Mobile Communication Systems

Introduction

The most popular mobile communication system is GSM (Global System for Mobile Communications). This is worldwide used second-generation (2G) cellular system which replaced first-generation (1G) analogue systems (e.g. NMT – Nordic Mobile Telephone). The GSM offers circuit switched voice calls, sending of short text messages (SMS – Short Message Service) and slow packet data transmission. The data transmission service was four times accelerated by the EDGE (Enhanced Data Rates for GSM Evolution).

The third-generation (3G) system is called UMTS (Universal Mobile Telecommunications System) and it was supposed to provide very fast data transmission. However, because of unwisely chosen radio access technology, i.e. WCDMA (Wideband Code Division Multiple Access), the practical downlink throughput speed was 80% lower than it was initially planned. However, mobile carriers invested a lot and they paid a fortune for 3G frequency licences. Therefore, extensions of UMTS technology were developed, i.e. HSPA (High Speed Packet Access) and its enhancements.

Data transmission in GSM system

The first voice call via GSM network was made in 1991, a year later the texting service was introduced. Firsts GSM networks were operating at 900 MHz frequency band. For higher network capacity, the 1800 MHz variant was developed. The first packet data transmission via cellular system was possible thanks to GPRS (General Packet Radio Service) which was commercially introduced in 2000. However, the maximum downlink transmission speed was about 56 kbps. This was slow, but comparable with classical modem connection. The GPRS is called also as 2.5G, because in contrast to the 2G system (i.e. GSM), it provides data transmission service.

The GSM is FDD (Frequency Division Duplex) system. It means, that the downlink and the uplink transmissions are carried at different frequencies. The channel bandwidth is only 200 kHz. However, the radio channel is divided into 8 time slots, so the TDMA (Time Division Multiple Access) technique is used. A single user uses a scheduled time slot, therefore up to 8 users can use the same frequency channel in the same time period.

Three years later (i.e. in 2003), the first commercial network with EDGE support was operational. The introduced enhanced allows to schedule 4 time-slot for a single user, therefore the downlink maximum data rate is about 236 kbps.

Improvements of 3G network – UMTS

The first UMTS standard was released in 1999 and it is called Release’99. It was supposed to offer up to 2 Mbps downlink peak rate. However, the maximum achievable result was only 384 kbps. UMTS system uses WCDMA technique in the radio interface. Thanks to this approach, all mobile users located in the same cell use the whole 5 MHz bandwidth at the same time. Each data stream is isolated from other streams by the scrambling codes. Unfortunately, these scrambling codes are not orthogonal in 100%, so a lot of interferences are generated by mobile phones located in the same cell.

The UMTS standard was developed by 3GPP (3rd Generation Mobile System) and it is fully compatible with GSM and EDGE mobile systems. It means that the mobile operator can have coexisting 2G and 3G networks. Also, the voice call can be switched between both networks without any connection drop.

Extended 3G networks: 3.5G, 3.75G, 3.9G

The speeds offered by the original UMTS R’99 were insufficient, therefore there was a need to develop enhancements for WCDMA system which will not require too much base station changes. The HSPA upgrade was introduced in two steps: HSDPA (High Speed Downlink Packet Access) introduced as 3GPP Release 5 in March 2002 and HSUPA (High Speed Uplink Packet Access) introduced as 3GPP Release 6 in December 2004. These two releases offer up to 14 Mbps downlink and 5.5 Mbps uplink throughput and they are very often called as 3.5G network.

The next step of 3G technology evolution is HSPA+, i.e. Evolved High Speed Packet Access, which was deployed as Release 7 in June 2007. The maximum downlink throughput speed is 21 Mbps with 64 QAM modulation and a single spatial data stream. In MIMO 2x2 mode only 16 QAM modulation is supported with the maximum downlink data rate 21 Mbps. The Release 8 capable mobile phones support MIMO 2x2 transmission mode with 64 QAM thanks to what up to 42 Mbps download throughput is achievable. In order to distinguish HSPA+ networks from HSPA networks, they are advertised as 3.75G.

The last step of WCDMA based network evolution is DC-HSPA (Dual Cell HSPA) which allows transmitting the downlink data with up to 84 Mbps using two 5 MHz cells and MIMO 2x2 spatial mode. This transmission mode is supported by Release 9 mobiles. The Release 10 supports up to four cells during carrier aggregation transmission, therefore 168 Mbps downlink throughput can be achieved in 20 MHz bandwidth.

The above-mentioned data rates are for the physical layer data rates, so the user data rate are transferred approximately 15% slower due to radio protocol and IP headers overheads. The DC-HSPA networks offer comparable throughputs with 4G LTE networks, so they are very often advertised as 3.9G.