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CELLULAR TECHNOLOGIES
History of Cellular Networks
The tremendous growth in wireless cellular industry reached to 4 billion over the past decades [9]. In 1981, the rst international mobile communication system, namely the Nordic Mobile Telephony (NMT) system was introduced in the Nordic countries. At the same time, the analog Advanced Mobile Phone System (AMPS) was introduced in North America. The First Generation (1G) analogue network only supported voice with limited roaming. With the introduction of digital communication during 1980s, the interest in developing a successor to the analog communication system appeared and provided the foundation towards the evolution of the 2G mobile communication system. The second generation digital network supported better quality voice, enhanced capacity and widespread roaming then did by the analogue system counterpart. Enhancement in roaming part was due to few standards and common spectrum allocation particularly in Europe. Global System for Mobile communication (GSM) and IS95 standards, 2G technologies are two widely deployed cellular systems. GSM is based on Frequency and Time Division Multiple Access (FDMA/TDMA) while IS95 is based on Code Division Multiple Access (CDMA) technique. The 2G cellular networks are mainly designed for voice communication; in later release they are made capable for data transmission but still data rates were lower than dialup. Both GSM and CDMA systems formed their own standards, 3G partnership projects (3GPP) and 3GPP2 respectively so that to develop newer technologies based on CDMA technology. The International Telecommunication Union-Radio (ITU-R) project on International Mobile Telecommunication IMT-2000 smoothed the way for 3G networks, the main features were high data rates i.e. 2 Mbps and vehicular mobility.
In 1980s, ITU-R initiated the Universal Mobile Telecommunication System (UMTS) which is referred to as the 3G mobile communication system. 3G system is based on Wideband CDMA (WCDMA). The 3G standard technology in 3GPP is referred to WCDMA which uses 5 MHz bandwidth while CDMA2000 in 3GPP2 uses 1.25 MHz bandwidth. Later on 3GPP2 also developed its own standard and the frequency band was extended to 5 MHz composed of three 1.25 MHz which is then called CDMA2000-3x. To di erentiate both standards, 5 MHz CDMA is called CDMA-3x and single carrier of 1.25 MHz CDMA is called CDMA-1x or 3G-1x [9].
The rst release of these standards didn’t ful ll their promises and the expecting data transmission was too lowered than the practical one. After serious e orts, 3GPP2 introduced High Rate Packet Data (HRPD) service which uses advance techniques for data optimization such as channel sensitive scheduling, fast link adaptation and hybrid ARQ etc. However, HRPD required a separate 1.25 MHz subcarrier for data transmission only with no voice on the same carrier and hence initially it was called CDMA2000-1x EVDO (evolution data only). 3GPP followed the same way and enhanced WCDMA; developed HSPA (High Speed Packet Access) and used the same access techniques, the only di erence was that the voice and data use the same bandwidth of 5 MHz, they are multiplexed in downlink. 3GPP2 also developed CDMA2000-1x EVDO to CDMA2000-1x EVDV which means (evolution data and voice). Both data and voice use the same subcarrier of 1.25 MHz but never used commercially. Later on, in HRPD Voice over IP (VoIP) was introduced to support both voice and data on the same carrier. Both of these new technologies ful lled the need for high data transmission in 3G and deployed in major markets of world [9].
Beyond 3G Networks
While HRPD and HSPA were in the process of deployment, in the meanwhile IEEE 802 LMSC (LAN/MAN Standard Committee) introduced a new standard that is IEEE 802.16e for mobile broadband wireless access which is the enhanced version of IEEE 802.16 for xed wireless broadband. This standard uses new access technology OFDMA (Orthogonal Frequency Division Multiple Access) and provides better data rates than HSPA and HRPD technologies.
The IEEE 802.16 family of standards is o cially called WirelessMAN in IEEE. It is also titled as Worldwide Interoperability for Microwave Access (WiMAX) by an industry group named the WiMAX forum. The duty of WiMAX forum is to check the compatibility and interoperability.
The WiMAX supported mobility just as in IEEE 802.16e standard is called mobile WiMAX. With the introduction of new standard speci cally Mobile WiMAX led both 3GGP and 3GPP2 to their own newer version beyond 3G by utilizing new access technology OFDMA and similar network architecture like Mobile WiMAX. The beyond 3G in 3GPP standard is called Evolved Universal Terrestrial Radio Access (EUTRA) technology. This technology in 3GPP is widely known as Long Term Evolution (LTE). While in 3GPP2 standard, similar is developed which is known as Ultra Mobile Broadband (UMB) [9]. Fig. 1 shows the evolution of 3GPP standards.
Long Term Evolution (LTE) Technology
HSPA is treated as 3.5G, beyond 3G or Super 3G. LTE is regarded as a pre-4G as it does not ful ll the International Telecommunication Union (ITU-R) requirements for data rate and heterogeneity of networks. However, businesses roll-out with LTE are often called 4G. LTE can operate in the frequency range from 900 MHz to 2.6 GHz. LTE is aimed to provide high data rate, low latency and packet optimized radio access technology supporting exible bandwidth deployment. LTE supports a wide range of bandwidth from 1.25 MHz to 20 MHz. The 20 MHz bandwidth gives peak data rate of 326 Mbps using 4×4 Multiple Input Multiple Output (MIMO). For uplink, MIMO is not yet implemented so the uplink data rate is limited to 86 Mbps [9]. It supports Orthogonal Frequency Division Multiple Access (OFDMA) which gives high robustness and spectral e ciency against multipath fading. While comparing to HSPA, LTE provides high spectral e ciency of two to four times. Moreover, LTE system in terms of its radio interface network is capable of providing low latency for packet transmission of 10 ms from network to User Equipment (UE). Similarly, there is some improvement in cell edge performance, utilizing the same macro network. LTE supports both unicast and multicast tra c in microcells up to 100 of meters and in macro cells more than 10 km in radius. LTE system also supports FDD (Frequency Division Duplex) and TDD (Time Division Duplex), in its Half-FDD, UE is not require to transmit and receive at the same time which avoids the requirement of costly duplexer in UE. Generally, it is optimized for 15 km/h but can be used up to 350 km/h with some tolerance to performance degradation. For its uplink it uses Single Carrier FDMA (SC-FDMA) access technique which gives greater coverage for uplink with the fact of low Peak to Average Power Ratio (PAPR).
For this purpose new network architecture is designed with the aim to support packet switched tra c with seamless mobility, low latency and high quality of service (QoS). Some basic LTE parameters related to air interface is summarized in Table I.
WCDMA Compatibility Issues With LTE
Currently 3G system is using Wideband Code Division Multiple Access (WCDMA) access technique with a bandwidth of 5 MHz both in uplink and downlink. In WCDMA di erent users have assigned di erent Walsh codes [9] which are multiplexed using same carrier frequency. In downlink, transmission is orthogonal, due to xed eNodeB with no multipaths, hence the Walsh codes received are synchronized at UEs. While in case of multipath, Walsh codes received are not orthogonal anymore and hence results in Inter Symbol Interference (ISI). The ISI can be eliminated with advance receiver such as Linear Minimum Mean Square Error (LMMSE) receiver.
In order to achieve high data rates, the interference problem increases in WCDMA when used with LTE due to multipath for larger bandwidths such as 10 MHz or 20 MHz. This is because of high chip rates in higher bandwidths, which is small in case of lower bandwidths. Similarly complexity of LMMSE also increases due to increased multipaths in large bandwidths. It is also possible to add multiple carriers of 5 MHz to support large bandwidth. However, it increases the complexity of eNodeB and UE. Another issue with WCDMA could be that a bandwidth less than 5 MHz will not be supported by LTE, as LTE supports smaller bandwidths as well. WCDMA only supports multiple of 5 MHz.
After carefull consideration of LTE exibility, scalability and compatibility issues associated with WCDMA, it was necessary to employ a new access technique for LTE system.
OFDMA Access Technique
OFDMA approach was rst proposed by R.W Chang [9], [20] a few decades ago and analyzed later by Slatzberg [9], [21]. In OFDMA the whole bandwidth is divided into small subcarriers or parallel channels which is then used for transmission with reduced signaling rate.
These subcarriers are orthogonal which means that they do not correlate with each other. The subcarrier frequency is shown in the equation given below, fk = k f (2.1) where f is the subcarrier spacing. Subcarrier is rst modulated with a data symbol of either 1 or 0, the resulting OFDMA symbol is then formed by simply adding the modulated carrier signal. This OFDM symbol has larger magnitude than individual subcarrier and thus having high peak value which is the characteristics of OFDMA technique.
Transmission Capacity
In this section we discuss the transmission capacity of OFDMA versus WCDMA for certain cases with high values of interference and nally come to the conclusion that OFDMA allows higher transmission capacity than WCDMA.
WCDMA Capacity
The Signal to Interference and Noise Ratio (SINR) in WCDMA technology for the signal received on nth multipath can be represented as, N 1 n = Pn=(f P + i=0X;i6=n Pi + N0) (2.2)
Where Pn in eq. 2.2 is the power of received signal on nth multipath for cell of the concentration, f is the ratio of other cell signals to own cell signal. To make our analysis simple we assume that all power, P/N is equally divided into N multipath, eq. 2.2 can then be modi ed into, n = P=N=(f P + (N 1)P=N + N0) (2.3)
For further simplicity we consider that single user is using all resources in Time Division Multiplexing (TDM) and hence there is no interference considered amongst users in the same cell. Furthermore, we consider Maximum Ratio Combining (MRC) signals which are received at di erent paths, so the average SINR can be represented in below equation as, WCDMA = n = (P=N=(f P + (N 1)P=N + N0) (2.4) n=0 =0
By taking the summation, P/N becomes P as previously considered that all power is equally divided into N multipaths. After re-arranging, eq. 2.4 can be modi ed as, W CDMA = P=(f P + (1 1=N)P + N0) = =(f + (1 1=N) + 1) (2.5)
Considering eq. 2.5 for certain cases, when N=1, mean that there is single path and at fading channel, eq. 2.5 can be simpli ed into, W CDMA = P=f P + N0 (2.6)
For case, when N is very large, N >> 1 then eq. 2.6 can be modi ed into, W CDMA = = (f + 1) + N0 (2.7)
After computing the SINR of WCDMA, we can now nd the capacity of WCDMA by the given equation below, CW CDMA = log2(1 + W CDMA) [b=s=Hz] (2.8) Where the bandwidth is considered to be 1 Hz.
OFDMA Capacity
With OFDMA, multipath fading does not a ect the transmission due to the use of Cyclic Pre x (CP) and 1-tap equalization of the subcarriers of OFDMA signal. The only source of degradation is background noise and the interference from neighboring cells which occures when users are at the edge of the cell. Using eq. 2.4 OFDMA SINR can be written as follows, OF DMA = P=f P + N0 (2.9) By considering a at fading channel with no multipath for WCDMA case, the above equation for both OFDMA and WCDMA is the same. The capacity of OFDMA can be represented by the following equation, COF DMA = log2(1 + P=(f P + N0)) = log2(1 + =( f + 1)) [b=s=Hz] (2.10) Cyclic Pre x (CP) is also considered due to multipath fading, eq. 2.10 can be modi ed into, COF DMA = (1 =T s):log2(1 + P=(f P + N0)) = log2(1 + =( f + 1)) [b=s=Hz] (2.11)
Where f is the ratio of other cell to own cell interference [9]. This value is greater for users at the edge of the cell. So the users at center of the cell get good SINR values in OFDMA access technique and hence better QoS is assured as compared to users at the edge of cell facing high interference from neighboring cells and hence poor QoS.
Fig. 2 and Fig. 3 show the performance evaluation comparison of both access technologies for the SINR value of 10 dB and 0 dB respectively. Fig. 2 shows that the performance of OFDMA is better than WCDMA for cases when N=2, N=4 and very large value, when N>>1.
By increasing the number of multipaths, the WCDMA capacity is also decreasing as shown clearly in Fig. 2. We also notice that OFDMA capacity is degrading when interference from neighboring cells also increases, that is f the ratio component in eq. 2.11.
Table of contents :
1 INTRODUCTION
1.1 Background
1.2 Future Challenges and Goals
1.3 Thesis Layout
2 CELLULAR TECHNOLOGIES
2.1 History of Cellular Networks
2.2 Beyond 3G Networks
2.3 Long Term Evolution (LTE) Technology
2.4 WCDMA Compatibility Issues With LTE
2.4.1 OFDMA Access Technique
2.4.2 Transmission Capacity
2.4.2.1 WCDMA Capacity
2.4.2.2 OFDMA Capacity
2.5 Evolution of 4G LTE Network
3 THE FEMTOCELL SOLUTION SERVING INDOOR USERS
3.1 Indoor Trac in Cellular Mobile Networks
3.1.1 Mobile Data Trac
3.1.1.1 New Applications and Usage Pattern
3.1.1.2 Emergence of New Devices
3.1.2 Impact of Indoor Data Trac
3.1.2.1 Capacity Degradation in Indoor
3.1.3 WiFi Technology
3.2 Solutions for Ecient Service of Indoor Users
3.2.1 Femtocells
3.2.1.1 Network Management with Femtocells
3.2.1.2 Business Impact on Operators
4 METHODOLOGY AND SIMULATOR DEVELOPMENT
4.1 Simulation Tools
4.1.1 OMNeT++
4.1.1.1 Scope of OMNeT++
4.1.2 MATLAB based LTE Simulators
4.1.2.1 LTE Link Level Simulator
4.1.2.2 LTE System Level Simulator
4.1.2.2.1 Simulator Structure
4.1.2.2.2 Simulator Development
5 PERFORMANCE EVALUATION
5.1 Simulation Scnearios for Performance Evaluation
5.2 Without Deployment of Femtocells
5.2.1 Simulation Results
5.3 Dense Deployment of Femtocells
5.3.1 Simulation Results
6 FUTURE WORK AND CONCLUSION
6.1 Conclusion
6.2 Future Work
REFERENCES
APPENDICES
