Layered video multicast using fractional frequency reuse over wireless relay networks
- Chung-Nan Lee^{1}Email author,
- Ying-Tsuen Chen^{1},
- Yung-Cheng Kao^{1},
- Hui-Hsiang Kao^{1} and
- Steve Haga^{1}
https://doi.org/10.1186/1687-1499-2012-187
© Lee et al; licensee Springer. 2012
Received: 3 October 2011
Accepted: 31 May 2012
Published: 31 May 2012
Abstract
Nowadays, multimedia services over wireless networks are increasingly popular. With multicast, many mobile stations can join the same video group and share the same radio resource to efficiently increase frequency utilization. However, users may be located at different positions, and so suffer different degrees of path loss and interference, and receive a different signal-to-interference and noise ratio (SINR). Users at the cell-edge receiving a lower SINR may degrade the multicast efficiency. In this article, we propose four schemes that consider fractional frequency reuse (FFR) over relay networks in multi-cells. With FFR, users close to a base station (BS) are given more resources to improve the video quality. An efficient resource allocation scheme is also proposed. Compared to the conventional relay scheme, the proposed schemes can provide over 10% more video layers for all users and give better video quality for users near the BS.
Keywords
1. Introduction
In coming years, the growing use of multimedia services and streaming, such as live TV and on-demand video, is expected to be one of the most attractive services of wireless networks. Compared to wired networks, wireless networks have a limited and more expensive bandwidth. Therefore, in order to efficiently utilize wireless resources, users accessing the same media program can join the same multicast group and be serviced simultaneously. As users within the same multicast group are located at different positions and experience various levels of path loss, they may receive a different signal-to-interference and noise ratio (SINR). For a high probability of correctly decoding video data transmitted from the base station (BS), a user requires a sufficient SINR. Traditionally, a lower transmission rate is needed to support a satisfactory multicast service for all users in the same group. Therefore, users at the cell-edge who suffer a larger path loss can only receive a lower SINR, resulting in poor multicast efficiency.
In addition, scalable video coding (SVC) [7, 8] can efficiently guarantee different video quality for users with heterogeneous bandwidth. By applying SVC, the original video is encoded into one base layer and several enhancement layers. Users with a poor SINR can receive fewer layers, while those with better SINR can receive more layers to enjoy better video quality.
Recent research into relay communications can be found in [2, 9, 10]. Jin and Li [2] exploited channel diversity for different users, studying cooperative multicast transmission combined with network coding to improve performance. Hou et al. [9] proposed a multicast scheduling scheme with user cooperation to achieve high performance and maintain fairness across groups. Elrabiei and Habaebi [10] presented a power-efficient cooperative transmission scheme that considers relay selection to significantly minimize the number of cooperative users and reduce power consumption. However, the IEEE 802.16j standard specifies a direct connection between the BS and users, or a connection between the BS and users with the help of RSs, but does not allow direct communication among users [11]. Therefore, the studies [2, 9, 11] require some modifications before it can be applied to a scenario with specified RSs.
Zhao and Su [12] analyzed the performance of cooperative and relay transmissions, and derived a closed-form formulation to model the average outage probabilities. Using this formulation, optimal cooperation strategies for the relay location and power allocation are provided. Multicast routing in relay networks has been investigated in [13, 14], where the protocol to find an appropriate routing path to increase the system performance was proposed. Sheen et al. [15] investigated optimal RS placement, path selection, resource allocation, and frequency reuse. However, only a few works consider multicasts over relay networks.
Kuo and Lee [16] proposed a multicast recipient maximization scheme to achieve near-optimal solutions. Research into video multicast scheduling over wireless relay networks was presented in [1, 17]. Alay et al. [1] considered both omni-directional and directional relays to analyze the optimal system parameters for maximizing the video quality, and Yu et al. [17] proposed a utility-based algorithm to maximize the system utility for scalable video multicasts. Different from [1, 17], the proposed schemes in this article increase frequency utilization by considering FFR in multi-cells.
In this article, the proposed schemes using frequency reuse in multi-cells have more efficient frequency utilization than the conventional scheme. With frequency reuse, the BS can use additional frequencies to transmit more video layers to inner users. In order to use frequency efficiently, a resource allocation scheme is proposed to provide as many video layers as possible to all users while keeping a satisfactory service, and to provide additional video layers to inner users who have a good SINR. Compared to the conventional scheme, the proposed schemes provide better video quality for cell-edge users and provide more video layers to all users.
The rest of this article is organized as follows. Section 2 introduces the background to FFR and related study. A description of the problem and the proposed schemes is given in Section 3, and the results of simulations are presented in Section 4. Finally, conclusions are drawn in Section 5.
2. Background and related study
2.1 Overview of FFR
Conventionally, FFR is used to increase frequency utilization efficiency and reduce interference. There are two main FFR types, hard and soft, which we now describe. The main difference is that soft FFR reuses more frequency for inner users, such that the BS needs to lower its transmission power to avoid interfering with users of neighboring cells.
2.2 Adaptive modulation and coding
To achieve a transmission rate close to the theoretical channel capacity, adaptive modulation is used to adjust different modulation schemes according to users' channel conditions. Both LTE-advanced and WiMAX support these modulation levels [18, 19], which are quadrature phase shift keying (QPSK), 16-state quadrature amplitude modulation (16-QAM), and 64-state QAM (64-QAM). Several channel coding schemes are supported. For example, in IEEE 802.16 [19], OFDMA architecture supports Convolution Code (CC), Block Turbo Code (BTC), Convolution Turbo Code (CTC), and Low Density Parity Check Code (LDPCC). BTC and CTC are optional. Channel coding can improve the error correction capability, but entails an additional delay and complex decoding. When a user's SINR is good, the modulation and coding scheme (MCS) with the highest transmission rate is used. Different modulation and coding levels would then be dynamically adjusted based on the user's channel condition. Each user measures channel state information and periodically feeds the information back to the BS, allowing the BS to select a suitable MCS for each user.
2.3 Channel model
3. Proposed schemes
3.1 FFR schemes with relay
The proposed schemes are suitable for multiple BSs. To simplify the system environment, we consider a central cell and six neighboring cells, as shown in Figure 6a. Each cell has six RSs, as shown in Figure 6b. We also assume that these systems operate in half duplex, i.e., RSs cannot receive and transmit data simultaneously. Several researchers [9], [21–23] have presented that a synchronization assumption is feasible. In this article, we assume that all RSs in a cell can synchronize their transmissions perfectly, so that synchronization errors are negligible. RSs in a cell can multicast data simultaneously without causing co-channel interference and degrading users' SINR. Cooperative transmission gain is not considered in this article. We apply FFR to two steps of multicast transmission. In the first step, the BS transmits video layers to inner users and RSs, and in the second step, all RSs simultaneously transmit video layers to outer users. With frequency reuse, the BS can transmit additional video layers to inner users. The four proposed FFR schemes are described as follows.
(4) Two-step power soft FFR with relay: This differs from the Soft FFR scheme in that, in order to increase the signal strength for inner users in the first step, the BS uses its original transmission power before lowering the power level in the second step. Similar to the Traditional FFR, this scheme must synchronize its transmission time with neighboring cells. Compared to the Soft FFR, the Two-step power soft FFR provides a better SINR to inner users in the first step, but the transmission time cannot be changed dynamically.
Main differences among the four FFR schemes
FFR scheme | Synchronization among neighboring cells | BS power | |
---|---|---|---|
Step 1 | Step 2 | ||
Hard | No | Normal | Normal |
Traditional | Yes | Normal | None |
Soft | No | Low | Low |
Two-step power soft | Yes | Low | Normal |
3.2 Resource allocation scheme
List of notation
Notation | Description |
---|---|
V | All videos played in multicast groups |
L | Number of video layers a video is encoded in |
X | Index of a video layer {1, 2,...,L} |
v _{ x } | x th layer of video v |
N _{ v } | Users request video v |
${m}_{1}^{v,x}$ | MCS used by the BS for the x th layer of video v in the first step |
${m}_{2}^{v,x}$ | MCS used by the RSs for the x th layer of video v in the second step |
${m}_{3}^{v,x}$ | MCS used by the BS for the x th layer of video v in the second step |
$R\left({m}_{j}^{v,x}\right)$ | Transmission rate of the MCS (bits/slot) |
T | Total multicast time |
t 1 | Duration of the first step |
t 2 | Duration of the second step |
S _{1} | Total resources available in the first step (slot) |
S _{2} | Resources available for the RSs in the second step (slot) |
S _{3} | Resources available for the BS in the second step (slot) |
bit_{v,x} | Bit rate of the x^{th} layer of video v (bits/OFDMA frame) |
${S}_{1}^{\mathsf{\text{used}}}$ | Total resources used in the first step (slot) |
${S}_{2}^{\mathsf{\text{used}}}$ | Resources used by the RSs in the second step (slot) |
${S}_{3}^{\mathsf{\text{used}}}$ | Resources used by the BS in the second step (slot) |
B | Total frequency band in the system (subchannel) |
C _{1} | Resources available for the BS in the first step within a basic service time |
C _{2} | Resources available for RSs in the second step within a basic service time |
Comparison of available frequencies
Conventional relay | Hard FFR | Traditional FFR | Soft FFR | Two-step power soft FFR | |
---|---|---|---|---|---|
Available frequencies for BS in step 1 | F1 = 10/30 × B | F1 + F2 = 14/30 × B | F1 + F2 + F3 = 30/30 × B | F1 + F2 + F3 = 30/30 × B | F1 + F2 + F3 = 30/30 × B |
Available frequencies for RS in step 2 | F1 = 10/30 × B | F2 = 8/30 × B | F1 = 10/30 × B | F1 = 10/30 × B | F1 = 10/30 × B |
Available frequencies for BS in step 2 | 0 | F1 = 6/30 × B | 0 | F2 + F3 = 20/30 × B | F2 + F3 = 20/30 × B |
Hard FFR: C_{1} = 14/30 × B, C_{2} = 8/30 × B.
Soft FFR: C_{1} = B, C_{2} = 1/3 × B.
Comparison of available resources
Conventional relay | Hard FFR | Traditional FFR | Soft FFR | Two-step power soft FFR | |
---|---|---|---|---|---|
S_{1} | 1/3 × B × t1 = 1/6 × B × T | 14/30 × B × t1 ≅ 5.09/30 × B × T | B × t1 = 1/4 × B × T | B × t1 = 1/4 × B × T | B × t1 = 1/4 × B × T |
S_{2} | 1/3 × B × t2 = 1/6 × B × T | 8/30 × B × t2 ≅ 5.09/30 × B × T | 1/3 × B × t2 = 1/4 × B × T | 1/3 × B × t2 = 1/4 × B × T | 1/3 × B × t2 = 1/4 × B × T |
S_{3} | 0 | 6/30 × B × t2 ≅ 3.82/30 × B × T | 0 | 2/3 × B × t2 = 1/2 × B × T | 2/3 × B × t2 = 2/4 × B × T |
Total | 1/3 × B × T | 14/30 × B × T | 1/2 × B × T | B × T | B × T |
Compared to a conventional relay scheme, the FFR schemes have more resources available to the BS in the first step. After providing more video layers to all users, the remaining resources of the BS in the first step of the FFR schemes are greater than in the conventional relay scheme. This remaining resource can be used to provide additional video layers to inner users. Based on the above observations, the proposed resource allocation scheme for use by all FFR methods is shown in Algorithms 1, 2, and 3. Algorithm 1 attempts to find the maximum number of video layers that can be provided to all users as a basic service and minimize resource consumption. The remaining resource is allocated to RSs in the second step by Algorithm 2, which determines the appropriate MCS of the BS and RSs to increase transmission efficiency, even if users with lower SINRs cannot receive data correctly. Algorithm 3 allocates the additional resources for the BS in the second step of the Hard FFR, Soft FFR, and Two-step power soft FFR, and determines which group should be scheduled as a higher priority and the MCS that should be employed to transmit video layers to inner users.
if the Hard FFR or the Soft FFR is used.
Equation (1) maximizes the number of video layers Q while minimizing the resource consumption, where bit_{v,x}is the bit rate of the x th layer of video v (denoted as v_{ x } ). The MCS ${m}_{1}^{v,x}$ used by the BS in the first step and MCS ${m}_{2}^{v,x}$ used by the RSs in the second step are found by Equation (1). The transmission rate $R\left({m}_{j}^{v,x}\right)$ is determined according to which ${m}_{j}^{v,x}$ is used. To transmit the video layer v_{ x } with the MCS ${m}_{j}^{v,x}$ requires at least $\u2308\mathsf{\text{bi}}{\mathsf{\text{t}}}_{v,x}/R\left({m}_{j}^{v,x}\right)\u2309$ resource slots, which is the ceiling value of $\mathsf{\text{bi}}{\mathsf{\text{t}}}_{v,x}/R\left({m}_{j}^{v,x}\right)$. In other words, this is the smallest integer that is greater than or equal to $\mathsf{\text{bi}}{\mathsf{\text{t}}}_{v,x}/R\left({m}_{j}^{v,x}\right)$. A higher transmission rate indicates that a user needs a higher SINR to receive data correctly. ${I}_{v,x}\left(i,{m}_{1}^{v,x},{m}_{2}^{v,x}\right)$ is equal to 1 if user i can receive and decode video layer v_{ x } correctly when the ${m}_{1}^{v,x}$ and ${m}_{2}^{v,x}$ MCSs are used, and is equal to 0 otherwise. Equation (2) checks whether all users can receive the Q* layers. Equations (3) and (4) check whether there are sufficient resources for the two steps, where ${s}_{1}^{\mathsf{\text{used}}}$ are the resources used by the BS in the first step, S_{1} are the resources available to the BS in the first step, ${s}_{2}^{\mathsf{\text{used}}}$ are the resources used by the RSs in the second step, and S_{2} are the resources available for the RSs in the second step. Equation (5) checks the resource constraint for the Hard FFR and the Soft FFR, where C_{1} are the resources available to the BS in the first step within a basic service time, and C_{2} are the resources available to RSs in the second step within a basic service time. Therefore, the minimum resource consumption and the maximum number of video layers Q for all users can be determined by Equations (1) (5).
if the Hard FFR or the Soft FFR is used.
Equation (6) is used to find the most efficient transmission rate for each step. Video layers with higher efficiency will be allocated a higher priority. In order to use resources efficiently, suitable MCSs ${m}_{1}^{v,x}$ and ${m}_{2}^{v,x}$ are found for each video group in order to increase the number of users who can successfully receive data per resource slot. In order to maintain fairness among groups, video layer v_{ x } would only be allocated after all lower layers (<x) of all videos had already been allocated. Equations (7) and (8) check the resource consumption of transmitting video layer v_{ x } . For the Hard FFR and the Soft FFR, the resource consumption is only checked by Equation (9).
In Equation (10), ${I}_{v,x}\left(i,{m}_{3}^{v,x}\right)$ is equal to 1 if user i can receive and decode video layer v_{ x } correctly when the ${m}_{3}^{v,x}$ MCS is used by the BS in the second step, and is equal to 0 otherwise. Equation (11) checks the resource consumption of transmitting video layer v_{ x } to inner users. ${s}_{3}^{\mathsf{\text{used}}}$ denotes the resources used by the BS in the second step, and S_{3} are the resources available to the BS in the second step.
4. Simulation
System parameters
Parameter | Value |
---|---|
Operating frequency | 2.5 GHz |
Duplex | TDD |
Channel bandwidth | 10 MHz |
Cell radius | 1200 m |
BS full power | 43 dBm |
BS lower power | 30 dBm |
RS power | 40 dBm |
BS height | 32 m |
RS height | 32 m |
MS height | 1.5 m |
BS antenna gain | 15 dBi |
RS antenna gain | 12 dBi |
MS antenna gain | 1 dBi |
MS noise figure | 7 dB |
OFDMA parameters
Parameter | Value |
---|---|
Permutation mode | PUSC |
FFT size | 1024 |
OFDMA symbol duration | 102.9° μ |
Frame duration | 5 ms |
PUSC mode | |
Number of data subcarriers | 720 |
Number of pilot subcarriers | 120 |
Number of null subcarriers | 184 |
Number of subchannels | 30 |
MCSs
MCS | Required SNR (db) | Normalized capacity | |
---|---|---|---|
QPSK | ½ | 5 | 1 |
QPSK | ¾ | 8 | 1.5 |
16-QAM | ½ | 10.5 | 2 |
16-QAM | ¾ | 14 | 3 |
64-QAM | ½ | 16 | 3 |
64-QAM | 2/3 | 18 | 4 |
64-QAM | ¾ | 20 | 4.5 |
Characteristics comparison of the four FFR schemes
FFR scheme | Video quality for inner users | Video quality for outer users | Drawbacks |
---|---|---|---|
Hard | Medium | Bad | |
Traditional | Medium | Best for outer users | Need to synchronize to neighboring cells |
Soft | High | Best for cell-edge users | Lower SINR for inner users as BS lowers its transmission power in both steps |
Two-step soft | High | Good | Must be synchronized to neighboring cells and has a more complicated power control as the BS lowers its transmission power in the second step only |
5. Conclusions
In this article, we have proposed four FFR schemes for layered video multicast over relay networks, namely Hard FFR, Traditional FFR, Soft FFR, and Two-step power soft FFR. A resource allocation scheme was also proposed for the four FFR schemes to provide as many video layers as possible to all users and provide additional video layers to inner users. Simulation results showed that cell-edge users could receive over 10% more video layers using the Traditional FFR, Soft FFR, and Two-step power soft FFR schemes than with the conventional relay scheme. These three schemes are each suited to a different distribution of users, and thus the selection of which scheme is most appropriate will depend on how the network of users is distributed. The Traditional FFR scheme performs better for users in the outer range, whereas Soft FFR performs better for cell-edge users, and the Two-step power soft FFR scheme gives better performance for the inner users.
Algorithm 1
1: Q* = 0//Number of video layers for all users
2: for x ≤ L // Index of all video layers
3: for all v ∈ V // v is the video played in multicast groups
4: for all available MCSs of BS (m_{ j } ) and RSs (m_{ k } )
5: if$\begin{array}{c}\u2308\mathsf{\text{bi}}{\mathsf{\text{t}}}_{v,x}/R\left({m}_{1}^{v,x}\right)\u2309+\u2308\mathsf{\text{bi}}{\mathsf{\text{t}}}_{v,x}/R\left({m}_{2}^{v,x}\right)\u2309\\ >\u2308\mathsf{\text{bi}}{\mathsf{\text{t}}}_{v,x}/R\left({m}_{j}\right)\u2309+\u2308\mathsf{\text{bi}}{\mathsf{\text{t}}}_{v,x}/R\left({m}_{k}\right)\u2309\end{array}$
and $\begin{array}{cc}\hfill \forall i\in {N}_{v},\hfill & \hfill {I}_{v,x}\left(i,{m}_{j},{m}_{k}\right)=1\hfill \end{array}$
6: Update ${m}_{1}^{v,x}\leftarrow {m}_{j}$, ${m}_{2}^{v,x}\leftarrow {m}_{k}$
7: next m_{ j } , m_{ k } pair
8: next v
9: if satisfy Equations (3), (4) and (5)
10: schedule v_{ x } with ${m}_{1}^{v,x}$, ${m}_{2}^{v,x}$, all v ∈ V
11: Q*++
12: else break
13: next x
14: Q*--
Algorithm 2
1: for each x, x > Q* // Index of unscheduled video layers
2: for all v ∈ V // v is the video played in multicast groups
3: for all available MCS pairs of BS (${m}_{1}^{v,x}$) and RSs (${m}_{2}^{v,x}$)
//Select ${m}_{1}^{v,x}$ and ${m}_{2}^{v,x}$ to achieve the highest effiency_{v,x}
4: if satisfy Equations (7), (8) and (9)
6: Store the highest effiency_{v,x}and its ${m}_{1}^{v,x}$ and ${m}_{2}^{v,x}$
7: end if
8: next${m}_{1}^{v,x}$, ${m}_{2}^{v,x}$ pairs
9: next v
10: Sort v_{ x } by efficiency_{v,x}in descending order
11: for each v_{ x } , v ∈ V
12: Schedule v_{ x } if satisfy Equations (7), (8) and (9)
14: next v_{x} has the highest efficiency_{v,x}
15: if resources are not enough break
16: next x
Algorithm 3
1: for x ≤ L // Index of unscheduled video layers
2: for all v ∈ V // each video belong to video multicast groups
3: for all available MCS of BS (${m}_{3}^{v,x}$)
// Select ${m}_{3}^{v,x}$ to achieve the highest effiency_{v,x}
4: if satisfy Equation (11)
6: Store the highest effiency_{v,x}and its ${m}_{3}^{v,x}$
7: end if
8: next${m}_{3}^{v,x}$
9: next v
10: Sort v_{ x } by efficiency_{v,x}in descending order
11: for each v_{ x } that are not scheduled, v ∈ V
12: Schedule v_{ x } if satisfy Equation (11)
13: Update ${s}_{3}^{\mathsf{\text{used}}}\leftarrow {s}_{3}^{\mathsf{\text{used}}}+\u2308\mathsf{\text{bi}}{\mathsf{\text{t}}}_{v,x}/R\left({m}_{3}^{v,x}\right)\u2309$
14: next v_{ x } has the highest efficiency_{v,x}
15: If resources are not enough break
16: Next x
Declarations
Authors’ Affiliations
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