Complexity and Overhead Gain Analysis

We evaluate the complexity and overhead of the proposed spectrum access presented in Section 4.2. The complexity is measured in terms of the average number of floating point operations to floating points, excluding the complexity and overhead of the operations defined in the original LTE protocol architecture. All real additions, subtraction, multiplications, and comparisons are treated equally, and divisions are weighted by two as shown in Table 4.2.

The complexity of the proposed scheme comprises three parts: the complexities of calculation for cost function, of comparison with threshold, and sorting, and is shown as follows:



 



FAP FUE aRB FAP FUE FAP aRB

N N  N  N N  N  N (3.1)

Step 1: All FUEs transmit random access requests to FAP Step 2: FAP feedbacks group index

Step 3: FUEs feedback interference reports Step 4: FAP broadcasts interference reports

Step 5: FAP sniffs interference reports from other FAPs Step 6: FAP sniffs RB spectrum usage from prior FAPs

Step 7: If all prior FAPs’ usage is obtained, FAP transmit access admittances and system information

Step 8: FUEs who have admittances start accessing

Step 9: If any two FUEs access the same RB, the FAP starts contention resolution

Step 10: Break the loop if RBs are all used up

Step 11: Go to Step 7 for different groups until all groups have already performed the access of RBs

Table 4.2 Complexity weight of different operations

The transmit overhead is measured as the overhead per second, and we assume the two phases of spectrum access should be renewed in a coherence time, and the coherence time of a object with the speed of 5 km/hour is 4 10 ^3 second.The major part of transmit overhead of the centralized exhaustive scheme is for the channel state information, which is shown as follows (in real numbers):





/ 4 10 250

FAP FUE aRB FAP FUE aRB

N N N  ^  N N N (3.2)

The transmit overhead of the proposed scheme comprises three terms: the terms for the group index and access admittance, the interference measurement reports, and the spectrum usage reports, and is shown as follows (in real numbers):





250 2 N_FAP N_FUE N_FAP N_FUE N_FAP N_FAP log N_aRB (3.3) The major difference of transmit overhead of both schemes is for the centralized scheme, the channel state information is needed on every RBs. In contrast, the number of overheads of the proposed scheme is independent of the number of RBs.

4.4 Computer Simulations

In this section, the simulation results of the proposed scheme are presented. The simulation environment is shown in Fig. 4.5, where the simulation parameters are listed in Table 4.3. In the environment, the FAPs are closely located, and the FUEs are

36

randomly distributed within the femtocell coverage. In order to verify the proposed method in a densely populated scenario, we adopt the maximum number of supported FUEs [9]. We increase the number of FUEs scattered in the FAP coverage from four to 32 by four each time. Fig. 4.6 shows comparison between multicell spectrum access with interference and without interference, and we can see that the inter-cell interference really degrades the capacity greatly, which leave a room for interference avoidance and capacity improvement. In Fig. 4.7, the capacity per channel (bits/sec/hertz) for the proposed spectrum access with inter-cell interference avoidance (IA) is shown, and is improved from the spectrum access without IA, and both of these are away from the lower bound of no coordination. Besides, the proposed spectrum access with IA also approaches the performances in both small and large numbers of FUEs obtained in the centralized scheme where the exhaustive search is adopted. The results show the ability of providing simultaneous supports to a larger number of FUEs in the two-tier femtocell networks. Fig. 4.8 shows the capacity per FUE (bits/sec), and we can see that with the increase in number of FUEs, the capacities provided by the proposed schemes and the centralized scheme decrease, and when the number of FUEs per RB is larger than two, the difference between them is small. However, the capacity of no coordination scheme is close to zero in both small and large numbers of FUEs, which shows the benefits in capacity from coordination in the multi-cell spectrum access.

The comparisons in complexity and overhead are shown in Fig. 4.9 and Fig. 4.10, respectively. Referring to the specification of spectrum release for LTE-Advanced in Taiwan [19], the bandwidth for an operator is up to 45 MHz, and the number of divisions of the total bandwidth by the minimum bandwidth configuration is 37.

Applying this figure into simulations, we can see that in Fig. 4.9 the complexity for the centralized scheme grows exponentially with the number of FUEs, while the

complexity for the proposed scheme grows linearly. In Fig. 4.10, the transmit overhead of the centralized scheme and the proposed scheme both grow linearly with the number of FUEs, and the exponent of the latter is less than the former by one, which means the transmit overhead of the proposed scheme is only tenth of the centralized scheme.

Table 4.3 Simulation parameters

Parameter Value

FUE transmit power 26 dBm

Path loss model [1] 3GPP TR 36.814 v9

Fading channel Rayleigh

Resource block bandwidth 180k Hz

The maximum number of FUE [9] 32

The number of available RBs 4

The number of femtocells 4

Thermal noise PSD [10] -174 dBm/Hz

-600 -400 -200 0 200 400 600

Fig. 4.5 Multiple femtocells in two-tier networks

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Fig. 4.6 Comparison in capacity per channel

1 2 3 4 5 6 7 8

Fig. 4.7 Comparison in capacity per channel

1 2 3 4 5 6 7 8

Fig. 4.8 Comparison in capacity per FUE

4 8 12 16 20 24 28 32

Fig. 4.9 Comparison in complexity

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4 8 12 16 20 24 28 32

10⁴ 10⁵ 10⁶ 10⁷

No. of FUEs

Transmit overhead (numbers/sec) Centralized scheme Proposed scheme

Fig. 4.10 Comparison in overhead

4.5 Summary

In this chapter, we first describe the requirement for distributed inter-cell interference coordination. To fulfill the requirement, we propose a multicell spectrum access with inter-cell interference avoidance. The new spectrum access scheme is based on the scheme proposed in Chapter 3 and can be adapted to the existing LTE protocol architecture. Analyses of complexity and overhead are also presented. Compared with the centralized scheme, the proposed scheme improves over the spectrum access without inter-cell interference avoidance and exhibits similar capacity performance to the centralized scheme.

Chapter 5

Conclusions and Future Works

To fulfill the increasing demands on the spectrum resource and to develop an efficient communication system in the next generation wireless communication systems, the femtocell network has been proposed. Femtocells feature no coordination center, low computation ability, and limited backhaul, which make the spectrum access hard to be efficient. Besides, uncoordinated network planning also introduces the interference issue. In this thesis, we propose a novel distributed multicell spectrum access with interference avoidance.

The network architecture and system model for the two-tier femtocell are introduced in Chapter 2, where two SON functionalities built in the system are also mentioned. The channel model of links between FUEs and FAPs is depicted, and the spectrum resource unit suggested by LTE is also adopted. We also briefly describe the mechanism of spectrum access in LTE and differences between femtocell networks and traditional macrocell networks. In the following Chapter 3 and 4, we propose a game theoretic distributed spectrum access in the femtocell networks.

In Chapter 3, we first give an introduction of the game theory, and also describe the vital requirement for a distributed mechanism. To meet the requirement, we propose a Bayesian game theoretic spectrum access with intra-cell interference avoidance. The new spectrum access scheme can be adapted to the existing LTE protocol architecture.

Compared with the centralized scheme, the proposed scheme improves greatly from the

42

original Bayesian game and exhibits similar utility performances to the centralized scheme.

In Chapter 4, we first describe the requirement for inter-cell interference coordination. To fulfill the requirement, we propose a multicell spectrum access with inter-cell interference avoidance. The new spectrum access scheme is based on the scheme proposed in Chapter 3. Analyses of complexity and overhead are also presented.

Compared with the centralized scheme, the proposed scheme improves over the spectrum access without inter-cell interference avoidance and exhibits similar capacity performance to the centralized scheme.

There are still some issues remaining to be further investigated. The inter-cell interference avoidance problem can be modeled as a game where FAP acts as player making actions, and each action chosen by each player is the power allocation. In the power allocation problem, each BS determines the power allocation using local information by iterative water-filling algorithms, i.e., SINR in each channel. The SINR measured at each iteration reflects transmission power changes of the other FAPs.

However, the water-filling algorithm may provide solution falling at some undesired equilibria [26]. How to develop an efficient scheme exploiting group spectrum access to pre-avoid this condition need to be further discussed.

Besides, the 3GPP proposed the incorporation of femtocells, picocells, microcell, and metrocells, and named the small cells. The deployment of small cells are managed by the operators and equipped with high speed fibre backbone [16]. Besides, the small cells are served as some hotspots deployed in some public areas to offload peak traffic, and the backhaul between small cells and macro cells is provided. In such scenario, inter-cell interference coordination plays a more important role and needs to be reconsidered to achieve better system performance. For instance, interference

coordination can be implemented by a victim detection procedure [17]. In this procedure, the interfered victim MUEs can be determined by eNodeBs, and their identities can be signaled to the FAP through the backhaul provided by the operators, so the spectrum efficiency can be further improved.

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