Difference analysis of WCDMA and GSM system planning

GSM system is the world's most widely-covered second-generation wireless network, it uses TDMA (time division multiple Access) approach, that is, in a carrier channel, at the same interval at the same intervals into a fixed cycle of several time slots, each time slot carries a business user information and necessary signaling. In this way, a carrier channel is occupied by a number of users in chronological order, with different time slots as a way of distinguishing different users.

Characteristics of GSM

In the GSM system, each carrier channel bandwidth is 200KHz, each frame 8 time slot, that is theoretically can provide the transmission channel for 8 users at the same time, the time slot length 0.577ms, each frame length 4.615ms. As one of the main standards of the third generation wireless network, WCDMA system combines the high-speed mobile access and the Internet Protocol Service, while improving the efficiency of wireless frequency utilization, providing the users with richer wireless communication service. WCDMA Multiple access mode is CDMA, i.e. Code Division multiple access mode, it is a mobile communication multiple access technology developed in spread spectrum communication technology. (Computer science)

The principle is based on spread spectrum technology, will need to transmit a certain signal bandwidth information data, with a bandwidth far greater than the information bandwidth of high-speed pseudo-random code modulation, so that the original data signal bandwidth is extended, and then transmitted by carrier modulation. The narrowband signal that the wideband signal is reduced to the original information data is amplified by using the autocorrelation and correlation characteristics between different pseudo-random codes and the receiving end as the correlation processing. Different users distribute different spread-spectrum codes as distinguishing marks, so as to achieve the goal of non-interference and simultaneous communication between multiple users in the same carrier. In WCDMA networks, the bandwidth of one carrier is 5MHz. Because the CDMA access mode has good anti-interference ability, each cell uses the same carrier frequency. WCDMA system is a network based on CDMA access mode, its technical features are closer to CDMA, so it is different from GSM. WCDMA's unique advantages of WCDMA system can provide a higher quality than GSM voice services, at the same time with high-speed business, good spectrum utilization and very strong mobile capabilities (soft and softer switching).

At the same time, because of the characteristics of the integrated data service of 3G network, it puts forward higher requirements for the network design. As one of the world's few 5 wireless access technologies (TDMA,GSM,IS-95,WCDMA,CDMA2000) and its products, Nortel Networks has established more than 300 mobile networks worldwide for customers, These include CDMA networks built by more than 30 CDMA operators in 9 countries, 59 TDMA networks in 18 countries and 31 GSM networks for 60 countries.

Meanwhile, Nortel Networks are also building WCDMA networks for 6 pan-European operators and North American operators. From these network construction, Nortel Network has accumulated very rich experience and expertise in network construction, more deeply realize the different connotation of WCDMA and GSM access technology, this article will expound these different points and their influence to wireless network performance from the design and planning angle of wireless network.

Link budget and uplink coverage

Link budget is the basic tool of wireless network planning, and the result of link budget determines the coverage radius of the cell. We first analyze the differences between WCDMA and GSM in the link budget.

In the GSM system, the uplink and downlink are basically balanced, and the cell radius can be determined by the threshold of the base station downlink signal level higher than a certain design requirement. In WCDMA system, the balance of uplink and downlink is not the goal of network design. Base station power in the downlink by the community all users and signaling sharing, so will not become covered by the restricted link. On the contrary, the power of mobile phone transmission is defined in the specification.

Due to the limited transmit power, uplink becomes the limited link covered by WCDMA system. In other words, the maximum radius of a cell depends on the cell phone with the smallest power limit. So the link budget of WCDMA system usually refers to the uplink budget, that is, the loss and gain of the path loss are removed from the maximum allowable uplink loss, and the maximum allowable path loss is obtained, then the maximum allowable path loss value is brought into the propagation model, and the expected area coverage radius and coverage area are obtained. It should be noted that such a plot radius is the maximum cell radius in the absence of load, that is, the cover radius of the empty isolated cell. Because the WCDMA coverage area is not determined by the absolute value of the signal level as GSM, its coverage is related to the load or interference level of the system. After adding the load and the neighborhood interference, the radius of the cell will shrink accordingly. In the WCDMA link budget, to introduce a parameter called load factor, Nortel Network proposed to take 50% to cover the design of the load margin. In the network design to all the community evenly added to the load allowance, so that the system in fact, under uneven load operation state can still through the cell breathing adjustment to maintain balance. This is the experience in the design, operation and optimization of CDMA network for many years in Nortel Networks. This is not considered in the GSM network design.