Overlaid Wireless Network
2.3.5 Interference management by optimum Subband Allocation : A Practical ViewView
An optimum subband allocation in two-tier femtocell network can be crucial from following considerations :
3 Interference minimization between different femtocells as well as femtocells and macrocell.
3 Smooth hand-off process for a user switching between macrocell and femtocell (or vice- versa) [75].
3 Efficient (re)use of spectrum.
However, in a heterogeneous (two-tier) network, implementation of conventional frequency plan- ning/allocation schemes is very difficult and less efficient due to random and opportunistic place- ment of femtocells by end user. Further, in overlaid network of femtocells, the assumption of absence of coordination/cooperation among femtocells and macrocell BS looks reasonable due to security, limited availability of backhaul bandwidth and scalability issues [84].
Few works are available on the analysis of subband allocation, frequency assignment, resource partitioning [75, 85, 86] for femtocell two-tier networks. In [85], a subband allocation analysis for FFR based OFDMA femtocells is presented along with the optimal power allocation. The study assumes a static channel scenario. Authors have concluded that optimality of a subband
2.3 Interference Analysis and Management in Heterogeneous/ Overlaid Wireless Network
for femtocell is highly dependent on the location of femtocell within the macrocell. A frequency assignment strategy is proposed in [75] for femtocells considering practical issues like hand-off, coverage and interference. In his work in [75], in order to get an optimal frequency assignment, authors have divided the macrocell into two regions : inner and outer. Co-channel operation is then allowed only in the outer region for more efficient cell search. Femtocells in inner region are not allowed to use the same subband which is used by macrocell. Different frequency planning schemes for OFDMA based cellular networks are discussed in [87]. A decentralized subband allocation scheme is proposed in [84] for two-tier network. Another decentralized algorithm for joint subband, rate and power allocation in OFDMA based two-tier femtocell network is reported in [88]. The resource allocation scheme in [88] is based on conventional FFR (also known as soft FFR) and assumes that femtocells are operating in closed access mode. A dynamic clustering based subband allocation scheme using (weighted) interference graph is investigated in [89]. Authors in [89] has provided a cognitive subband self-management mechanism to allocate the subband for the newly added femtocells. As opposed to closed access, open access may be an efficient way to mitigate cross-tier interference in femtocell networks. A resource allocation method for OFDMA femtocells is proposed in [90], assuming femtocells are operating in open access mode. The resource allocation method in [90] ensures QoS in neighboring macrocell user (MU) in dead zone as well as limits cross-tier interference to other MUs.
However, these works are carried out under the assumption of ideal network environment and perfect coordination among the femtocells and macrocell. This assumption resulted in the synchro- nized interference scenario, as discussed in Sec. 2.2.1. Such assumptions make the modeling simple and bring tractability from mathematical aspects but do not reflect the true practical conditions in the analysis of various network parameters and metrics. It has been recently pointed out that timing asynchronization issues often appears among various users due to practical constraint like propagation delays and timing offsets [34, 91].
Timing asynchronization issues become even more imperative in subband allocation analysis, since it consists of channel sensing to predict the level of interference present. In order to decide the optimum subband for communication purposes in an interference limited/affected environment, femtocell unit must check the level of interference present in each subband. As discussed earlier in Sec. 2.2.1, the total interference depends on the overlap mechanism of frames also. Therefore, to get
2.3 Interference Analysis and Management in Heterogeneous/ Overlaid Wireless Network
an accurate statistics about subband selection, the asynchronization issues must be included in the analysis as they result in partial overlaps of frames, which ultimately affects the total interference.
Table 2.4: Summary of available works for heterogeneous/overlaid wireless networks in exiting literature.
Reference Work summary Resource alloca-
tion
Scenario Propagation
& timing offsets
Guvencet al. [75]
Proposed frequency assignment method for femtocells
Sub-band Synchronized Not considered
Chandrasekhar et al.[81]
Proposed a distributed utility based SINR adaptation for femtocells
Power Synchronized Not considered
Hanet al.[82] Proposed power control strategies for interference mitigation in
femtocells
Power Synchronized Not considered
Choiet al.[83] Discussed tradeoffs relating to access policies for femtocells
None Synchronized Not considered
Chandrasekhar et al.[84]
Proposed decentralized spectrum allocation policies for two-tier
networks
Sub-band Synchronized Not considered
Leeet al.[85] Sub-band and optimal power allocation for OFDMA based
femtocell networks
Sub-band power
Synchronized Not considered
Jeonet al.[86] proposed a downlink radio resource partitioning scheme for two-tier
cellular networks
Frequency band
Synchronized Not considered
Salatiet al.[88]
Proposed decentralized algorithm for joint subband, rate and power
allocation
Sub-band, rate and
power
Synchronized Not considered
Liet al.[89] Proposed a dynamic
clustering-based sub-band allocation scheme
Sub-band Synchronized Not considered
Tarasaket al.[91]
Capacity and symbol error rate evaluations under interference and
timing misalignment
None Asynchronized Considered
Liet al.[90] Proposed a resource allocation method for open access OFDMA
femtocells
Sub-band Synchronized Not considered
Some of the important existing works are summarized in Table 2.4.
2.4 Summary