3G Evolution. HSPA and LTE for Mobile Broadband

general context in Part II of this book. The technologies under consideration in 3GPP for GERAN Evolution are:

•Dual-antenna terminals.

•Multi-carrier EDGE.

•Reduced TTI and fast feedback.

•Improved modulation and coding.

•Higher symbol rates.

The specific application of these technologies to GERAN is discussed below. All of the proposed solutions are already undergoing standardization for GERAN in 3GPP (November 2006). Improved modulation and channel coding are being standardized for the uplink, but are still in a feasibility study phase for the downlink.

20.3.2Dual-antenna terminals

As was shown in Chapter 6, multiple receiver antennas are an effective means against multipath fading and to provide an improved signal-to-noise ratio through ‘power gain’ when combining the antenna signals. There will thus be improvements for both interference-limited and noise-limited scenarios. There are also possibilities for interference cancellation through multiple antennas.

For GSM/EDGE, a dual-antenna solution called Mobile Station Receive Diversity (MSRD) is being standardized. Analysis shows that a dual-antenna solution in GSM terminals can give a substantial coverage improvement of up to 6 dB. In addition, the dual-antenna terminals could potentially handle almost 10 dB more interference [20, 85].

20.3.3Multi-carrier EDGE

The GSM radio interface is based on a TDMA structure with 8 time slots and 200 kHz carrier bandwidth. To increase data rates, today’s GPRS and EDGE terminals can use multiple time slots for transmission, reception, or both. Using 8PSK modulation (Figure 20.6) and assigning the maximum of eight time slots, the GSM/EDGE standard gives a theoretical peak data rate of close to 480 kbps. However, from a design and complexity point of view, it is best to avoid simultaneous transmission and reception. Today’s terminals typically receive on a maximum of five time slots because they must also transmit (on at least one time slot) as well as measure the signal strength of neighboring cells.

To increase data rates further, multiple carriers for the downlink and the uplink can be introduced, similar to what is discussed in Chapter 3. This straightforward

Figure 20.6 Existing and proposed new modulation schemes for GSM/EDGE. The peak radio interface data rate using GPRS is shown for each scheme [20]. Note that the view of the non-linear binary GMSK scheme is simplified in the figure.

enhancement increases peak and mean data rates in proportion to the number of carriers employed. For example, given two carriers with eight time slots each, the peak data rate would be close to 1 Mbps. The limiting factor in this case is the complexity and cost of the terminal, which must have either multiple transmitters and receivers or a wideband transmitter and receiver. The use of multiple carriers has only a minor impact on base transceiver stations. A solution with two downlink carriers is being standardized in GERAN (November 2006).

20.3.4Reduced TTI and fast feedback

Latency is usually defined as the roundtrip time over the radio access network. It has a major influence on user experience and especially conversational services such as voice and video telephony require low latency. A major parameter in the radio interface that has impact on latency is the Transmission Time Interval (TTI) as discussed for HSPA and Enhanced Uplink in Chapters 9 and 10. It is difficult to substantially improve latency without reducing the TTI.

The roundtrip time (RTT) in advanced GSM/EDGE networks can be 150 ms [20], including network delays but not retransmissions over the radio interface. Radio blocks are transmitted interleaved over four consecutive bursts on one assigned

time slot over 20 ms. The TTI can be reduced by using fewer than four bursts for a radio block, or by transmitting the bursts for a radio block over several time slots assigned to the mobile or over more than one carrier. A reduction of the TTI from 20 to 10 ms can reduce the roundtrip time from 150 to 100 ms [20].

The receiver sends feedback information via the Radio Link Control (RLC). This informs the transmitter about the radio environment and acknowledges radio transmissions. More stringent requirements and fast responses to incorrectly received radio blocks speed up the retransmission of radio blocks and can also help in reducing the latency.

A work item for reduced TTI and fast feedback is initiated in 3GPP for GERAN (November 2006).

20.3.5Improved modulation and coding

The main evolution step taken for GSM when EDGE was introduced was the higher-order modulation to enhance the data rates over the radio interface – Enhanced Data Rates for GSM Evolution. 16QAM was considered for EDGE, but 8PSK was finally chosen as a smooth evolution step that gives 3 bits per modulated symbol instead of one. This increases the peak data rate from approximately 20 to 60 kbps.

Figure 20.6 shows two further steps that are considered for Evolved EDGE, giving 4 bits/symbol for 16QAM and 5 bits/symbol for 32QAM. Since the signal points in the new schemes move more closely together, they are more susceptible to interference. With more bits per symbol, however, the higher data rates allow for more robust channel coding which can more than compensate for the increased susceptibility to interference.

Both GSM and EDGE use convolutional codes, while Turbo codes are being considered for Evolved EDGE. Decoding Turbo codes is more complex than regular convolutional codes. However, since Turbo codes are already used today for WCDMA and HSPA, and many terminals support both GSM/EDGE and WCDMA/HSPA, the decoding circuitry for Turbo codes already exists in the terminals and can be reused for EDGE.

Turbo codes perform well for large code block sizes, making them better suited for higher data rate EDGE channels using 8PSK or higher-order modulation. It is estimated that compared to the existing EDGE scheme with 8PSK modulation, the combination of Turbo codes and 16QAM improves the user data rates 30–40% for the median user in a system [87].

IEEE 802.16 is an IEEE Standard for Wireless Metropolitan Area Networks (MAN). The first version was developed for fixed wireless broadband access in the 10–66 GHz bands and line-of-sight communication in 2001. The next step called IEEE 802.16a that was completed in 2003 supports also non-line- of-sight communication, focusing on the 2–11 GHz bands. The first products based on WiMAX will be based on the revised and consolidated version of the standard published in 2004 called 802.16-2004. The most recent addition to the WiMAX family of standards is 802.16e, which is also called ‘Mobile WiMAX.’

The IEEE standards specify the physical layer (PHY) and the Medium Access Layer (MAC), with no definition of higher layers. For IEEE 802.16, those are addressed in the WiMAX Forum Network Working Group. There is a range of options specified in IEEE 802.16, making the standards much more fragmented than what is seen in 3GPP and 3GPP2 standards. The 802.16 standard defines four different physical layers, of which two are certified by the WiMAX forum:

•OFDM-PHY: based on an FFT size of 256 and aimed at fixed networks.

•OFDMA-PHY (scalable): based on an FFT size from 128 to 2048 for 802.16e.

In addition to the multiple physical layers, the 802.16 standards support a range of options, including:

•TDD, FDD, and half-duplex FDD (H-FDD) operation.

•TDM access with variable frame size (2–20 ms).

•OFDM with a configurable cyclic prefix length.

•A wide range of bandwidths supported (1.25–28 MHz).

•Multiple modulation and coding schemes: QPSK, 16QAM, and 64QAM combined with convolutional codes, convolutional Turbo codes, block Turbo codes, and LDPC (Low-Density Parity Check) codes.

•Hybrid ARQ.

•Adaptive antenna system (AAS) and MIMO.

There is also a range of Radio Resource Management (RRM) options, MAC features and enhancements in the standards. The WiMAX forum defines system profiles that reduce all the optional features to a smaller set to allow interoperability among different vendors. This is done through an industry selection of features for MAC, PHY, and RF from 802.16 specifications and forms the basis for testing conformance and interoperability. Products certified by the WiMAX forum adhere to a Certification Profile that is based on a combination of band of operation, duplexing option and bandwidth.

The intended applications with the original 802.16 standard were fixed access and backhaul, mainly for line-of-sight operation. The addition of a physical layer for non-line-of-sight applications in IEEE 802.16-2004 and support for mobility in IEEE 802.16e opens up the standard for nomadic and mobile use. In addition, provisions for multicast and broadcast services (MBS) are also included. This makes the standard more similar to the evolved 3G standards, but coming from a completely different direction. The IEEE standards such as 802.16 are driven by the datacom industry as Layer 1 and 2 standards, starting with line-of-sight use for limited mobility, targeting best-effort data applications and now moving to higher mobility and encompassing also other applications such as conversational services. The evolved 3G standards are driven by the telecom industry, targeting non-line-of-sight use and mobility from the beginning, optimized end-to-end standards for voice and later also data services, now moving to broader data applications including best-effort services.

There are several versions of the IEEE 802.16 standards published from 2001 to 2005, with multiple physical layers and features as described above. The description of the WiMAX technology and its features below is based on the amended IEEE 802.16e-2005 version of the standard [2] and what is referred to as the Release 1 Mobile WiMAX system profiles [116] unless otherwise stated.

20.4.1Spectrum, bandwidth options and duplexing arrangement

The Release 1 WiMAX profiles cover operation in licensed spectrum allocations in the 2.3, 2.5, 3.3, and 3.5 GHz bands. The channel bandwidths supported are 5, 7, 8.75, and 10 MHz.

While WiMAX supports TDD, FDD, and half-duplex FDD, the first release only supports TDD operation. TDD enables adjustment of the downlink/uplink ratio for asymmetric traffic, does not require paired spectrum, and has a less complex transceiver design. To counter interference issues, TDD does however require system-wide synchronization and use of the same uplink/downlink ratio in neighbouring cells [116]. The reason is the potential for mobile-to-mobile and base station-to-base station interference if uplink and downlink allocations overlap, which becomes an issue in multi-cell deployments. Because of adjacent channel interference, system-wide synchronization may also be required for TDD operators deployed on adjacent or near-adjacent channels.

While the initial profiles and deployment of WiMAX use TDD as the preferred mode, FDD may be introduced in the longer term. The reason may be local

transmission ordering and scheduling on the air interface and can be negotiated statically or dynamically through MAC messages.

Applications supported through the WiMAX QoS mechanism are:

•Unsolicited Grant Service (UGS): VoIP

•Real-Time Polling Service (rtPS): Streaming Audio or Video

•Extended Real-Time Polling Service (ErtPS): Voice with Activity

•Non-Real-Time Polling Service (nrtPS): File Transfer Protocol (FTP)

•Best-Effort Service (BE): Data Transfer, Web Browsing, etc.

20.4.6Mobility

Mobile WiMAX supports both sleep mode and idle mode for more efficient power management. In sleep mode, there is a pre-negotiated period of absence from the radio interface to the serving base station, where the mobile station may power down or scan other neighboring base stations. There are different ‘power saving classes’ suitable for applications with different QoS types, each having different sleep mode parameters. There is also an idle mode, where the UE is not registered to any base station and instead periodically scans the network at discrete intervals.

There are three handover methods supported, with Hard Handover (HHO) being mandatory and Fast Base-Station Switching (FBSS) and Macro-Diversity handover (MDHO) being optional [2, 116]. Soft handover is not supported:

•Hard Handover (HHO): The mobile station scans neighboring Base Stations (BS) and maintains a list of candidate BS to make potential decisions for cell re-selection. The decision to handover to a new BS is made at the MS or at the serving BS. After decision, the MS immediately starts synchronization to the downlink of the target BS and obtains uplink and downlink transmission parameters and establishes a connection. Finally, the context of all connections to the previous serving BS are terminated.

•Fast Base-Station Switching (FBSS): The MS can maintain a diversity set (same as ‘active set’ for WCDMA) of multiple suitable Base Stations. One BS in the active set is selected as Anchor BS through which all uplink and downlink communication is done, both traffic and control messages. The MS selects the best BS in the diversity set as its Anchor and can change Anchor BS without any explicit handover signaling by reporting the selected BS on the CQICH channel.

•Macro-Diversity Handover (MDHO): An Anchor BS and a diversity set of the most suitable BSs are maintained also in MDHO. However, the MS communicates with all BSs in the diversity set in case of MDHO. In the downlink, multiple BS provides synchronized transmission of the same messages and in the uplink, multiple BS receive messages from the MS and provide selection diversity.

A requirement for FBSS and MDHO is that all BS in the active set need to operate on the same frequency, be time synchronized and also share and transfer all MAC contexts between BSs.

20.4.7Multi-antenna technologies

In addition to conventional receive diversity, WiMAX supports additional options using smart-antenna technologies:

•Beam-forming or Adaptive/Advanced Antenna System (AAS): Provides increased coverage and capacity through uplink and downlink beam-forming. AAS and non-AAS mobiles are time division multiplexed using a different time zone for the different options. Other MIMO schemes may be used in the non-AAS zones.

•Space–time coding (STC): With two transmit antennas, the use of transmit diversity through Alamouti code is supported to achieve spatial diversity. It is also possible to have transmit diversity using Linear Dispersion Codes (LDC) for more than two transmit antennas.

•Spatial multiplexing through MIMO: Higher peak rates and throughput in favorable propagation environments are provided through a downlink 2 × 2

MIMO scheme. The uplink supports ‘Uplink collaborative MIMO,’ where two users transmit with one antenna in the same slot and the BS receives the two multiplexed streams as if it were a MIMO transmission from a single user.

20.4.8Fractional frequency reuse

WiMAX can operate with a frequency reuse of one, but co-channel interference may in this case degrade the quality for users at the cell edge. However, a flexible sub-channel reuse is made possible by dividing the frame into permutation zones as described above. In this way, it is possible to have a sub-channel reuse by proper configuration of the sub-channel usage for the users. For users at the cell edges, the Base Station operates on a zone with a fraction of the sub-channels, while users close to the Base Station can operate on a zone with all sub-channels. As shown in the example in Figure 20.8, there can be an effective reuse of frequencies for users at the cell edge, while still maintaining a reuse of one for the OFDMA carrier as a whole.

20.5Mobile Broadband Wireless Access (IEEE 802.20)

Within the IEEE 802 LAN/MAN Standards Committee, the 802.20 group specifies a Mobile Broadband Wireless Access (MBWA) standard. The group is targeting a system optimized for IP-data transport, operating in licensed bands below 3.5 GHz with peak data rates per user in excess of 1 Mbps. High mobility with mobile speeds