Slavik Bryksin [email protected] CSE237a Fall 08 Topic research Data transmission standards over GSM/UMTS networks 1. Introduction There are a lot of emerging and existing standards that are used for data transmission over cellular networks. This paper is focused on the GSM/UMTS networks technologies that are marketed as 2G through 3G, their underlying technologies and concepts (channel access methods, duplexing, coding schemes, etc), data transmission rates, benefits and limitations. The generation that preceded 2G GSM was analog, whereas all following generations are digital. Generation labeling is mostly for marketing purposes, thus some technologies that existed in 2G are carried over and labeled 3G (i.e. EDGE versions), moreover, the timeline of adoption of the protocols and their inclusion under the umbrella of a certain generation might not align with the technology inception and certification. 2. (2G) Technologies 2.1. GSM (Global System for Mobile communications) GSM data transmission protocol is circuit switched with a fixed rate of 9.6Kbps and uses TDMA (Time Division Multiple Access) to assign static downlink and uplink timeslots for data.[16] The fact that data rate is fixed leads to inefficient usage of the available bandwidth due to the bursty network traffic.[1] 2.2. GPRS (General Packet Radio Service) GPRS standard is marketed as 2.5G and was the next step after circuit switched GSM standards. It is packet switched, which implies better bandwidth utilization, however packetization of data incurs the cost of extra information included in the packet, and the overhead of negotiation of transmission with the base station. GPRS employs hybrid channel access method: FDMA (Frequency Division Multiple Access) to allocate 124 channels, and TDMA to divide each frame into 8 slots [16]. The time slots are allocated on demand (upon data availability), thus, bandwidth is used more efficiently, and downlink/uplink bandwidth is dynamically adjusted for asymmetric traffic.[1] Depending on the quality of the radio channel, different coding schemes can be used (CS-1 to CS-4) with maximum data rate of 21.2 Kbps for each channel, which gives a theoretical maximum bandwidth of 171.2 Kbps for 8 channels. However, practical bandwidth is around 57 Kbps. 2.3. EDGE (Enhanced Data rates for GSM Evolution) EDGE, or EGRPS (Enhanced GPRS) is marketed as 2.75G or 3G technology, and can be deployed on existing GPRS networks with minimal cost.[3] It is a packet switched standard that uses a combination of 8-PSK (Phase Shift Keying) modulation with GMSK (Gaussian Minimum Shift Keying), whereas GSM and GPRS rely on GMSK.[2] Channel access method is a combination of TDMA and FDMA similar to GPRS. The enhancements also include different coding scheme (MSC1-MSC4 use GMSK, MSC5-MSC9 use 8-PSK), and link adaptation with incremental redundancy. Thus, link adaptation allows to set the best modulation and coding for a given channel quality, whereas incremental redundancy starts off with a high bit rate and small redundancy and increases redundancy coding upon failures. The theoretical maximum bandwidth is 473.6 Kbps (using 8-PSK, 8 time slots x 59.2Kbps per time slot), while typical practical bandwidth is up to 384 Kbps. 2.4.1 HSCSD (High Speed Circuit Switched Data) HSCSD is an enhancement of CSD (Circuit Switched Data) standard (where a single dedicated time slot is allocated). It is a TDMA-based standard that is circuit switched and uses multiple time slots that are reserved to a user.[4] Coding scheme uses ALA (Automatic Link Adaptation) that selects the most appropriate coding. The maximum data rate per time slot is 14.4 Kbps with 144CC coding, which gives a theoretical maximum bandwidth of 115.2Kbps (8 time slots x 14.4Kbps). One of the advantages of HSCSD is that it has lower latency then packet switched technologies: since there is no need for request transmission negotiations with the base station. However a dedicated allocation of several channels inefficiently uses bandwidth in case of idle channel and limits the total available bandwidth on the network. 3. (3G) Technologies 3.1. UMTS (Universal Mobile Telecommunications System) UMTS standards use W-CDMA as a primary air interface and FDD (Frequency Division Duplexing) with the exception of UMTS-TDD which relies on TD-CDMA (Time Division – Code Division Multiple Access) standard. UMTS is compatible with GSM, but uses different frequency bands, and, given wider spectrum of W-CDMA, bears an expense of spectrum licenses and interference. 3.1.1. W-CDMA (Wideband Code Division Multiple Access) This standard was developed by NTT Docomo and was first introduced in Docomo FOMA network in Japan and later was standardized by UMTS.[3] It relies on FDD for duplexing and uses a wide spectrum: 2x5Mhz channels for downlink and uplink[4], which is advantageous to the capacity of the network, however it is criticized for deployment issues in limited available spectrum. W-CDMA is not a data transmission standard, but is an underlying air interface in many emerging and existing 3G standards. 3.1.2 FOMA (Freedom of Mobile Multimedia Access) FOMA was the first 3G service by NTT Docomo that was launched in 2001 in Japan and used W- CDMA channel access method. Practical bandwidth (cited by Docomo and possibly capped is 3.6 Mbps downlink and 384Kbps uplink.[17] The initial implementation was power inefficient, however this improved after the inception of FOMA. Currently Docomo restructured its network to use HSDPA/ HSUPA standard over W-CDMA. 3.1.3 HSPA (High Speed Packet Access) HSPA is a collective name for HSDPA/ HSUPA standards and HSPA+, which is a part of UMTS and uses W-CDMA channel access. 3.1.3.1 HSDPA (High Speed Downlink Packet Access) HSDPA uses existing 3G networks infrastructure and is gaining foothold quickly. The key success factors of HSDPA include High Speed Downlink Shared Channel (HS-DSCH) combined with AMC (Adaptive Modulation and Coding), where the modulation is changed based on signal quality. The modulation scheme is a hybrid of 16QAM (Quadrature Amplitude Modulation) and QPSK (Quadrature Phase Shift Keying).[7,8] By employing adaptive higher order modulation and coding, higher data transfer rate can be achieved without increasing the bandwidth. HS-DSCH is a channel with constant Spreading Factor 16 (SF-16) CDMA codes that is transmitted at constant power while modulation, coding and number of codes change according to signal conditions. Another factor is Fast Packet Scheduling, where the base station determines how much data to send to devices based on their link strength, thus resulting in better utilization of base station resources.[10] HARQ (Hybrid Automatic Repeat reQuest) reduces latency of retransmission by storing erroneous packets. Then retransmit of corrupt packets is requested, and received packets are combined with the erroneous data to recover the packet. Thus, the possibility of successful recovery faced with low link quality is increased. HSDPA TTI (Transmission Time Interval) is reduced to 2ms, therefore reducing the latency and making the system react faster to changing link or user conditions. Theoretical bandwidth is 14.4Mbps.[10,11] 3.1.3.2 HSUPA (High Speed Uplink Packet Access) HSUPA or EUL (Enhanced UpLink) is a complimentary uplink standard to HSDPA that uses the same techniques as HSDPA with some differences. Higher order modulations are not used since uplink power budget is lower.[9] The E-DCH (Enhanced Dedicated Channel) uplink channel is dedicated. Also, scheduling has 2 options to achieve better power efficiency: scheduled, where the base station dynamically sets power level of the device transmission, and non-scheduled, where constant power level set by the device. Theoretical bandwidth of HSUPA is 5.76Mbps [9,11] 3.1.3.2 HSPA+ (High Speed Packet Access Evolved) HSPA+ is an enhancement of HSDPA/HSUPA standards that is currently developed and deployed. The data rates are increased by addition of MIMO antennas (multiple transmit/receive antennas) and higher orders of modulation (64QAM modulation on downlink, 16QAM modulation on uplink). A power saving feature of HSPA+ is Continuous Packet Connectivity, where both the device and the base station shut down the control channel transmission when there is no data channel transmissions. [11] Theoretical bandwidth of HSPA+ is 42Mbps downlink and 11.5Mbps uplink [12] 3.1.4 UMTS-TDD (Universal Mobile Telecommunications System – Time Division Duplex) UMTS-TDD uses wideband TD-CDMA air interface which is similar to W-CDMA, however the duplexing is TDD, unlike FDD in W-CDMA based standards. It is designed to work in a single unpaired frequency band. [13,15] Time multiplexing allows for a fine control over downlink/uplink bandwidth, and for better handling of asymmetric data [14], however the limiting factors are problems with interference and synchronization. Moreover, this standard is not compatible with UMTS W- CDMA networks because of different duplexing scheme. The theoretical maximum bandwidth is 16Mbps downlink, 16Mbps uplink. 4. Conclusion GSM/UTMS networks as well as cdmaOne/CDMA2000 networks are converging on CDMA based technologies. Another common factor is increasing spectrum usage, which allows higher data rates with the higher cost of licensing and power usage. Currently, as new 3G+ data transmission standards are being developed, the 3GPP longterm evolution (LTE) goals are to enable data rates up to 80Mbps, which is comparable with the 802.11 wireless data rates. These advances might be an opportunity for cell phone service providers to extend phone and data solutions beyond cell phones and PDAs. References: 1. C. Foh', B. Meini, B. Wvdrowski', M Zukerman. ”Modeling and Performance Evaluation of GPRS”, IEEE Vehicular Technology Conference, 2001.vol. 3, pp 2108-2112 2. A. FURUSKAR, S. MAZUR, F. ULLER, H OLOFSSON, “EDGE: Enhanced Data Rates for GSM and TDMA/736 Evolution”,IEEE Personal Communications, Jun 1999, vol.