Hence, BB signalling may serve as an alternative control channel.
4.1. Two Competing Links
To mathematically describe BB-enabled interference avoidance, let define a transmitter or receiver (either BS or MS) of user within cell . With this notation, the channel gain of the intended link at chunk becomes . The channel gain of an interfering link, between transmitter of user located in an adjacent cell and receiver , is denoted by . In case two links compete for resources, the CCI between transmitter and receiver amounts to . (The term is equivalent to the CCI as defined in (1). While the notation is preferred for intercellular interference management, the latter is used for intracell resource allocation. The same rule applies for related quantities that denote transmitted and received signal powers.) Likewise, and are the transmit power of the BB transmitter (data receiver) and the interfering BB power received at data transmitter (BB receiver), respectively.
4.2. Extension to Multiple Cells
Unlike the case when two links compete for resources, is no longer equivalent to in the threshold test (8). This is because in (9) the interference powers from multiple transmitters add up. Consequently, the total CCI at data receiver may exceed the tolerable threshold such that , although the BB power (10) observed by the individual interferers is below the threshold, . On the other hand, in (10) the interfering BB powers from multiple simultaneously active receivers observed at add up. It is, therefore, possible that , so that link is prohibited from accessing chunk , although its individual CCI contribution, would be below . Note that the former effect partly compensates the latter. Moreover, in many cases the interference is dominated by one strong interfering source. Therefore, the threshold test (8) provides a good approximation to the level of interference potentially caused to the active receivers.
4.3. Initial Access in Contention
Initial access of unreserved slots in BB-OFDMA is carried out in contention. During contention, two or more transmitters from adjacent cells may access chunk simultaneously. As a result, one or several links may encounter a collision on chunk where the SINR target is not met. To reduce the occurrence of simultaneously accessed chunks in contention, a -persistent chunk allocation procedure is applied to BB-OFDMA, where chunk in cell is accessed with probability . Denoting the outcome of the -persistent chunk allocation with the binary random variable , the access probability yields . The impact of on the system performance is investigated in Section 6.1.
4.4. Decentralised Chunk Reservation with BB Signalling
Chunks, where in (4), are allocated to user . Those chunks where the achieved SINR is above a required SINR target, , are reserved by setting the reservation indicator in (4), and are subsequently protected from CCI by BB broadcast. The BB broadcast from the intended data receiver is observed as a surge in the received BB power , which effectively notifies the transmitter that the data of chunk has been correctly received. User then reserves chunk in the next frame by setting in (5). On the other hand, if the transmitter does not detect a BB surge, it is understood that the SINR target was not met due to high CCI. These chunks are released by a reset of the reservation indicator to and setting , so that chunk may be assigned to other users.
4.5. Balancing System Throughput and Fairness
Cell-edge users are particularly affected by CCI for two reasons. First, the desired signal levels are, on average, much weaker compared to users in close vicinity to the desired BS due to relatively low channel gains on their intended links . Second, cell-edge users suffer from high CCI in the downlink, or cause high CCI to the adjacent cells in the uplink.
By tuning the interference threshold in (8), the amount of CCI caused to the receiver of a preestablished and coexisting link is adjusted. Lowering enforces a larger exclusion region around a vulnerable receiver. This enables cell-edge users to meet their SINR target with a greater likelihood. On the other hand, by augmenting , the number of simultaneously served links increases, giving rise to an enhanced system throughput. However, the cell-edge users are less likely to maintain their SINR target as interference protection is gradually eliminated. The chunks are released where the SINR target is not met, which means that these chunks are no longer reserved. Since the cell-centre users are less exposed to CCI, the chunks released by the cell-edge users are likely to be reallocated to the cell-centre users. As the allocation of the resources is shifted from the cell-edge users towards the cell-centre users, fairness is compromised. Hence, by adjusting , system throughput is traded off for fairness.
where is the number of users in a given cell . The user throughput accounts for the number of successfully transmitted/received bits by user , as defined in (5). A fairness index of represents a perfectly fair system where all users achieve the same throughput. On the other extreme, a fairness index of represents an unfair system where one user is served while all other users starve. We note that the fairness definition (12) is a relative measure, which ignores the absolute achieved throughput per user. To this end, a good fairness index may coincide with poor spectrum utilisation. For instance, a system where two users achieve Mbps and Mbps would result in a poorer fairness index than a system where both users achieve only Mbps. When analysing fairness, the fairness definition (12) should therefore be considered jointly with user throughput results.
(1) Consequences for the Downlink In the downlink, MSs at the cell edge are exposed to high CCI from transmitters in adjacent cells (see Figure 2(a)). Note that the CCI observed at a given cell (cell 1 in Figure 2(a)) is independent of the user distribution in adjacent cells (cell 2 in Figure 2(a)), assuming a constant transmit power . This implies that if BS2 lies within the exclusion region of MS1, resources reserved by MS1 cannot be spatially reused by any of the links in cell 2. However, if is increased such that BS2 is located outside the exclusion region of MS1, all links in cell 2 qualify for a spatial reuse of the resources reserved by MS1. However, the SINR target at MS1 is less likely to be met. Should the SINR target at MS1 not be met, this would cause the chunk allocated to MS1 to be released and reallocated to another user served by BS1- possibly a user that is located closer to the the serving BS1. Therefore, the cell-edge throughput would suffer.
(2) Consequences for the Uplink In the uplink, the transmitters (MSs) are distributed uniformly over the coverage area of the BS (see Figure 2(b)). Unlike the downlink, the CCI at the tagged BS depends on which MS transmits in the adjacent cell. To this end, the CCI observed at BS1 in Figure 2(b) depends on whether MS2 or MS3 transmits to BS2. Suppose that in cell 2 both MS2 and MS3 contend with MS1 in cell 1 for chunks and . In case MS2 and MS1 simultaneously access chunk , while MS3 and MS1 simultaneously access chunk , the SINR at BS1 tends to be superior on chunk due to the lower CCI caused by MS3. While MS2 causes excessive CCI to BS1, MS1 and MS3 may share chunk , although both users might be located near the cell boundary. Thus the uplink benefits from interference diversity due to the distributed location of mobile users. As a result, the degradation of performance at the cell edge at high in uplink mode is less severe compared to the downlink.
4.6. Interference Tolerance Signalling via Busy Bursts
where is the maximum BB transmit power. The operator ensures that . Note that when , we get . In this situation, the chunk is released and no BB is transmitted. Therefore, it is ensured that in (14) always has a positive value. We note that and . It can be checked by plugging (14) into (8) that the threshold test (8) effectively checks if , regardless of the threshold used, as long as the BB transmit power is not clipped. In this paper, we choose dBm because the probability of BB transmit power being clipped was found to be lower than 0.05 for the given deployment scenario with dB used. Furthermore, with this threshold, the received BB at the intended transmitter (the lower bound of which is approximated by ) remains well above the noise floor dBm, such that it can be reliably detected.
4.7. Link Adaptation with BB Signalling
Receiver feedback based on BB-ITS (see Section 4.6) allows for receiver-driven link adaptation, where the chosen transmission rate is adapted to the instantaneous channel conditions. Let be the set of supported modulation schemes. Associated to each modulation scheme is an SINR target that must be achieved to satisfy a given frame error rate (FER).
Provided that the channel response does not change between successive frames, changes in may be signalled from receiver to transmitter through (14), since any fluctuation in received BB power is due to a change of in (14). In summary, BB-ITS serves two important objectives. First, by adjusting the SINR target , the receiver implicitly signals to the transmitter through BB-ITS that the transmission format should be changed; second, by varying the BB power in (14), the size of the exclusion region around the active receiver is adjusted, so that the required SINR target is met in successive frames.
Link adaptation with BB-ITS is carried out in two phases: the contention phase, where the link is established and the link adaptation (LA) phase, where the receiver adjusts its transmission format to the current channel conditions.
In contention, multiuser chunk allocation is carried out as described in Section 4.3. To contend for an unreserved chunk , transmitter initially uses the modulation scheme with the lowest spectral efficiency . Chunks that satisfy are reserved in the next frame by BB signalling (see Section 4.4), where the transmit power in (14) is set using . Then the transmission proceeds to the link adaptation phase.
Link Adaptation Phase
Estimation errors due to channel variations and noise may cause detection errors, so that . Mismatch between the selected modulation schemes at transmitter and receiver can be mitigated if the transmitter announces together with payload data on chunk .
4.8. Benchmark System
5. Manhattan Grid Deployment
On average MSs are served by one cell, uniformly distributed in the streets and moving with a constant velocity of 5 km/h. BSs are placed in the middle of the street canyons with an inter-BS distance of 4 building blocks, as depicted in Figure 4. Distance dependent path loss, log-normal shadowing, and frequency selective fading are taken into account, as specified in , channel model B1. While the effect of user mobility on the channel response due to the Doppler effect is taken into account, movement of users along the streets is not considered during the duration of one snapshot. Links where transmitter and receiver are located on the same street are modelled as line-of-sight (LoS) channels, with significantly lower path loss attenuation than nonline-of-sight (NLoS) links . WINNER channel models B1-LOS and B1-NLOS  are used to model the LoS and NLoS channels, respectively. MSs are connected to the BS with the least path loss. A network synchronised in time and frequency is assumed.
Carrier centre frequency
No. of subcarriers (SCs)
No. of sectors/cell
No. of snapshots
6. Results and Discussion
6.1. Collisions Based on Access Probability
6.2. Setting the Threshold for Fixed Power BB Signalling
6.3. Impact of Interference Threshold on Fairness
On the other hand, with BB-ITS, median fairness indices of 0.7 are achieved. The corresponding average uplink and downlink user throughputs at the cell edge amount to 2.57 Mbps and 2.99 Mbps, respectively. The corresponding reduction in system throughput compared to the respective optimal thresholds with fixed power BB is only 1% in the uplink and 8% in the downlink. Note that BB-OFDMA with fixed BB power requires a 22% reduction in system throughput for a comparable performance at the cell edge. In light of this, BB-ITS results in a better tradeoff between system throughput and fairness.
For comparison, the median fairness resulting from ASCA is in the uplink and in the downlink. The corresponding average user throughputs at the cell edge are 2.278 Mbps and 208 kbps, respectively. This means that ASCA is more fair in the uplink compared to the downlink. The reason is that in the downlink cell-edge users are exposed to high CCI, while in the uplink cell-edge users cause high CCI to adjacent cells. Hence the detrimental effects of interference on the uplink tend to be more equally distributed among all users.
6.4. Comparison between BB-OFDMA and ASCA
Figures 9(c)-9(d) show the cdf of the user throughput for BB-OFDMA and ASCA. When fairness is the primary concern, dBm in the uplink and dBm in the downlink are preferable. Then the 10%-ile of the achieved user throughput amounts to Mbps in the uplink (see Figure 9(c)) and Mbps in the downlink (see Figure 9(d)). In contrast, ASCA fails to deliver any downlink throughput to more than 20% of the users. In the uplink, the 10%-ile of the user throughput of BB-OFDMA is improved by 40% compared to ASCA. With these uplink and downlink thresholds of dBm and dBm, the median system throughput of BB-OFDMA is still 15% and 18% higher than that achieved with ASCA (see Figures 9(a)-9(b)).
The results of BB-OFDMA with variable BB power, termed BB-ITS, are also included in Figures 9(a)–9(d). With BB-ITS, the lower 10%-ile of user throughput achieved is 1.04 Mbps in uplink and 1.416 Mbps in downlink (see Figures 9(c)-9(d)), at a modest degradation in system throughput (see Figures 9(a)-9(b)) compared to BB-OFDMA with fixed threshold that maximises the respective system throughput. BB-ITS, therefore, not only avoids the need for tuning the interference threshold so as to match a certain interference scenario (e.g., in uplink or downlink), but also achieves a preferable compromise between maximising system throughput and maintaining fairness.
6.5. Link Adaptation with BB-Signalling
By performing link adaptation with BB-ITS, the cell-edge users benefit in the downlink, whereas the users that are closer to their desired BS benefit in the uplink. The reason for this opposite trend for the uplink and the downlink is elaborated in the following. Due to the specific point-to-multipoint structure in the downlink, the CCI observed by the cell-edge users is dominated by the interference originating from the closest BS. When a chunk is assigned to a cell-edge user in the downlink, interference tolerance signalling enforces that this chunk cannot be spatially reused by the closest BS in an adjacent cell. By ensuring that, this dominant interferer does not access this chunk, the achieved SINR is greatly improved, potentially enough to meet the higher SINR target(s), thus allowing for the higher-order modulation schemes. In the uplink, on the other hand, the chunks assigned to the cell-edge users are more likely to be reused in the adjacent cells due to the distributed location of the MSs transmitters (see Section 4.5). Consequently, it is less likely that a more spectrally efficient modulation scheme can be used by the cell-edge users. Furthermore, in the uplink, the distance between the MSs (transmitters) and the victim BSs (receivers) in neighbouring cells is larger for the cell-centre MSs than the cell-edge users. Hence the cell-centre users are more likely to be located outside the exclusion range of BSs receivers (BB transmitters). This results in a larger number of chunks that are available to be spatially reused for the cell-centre users. Lastly, the cell-centre users also benefit from higher SINRs as a result of which throughput is particularly boosted by performing link adaptation.
In this paper, the busy signal concept for decentralised and self-organised interference aware medium access has been applied to OFDMA-TDD systems operated in Manhattan grid deployment scenarios. An exclusion zone around victim receivers is established by means of receiver feedback in the form of time-multiplexed busy bursts (BBs), wherein no active transmitter from an adjacent cell may be located. BB enabled interference avoidance exhibits impressive gains in system and user throughputs compared to the benchmark system, with full-frequency reuse without interference avoidance, both in the uplink and the downlink. The impact of the BB specific threshold parameter that controls the interference imposed on coexisting links in neighbouring cells has been studied.
By adjusting this threshold parameter, the system benefits from flexible operation of either achieving high system throughput or enhanced fairness in terms of cell-edge user throughput. A onefold (uplink) and tenfold (downlink) improvement in average cell-edge user throughput is achieved at a reduction in system throughput of about ( 20 Mbps/cell) in both cases. BB-enabled interference avoidance is therefore particularly powerful in enhancing downlink cell-edge user throughput, since in the downlink high interference is coupled with low-desired signal levels, resulting in poor average SINRs at the cell edge. In the uplink, on the other hand, cell-edge users cause high CCI, so that the detrimental effects of uplink interference are distributed more equally among all users, giving rise to interference diversity.
By allowing each receiver to signal the amount of interference it can tolerate, by using a variable busy burst power, an even better tradeoff between system throughput and fairness is achieved. Especially in the downlink, a tenfold improvement has been achieved at the cost of only 8% reduction in maximum system throughput. Furthermore, this scheme also alleviates the need to adjust the BB threshold parameter. The latter property is particularly important for self-organising wireless networks, as the optimum choice of the BB threshold is sensitive to changes in the network topology, and may not be known a priori.
Finally, link adaptation has been combined with busy burst-enabled interference avoidance, where changes in the transmission format are implicitly signalled to the transmitter by virtue of a variable BB power. BB signalling with link adaptation attained a superior performance than the benchmark system with link adaptation, both in terms of system throughput and user throughput. Due to the particular interference scenario, the cell-edge users achieved larger gains in the downlink whereas the cell-centre users benefitted more in the uplink. Consequently, larger gains in system throughput in the uplink mode were achieved compared to the gains achieved in the downlink mode.
Initial parts of this work have been supported by DFG Grant HA 3570/1-2 within the program SPP-1163 (Techniken, Algorithmen und Konzepte für zukünftige COFDM Systeme-TakeOFDM) while some latter parts of this work have been performed within the framework of the IST Project IST-4-027756 WINNER, which is partly funded by the European Union. Harald Haas acknowledges the Scottish Funding Council support of his position within the Edinburgh Research Partnership in Engineering and Mathematics between the University of Edinburgh and Heriot Watt University. This work was presented in part at the IEEE International Symposium of Personal, Indoor and Mobile Radio Communications (PIMRC) 2008, Cannes, France.
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