3G Evolution. HSPA and LTE for Mobile Broadband

UTRA TDD

IEEE 802.20

MBWA

GSM/EDGE

Figure 20.1 The wireless technologies discussed in this book. The evolution tracks for UTRA FDD are described in Parts III and IV of this book, while this chapter gives an overview for the other technologies.

ideas have been applied as for WCDMA/HSDPA (UTRA FDD), but they were completed at a later stage in the 3GPP standardization process:

•HSDPA: Development of high-speed downlink packet access for UTRA TDD was performed as a common work item for WCDMA and UTRA TDD, but was performed as a separate task in practice [84]. HSDPA for TDD has many similarities with the FDD solution, such as the use of 16QAM higher-order modulation and the use of HARQ. Some main differences are that due to the TDD frame structure, timing relations are very different and a much larger TTI of 10 ms is used. Also, ACK/NACK information is not sent on a dedicated channel, but rather on a shared uplink resource.

•Enhanced Uplink: There is a corresponding Enhanced Uplink specified for UTRA TDD [82], also with some similarities to the FDD Enhanced Uplink. Some main differences are the 10 ms TTI due to the TDD frame structure and the use of a common scrambling sequence for uplink resources in the same time slot. This implies that the number of codes available in a time slot is limited, requiring that the NodeB scheduler dynamically reassigns the available uplink code resources between users. A second implication is that the TDD uplink can be code limited and therefore both QPSK and 16QAM modulation schemes are supported.

•MBMS: As described in Chapter 11, the main impact of MBMS is not on the radio access network [102]. There are however some additional transport channels. For MBMS in TDD, an additional feature added is simulcast combining. Unlike for FDD, selective or soft combining in the UE for macro-diversity reception is not mandatory for TDD prior to the introduction of MBMS. Since macro-diversity is essential for efficient broadcast service, a combination of simulcast reception with time slot reuse between cells is introduced. Time slots exclusively used for MBMS are assigned to groups of cells in a reuse pattern and the UE attempts to receive the time slots from each set and then combine them to enhance reception.

Figure 20.2 The evolution from IS-95 to CDMA2000 1x and 1x EV-DO.

and Voice) was developed to give parallel support of data and circuit-switched services on the same carrier. It is at the moment not developed further within 3GPP2.

20.2.1CDMA2000 1x

The CDMA2000 standard, that originally supported both a single-carrier (1x) and a multi-carrier (3x) mode, was adopted by ITU-R under the name IMT-2000 CDMA Multi-carrier (MC) [46]. It offers several improvements over the earlier versions of IS-95 that give better spectral efficiency and higher data rates. The most important aspect from a 3G evolution perspective however is that 1x has been a platform for further evolution of packet-data modes as shown in Figure 20.2. While 1x EV-DV is not undergoing any new development, 1x EV-DO is deployed today and is going through several new evolution steps in Rev 0, Rev A, Rev B, and Rev C that are further described below. Rev C is also called Ultra Mobile Broadband (UMB). While all steps up to Rev A are fundamentally based on direct sequence spread spectrum and a 1.25 MHz carrier bandwidth, Rev B and Rev C diverge from this by including wider carrier bandwidths and for Rev C also OFDM operation.

The 3x mode of CDMA2000 was never deployed in its original form, but was still an essential component of the submission of CDMA2000 to the ITU-R. However, multi-carrier CDMA is today once again included in the CDMA2000 evolution through CDMA2000 Rev B.

20.2.21x EV-DO Rev 0

EV-DO Rev 0 defines a new uplink and downlink structure for CDMA2000 1x, where DO originally implied ‘Data Only.’ The reason is that an EV-DO carrier has a structure optimized for data that does not support the voice and circuit-switched data services of a CDMA2000 1x carrier. In this way, the whole carrier works as a shared downlink resource for data transmission. An operator would deploy an additional carrier for EV-DO, thereby separating circuit-switched and packetswitched connections on different carriers. A drawback is a reduced flexibility in that there cannot be a simultaneous packet data and legacy circuit-switched service (such as voice) to the same user on one carrier.

With EV-DO Rev 0, a peak data rate of 2.4 Mbps is supported in the downlink on a 1.25 MHz carrier. There are several components in EV-DO Rev 0, some of which have similarities with the HSPA evolution described in Chapter 9:

•Shared-channel transmission: EV-DO Rev 0 has a Time Division Multiplexing

(TDM) downlink, with transmission to only one user at the time with the full power of the Base Station (BS). This makes the downlink a resource that is shared between the users in the time domain only. This is similar to the ‘sharedchannel transmission’ for HSDPA, with the difference that HSDPA can also share the downlink in the code domain, primarily with non-HSDPA users.

•Channel-dependent scheduling: The adaptive data scheduler of EV-DO Rev 0 takes into account fairness, queue sizes, and measures of the channel state. It thereby exploits multi-user diversity in fading channels in a way similar to the channel-dependent scheduling used in HSPA

•Short TTI: The transmission time interval is reduced from 20 ms in CDMA2000 to 1.6 ms in EV-DO Rev 0. This is important to enable fast channel-dependent scheduling and rapid retransmissions, resulting also in lower latency. The TTI for EV-DO Rev 0 is thereby of the same order as the 2 ms TTI used for HSPA.

•Rate control: EV-DO Rev 0 employs rate control through adaptive modulation and coding, thereby maximizing the throughput for a given channel condition. This is similar to HSPA, but the system in CDMA2000 follows the rate request from the mobile, while in HSPA the feedback is a recommendation and the NodeB takes the final decision.

•Higher-order modulation: EV-DO Rev 0 supports 16QAM modulation in the downlink, which is the same as for the HSPA downlink.

•Hybrid ARQ: The EV-DO Hybrid ARQ scheme is similar to the scheme used for HSPA.

•Virtual SOHO: EV-DO Rev 0 does not use soft handover in the downlink like CDMA2000 does. Instead the terminal supports ‘virtual soft handover’ via adaptive server selection initiated by the terminal, which can be seen as a fast

cell selection within the ‘active set’ of base stations. These server changes may result in some packet transmission delays [12].

•Receive diversity in the mobile: EV-DO Rev 0 has terminal performance numbers specified assuming receive diversity, similar to the ‘Type 1’ performance requirements specified for HSPA advanced receivers as discussed in Chapter 12.

20.2.31x EV-DO Rev A

The next step in the evolution of CDMA2000 is 1x EV-DO Rev A. The focus is on an uplink improvement similar to the Enhanced Uplink of HSPA, but it also includes an updated downlink, a more advanced quality-of-service handling, and an add-on multicast mode [12].

The downlink of EV-DO Rev A is based on the EV-DO Rev 0 downlink, with the following differences:

•Higher peak rates: EV-DO Rev A downlink supports 3.1 Mbps as compared to the 2.4 Mbps of EV-DO Rev 0. Rev A also offers a finer quantization of data rates.

•Shorter packets: New transmission formats for EV-DO Rev A enable 128-, 256-, and 512-bit packets. This together with new multi-user packets, where data to multiple terminals share the same packet in the downlink, improves support for lower rate, delay-sensitive services.

The major enhancements in EV-DO Rev A compared to Rev 0 are in the uplink. This results in a more packet-oriented uplink with higher capacity and data rates. Peak uplink data rates of up to 1.8 Mbps are supported:

•Higher-order modulation: In addition to BPSK modulation in EV-DO Rev 0, the uplink physical layer of Rev A supports QPSK and optionally 8PSK modulation.

•Hybrid ARQ: Improved performance is achieved through an uplink hybrid ARQ scheme, similar to HSPA.

•Reduced latency: The use of smaller packet sizes and a shorter TTI enables a reduced latency of up to 50% compared to EV-DO Rev 0 [8].

•Capacity/latency trade-off : Each packet can be transmitted in one of two possible transmission modes; LoLat gives low latency through higher power level ensuring that the packet is received within the latency target, while HiCap gives higher total capacity by allowing for more retransmissions and lower transmit power levels. This is similar to the use of hybrid ARQ profiles for Enhanced Uplink in HSPA as described in Chapter 10.

cards do not have to communicate with each other and can have independent scheduling and physical layer processes on each card.

Carriers do not have to be symmetrically allocated in the uplink and the downlink. For asymmetric applications such as file download, a larger number of carriers can be set up for downlink than for uplink. This reduces the amount of overhead for the uplink transmission. One uplink channel can carry feedback information for multiple forward link channels operating on multiple downlink carriers to a single terminal in asymmetric mode.

20.2.51x EV-DO Rev C (UMB)

A further step of CDMA2000 development is currently in progress in 3GPP2 as a Revision C of the 1x EV-DO standard. It includes work on smart-antenna technologies and fundamental channel bandwidths up to 5, 10, or 20 MHz [8]. 3GPP2 is investigating what is called a Loosely Backward Compatible (LBC) option for EV-DO Rev C that is also called Ultra Mobile Broadband (UMB). This evolution step is thus not backward compatible with the previous revisions of the CDMA2000 standard.

A joint proposal for Rev C was made in 3GPP2 [1], detailing a physical layer and some MAC layer aspects for FDD operation. Objectives for the proposed framework are introduction of higher peak date rates, better spectral efficiency, lower latency, improved terminal battery life and higher capacity and enhanced user experience for delay-sensitive applications. This joint proposal is very similar to LTE in terms of technologies and features used, with one main difference that Rev C uses OFDM in the uplink while LTE uses a single-carrier modulation. Rev C also supports a CDMA mode in the uplink. The theoretical peak data rates for a 20 MHz downlink is 260 Mbps and for the uplink 70 Mbps.

The downlink is OFDM based while for low-rate traffic there is also a CDMA mode in the uplink. Several varieties of multi-antenna techniques including multi-layer transmission are supported. Some important features of the downlink are:

•OFDM data transmission with 9.6 kHz subcarrier spacing and assuming an FFT size of 128, 256, 512, 1024, or 2048, depending on the sampling rate and bandwidth. There is a variable length cyclic prefix with a length of between 6% and 23% of the OFDM symbol duration.

•The downlink dedicated channel supports QPSK, 8PSK, 16QAM, and 64QAM modulations.

•Downlink scheduling and rate adaptation are based on CQI reports from the terminal. The CQI report also indicates the desired downlink for handover and can be used for power control purposes.

OFDMA

Pilot + data

CDMA

Pilot + control + data

Figure 20.4 1x EV-DO Rev C enables multiplexing of OFDMA and CDMA traffic on the uplink.

•MIMO (spatial multiplexing) with Single Codeword (SCW) and MultiCodeword (MCW) design is supported.

•Space Time Transmit Diversity (STTD) with up to four transmit antennas are used in place of MIMO when the propagation environment is not suitable for MIMO.

•Spaced Division Multiple Access (SDMA) schemes are under study.

The uplink is OFDMA based with optional CDMA transmission:

•OFDMA and CDMA uplink traffic are frequency multiplexed as shown in Figure 20.4.

•CDMA is mainly targeting bursty low-rate traffic and delay-sensitive applications. The lower Peak-to-Average Ratio (PAR) of CDMA is also beneficial for power-limited users.

•Uplink power control is used for both OFDMA and CDMA. In OFDMA mode the control channels are closed loop controlled, while the traffic has a delta power setting relative to the control channels. A fast Other Cell Interference Channel (OSICH) from base stations in the active set is used to further control the uplink power.

•The OFDMA mode includes frequency hopping which can be used universally across the band for full frequency diversity. It can also be done on a subband level to achieve multiplexing diversity in combination with the subband scheduling.

•There is also a ‘quasi-orthogonal’ uplink scheme that uses spatial processing (with multiple receive antennas) to demodulate the individual uplink signals. Frequency hopping is done with hopping sequences randomized between sectors to achieve intra-sector interference diversity.

Other prominent features of EV-DO Rev C are:

•A half-duplex mode is supported for use in fragmented spectrum. This may be scenarios where the mobile receive and transmit bands are too close, making the

implementation of a duplexer in the mobile too complex. In half-duplex mode, non-overlapping frames for uplink and downlink transmission are interlaced in two ‘half-duplex interlaces,’ with control information replicated on both interlaces.

•1xEV-DO Rev C [1] can be deployed with single frequency reuse, but frequency planning can optionally be used to enhance coverage and QoS. The trade-off is a bandwidth reduction with lower peak data rates and usually lower capacity.

20.3GSM/EDGE

Worldwide GSM deployment started in 1992 and GSM quickly became the most widely deployed cellular standard in the world with more than 2 billion subscribers today. A major evolution step for GSM was initiated in late 1996, at the same time as UTRA development started in ETSI. This step sprung out of the study of higherorder modulation for a 3G TDMA system that was performed within the European FRAMES project (see also Chapter 1).

The enhancement of GSM is called EDGE (Enhanced Data Rates for GSM Evolution). The EDGE development was initially focused on higher end-user data rates by introducing higher-order modulation in GSM, both for circuit-switched services and the GPRS packet-switched services. During the continued work, focus moved to the enhancement of GPRS (EGPRS), where other advanced radio interface components were added, including link adaptation, HARQ with soft combining, and advanced scheduling techniques as described in Part II of this book. In this way GSM became the first cellular standard to add such enhancements,2 later followed by WCDMA/HSPA, CDMA2000, and other technologies. GSM/EDGE is however a more narrowband technology than WCDMA/HSDPA and CDMA2000, implying that the peak data rates achievable are not as high. HSDPA and CDMA2000 have however added a time division structure to make full use of advanced scheduling techniques for high-rate data services. Being a TDMA system, EDGE already has a time division structure.

20.3.1Objectives for the GSM/EDGE evolution

The standardization work on GSM/EDGE in 3GPP involves a feasibility study for an ‘Evolved GSM/EDGE Radio Access Network (GERAN)’ [87]. The study objective is to improve service performance for GERAN for a range

2 The first cellular technology to deploy higher-order modulation was iDEN that uses ‘M-16QAM.’ It is however a proprietary technology and is as such not standardized.

Figure 20.5 GSM/EDGE network structure.

of services, including interactive best-effort services as well as conversational services including Voice-over-IP (VoIP).

The evolved GERAN should be compatible with the existing GSM/EDGE in terms of frequency planning and coexistence with legacy terminals. It should also reuse the existing network architecture (Figure 20.5) and have minimum impact on the BTS, BSC, and core network hardware. There are also a number of performance target for Evolved GERAN [87]:

•Improved spectrum efficiency: The target is a 50% increase in an interferencelimited scenario (measured in kbps/MHz/cell for data or Erlang/MHz/cell for voice).

•Increased peak data rates: The target is a 100% increase in both downlink and uplink.

•Improved coverage for voice and data: The target is a sensitivity increase of 3 dB in downlink (noise-limited scenario).

•Improved service availability: The target is a 50% increase in mean data rate

for uplink and downlink at cell edges (when cells are planned for voice).

• Reduced latency: The target is a roundtrip time (RTT) of less than 450 ms for initial access and less than 100 ms after initial access (in non-ideal radio conditions, counting across the radio access network as shown in Figure 20.5).

A number of alternative solutions are studied in 3GPP to achieve the performance and compatibility targets listed above. These are not fundamentally different from the solutions selected for LTE or other technologies and are all described in a more